'use strict';

function _interopDefault (ex) { return (ex && (typeof ex === 'object') && 'default' in ex) ? ex['default'] : ex; }

var cloneBuffer = _interopDefault(require('clone-buffer'));
var crypto = _interopDefault(require('crypto'));
var nodeFetch = require('node-fetch');
var nodeFetch__default = _interopDefault(nodeFetch);
var fetchCookie = _interopDefault(require('fetch-cookie'));
var AbortController = _interopDefault(require('abort-controller'));
var uuid = require('uuid');
var levelup = _interopDefault(require('levelup'));
var ltgt = _interopDefault(require('ltgt'));
var Codec = _interopDefault(require('level-codec'));
var ReadableStreamCore = _interopDefault(require('readable-stream'));
var Deque = _interopDefault(require('double-ended-queue'));
var bufferFrom = _interopDefault(require('buffer-from'));
var vuvuzela = _interopDefault(require('vuvuzela'));
var fs = _interopDefault(require('fs'));
var path = _interopDefault(require('path'));
var level = _interopDefault(require('level'));
var through2 = require('through2');
var LevelWriteStream = _interopDefault(require('level-write-stream'));
var vm = _interopDefault(require('vm'));
var EE = _interopDefault(require('events'));

function mangle(key) {
  return '$' + key;
}
function unmangle(key) {
  return key.substring(1);
}
function Map$1() {
  this._store = {};
}
Map$1.prototype.get = function (key) {
  var mangled = mangle(key);
  return this._store[mangled];
};
Map$1.prototype.set = function (key, value) {
  var mangled = mangle(key);
  this._store[mangled] = value;
  return true;
};
Map$1.prototype.has = function (key) {
  var mangled = mangle(key);
  return mangled in this._store;
};
Map$1.prototype.keys = function () {
  return Object.keys(this._store).map(k => unmangle(k));
};
Map$1.prototype.delete = function (key) {
  var mangled = mangle(key);
  var res = mangled in this._store;
  delete this._store[mangled];
  return res;
};
Map$1.prototype.forEach = function (cb) {
  var keys = Object.keys(this._store);
  for (var i = 0, len = keys.length; i < len; i++) {
    var key = keys[i];
    var value = this._store[key];
    key = unmangle(key);
    cb(value, key);
  }
};
Object.defineProperty(Map$1.prototype, 'size', {
  get: function () {
    return Object.keys(this._store).length;
  }
});

function Set$1(array) {
  this._store = new Map$1();

  // init with an array
  if (array && Array.isArray(array)) {
    for (var i = 0, len = array.length; i < len; i++) {
      this.add(array[i]);
    }
  }
}
Set$1.prototype.add = function (key) {
  return this._store.set(key, true);
};
Set$1.prototype.has = function (key) {
  return this._store.has(key);
};
Set$1.prototype.forEach = function (cb) {
  this._store.forEach(function (value, key) {
    cb(key);
  });
};
Object.defineProperty(Set$1.prototype, 'size', {
  get: function () {
    return this._store.size;
  }
});

// Based on https://kangax.github.io/compat-table/es6/ we can sniff out
// incomplete Map/Set implementations which would otherwise cause our tests to fail.
// Notably they fail in IE11 and iOS 8.4, which this prevents.
function supportsMapAndSet() {
  if (typeof Symbol === 'undefined' || typeof Map === 'undefined' || typeof Set === 'undefined') {
    return false;
  }
  var prop = Object.getOwnPropertyDescriptor(Map, Symbol.species);
  return prop && 'get' in prop && Map[Symbol.species] === Map;
}

// based on https://github.com/montagejs/collections

var ExportedSet;
var ExportedMap;

{
  if (supportsMapAndSet()) { // prefer built-in Map/Set
    ExportedSet = Set;
    ExportedMap = Map;
  } else { // fall back to our polyfill
    ExportedSet = Set$1;
    ExportedMap = Map$1;
  }
}

function isBinaryObject(object) {
  return object instanceof Buffer;
}

// most of this is borrowed from lodash.isPlainObject:
// https://github.com/fis-components/lodash.isplainobject/
// blob/29c358140a74f252aeb08c9eb28bef86f2217d4a/index.js

var funcToString = Function.prototype.toString;
var objectCtorString = funcToString.call(Object);

function isPlainObject(value) {
  var proto = Object.getPrototypeOf(value);
  /* istanbul ignore if */
  if (proto === null) { // not sure when this happens, but I guess it can
    return true;
  }
  var Ctor = proto.constructor;
  return (typeof Ctor == 'function' &&
    Ctor instanceof Ctor && funcToString.call(Ctor) == objectCtorString);
}

function clone(object) {
  var newObject;
  var i;
  var len;

  if (!object || typeof object !== 'object') {
    return object;
  }

  if (Array.isArray(object)) {
    newObject = [];
    for (i = 0, len = object.length; i < len; i++) {
      newObject[i] = clone(object[i]);
    }
    return newObject;
  }

  // special case: to avoid inconsistencies between IndexedDB
  // and other backends, we automatically stringify Dates
  if (object instanceof Date && isFinite(object)) {
    return object.toISOString();
  }

  if (isBinaryObject(object)) {
    return cloneBuffer(object);
  }

  if (!isPlainObject(object)) {
    return object; // don't clone objects like Workers
  }

  newObject = {};
  for (i in object) {
    /* istanbul ignore else */
    if (Object.prototype.hasOwnProperty.call(object, i)) {
      var value = clone(object[i]);
      if (typeof value !== 'undefined') {
        newObject[i] = value;
      }
    }
  }
  return newObject;
}

function once(fun) {
  var called = false;
  return function (...args) {
    /* istanbul ignore if */
    if (called) {
      // this is a smoke test and should never actually happen
      throw new Error('once called more than once');
    } else {
      called = true;
      fun.apply(this, args);
    }
  };
}

function toPromise(func) {
  //create the function we will be returning
  return function (...args) {
    // Clone arguments
    args = clone(args);
    var self = this;
    // if the last argument is a function, assume its a callback
    var usedCB = (typeof args[args.length - 1] === 'function') ? args.pop() : false;
    var promise = new Promise(function (fulfill, reject) {
      var resp;
      try {
        var callback = once(function (err, mesg) {
          if (err) {
            reject(err);
          } else {
            fulfill(mesg);
          }
        });
        // create a callback for this invocation
        // apply the function in the orig context
        args.push(callback);
        resp = func.apply(self, args);
        if (resp && typeof resp.then === 'function') {
          fulfill(resp);
        }
      } catch (e) {
        reject(e);
      }
    });
    // if there is a callback, call it back
    if (usedCB) {
      promise.then(function (result) {
        usedCB(null, result);
      }, usedCB);
    }
    return promise;
  };
}

function logApiCall(self, name, args) {
  /* istanbul ignore if */
  if (self.constructor.listeners('debug').length) {
    var logArgs = ['api', self.name, name];
    for (var i = 0; i < args.length - 1; i++) {
      logArgs.push(args[i]);
    }
    self.constructor.emit('debug', logArgs);

    // override the callback itself to log the response
    var origCallback = args[args.length - 1];
    args[args.length - 1] = function (err, res) {
      var responseArgs = ['api', self.name, name];
      responseArgs = responseArgs.concat(
        err ? ['error', err] : ['success', res]
      );
      self.constructor.emit('debug', responseArgs);
      origCallback(err, res);
    };
  }
}

function adapterFun(name, callback) {
  return toPromise(function (...args) {
    if (this._closed) {
      return Promise.reject(new Error('database is closed'));
    }
    if (this._destroyed) {
      return Promise.reject(new Error('database is destroyed'));
    }
    var self = this;
    logApiCall(self, name, args);
    if (!this.taskqueue.isReady) {
      return new Promise(function (fulfill, reject) {
        self.taskqueue.addTask(function (failed) {
          if (failed) {
            reject(failed);
          } else {
            fulfill(self[name].apply(self, args));
          }
        });
      });
    }
    return callback.apply(this, args);
  });
}

// like underscore/lodash _.pick()
function pick(obj, arr) {
  var res = {};
  for (var i = 0, len = arr.length; i < len; i++) {
    var prop = arr[i];
    if (prop in obj) {
      res[prop] = obj[prop];
    }
  }
  return res;
}

// Most browsers throttle concurrent requests at 6, so it's silly
// to shim _bulk_get by trying to launch potentially hundreds of requests
// and then letting the majority time out. We can handle this ourselves.
var MAX_NUM_CONCURRENT_REQUESTS = 6;

function identityFunction(x) {
  return x;
}

function formatResultForOpenRevsGet(result) {
  return [{
    ok: result
  }];
}

// shim for P/CouchDB adapters that don't directly implement _bulk_get
function bulkGet(db, opts, callback) {
  var requests = opts.docs;

  // consolidate into one request per doc if possible
  var requestsById = new ExportedMap();
  requests.forEach(function (request) {
    if (requestsById.has(request.id)) {
      requestsById.get(request.id).push(request);
    } else {
      requestsById.set(request.id, [request]);
    }
  });

  var numDocs = requestsById.size;
  var numDone = 0;
  var perDocResults = new Array(numDocs);

  function collapseResultsAndFinish() {
    var results = [];
    perDocResults.forEach(function (res) {
      res.docs.forEach(function (info) {
        results.push({
          id: res.id,
          docs: [info]
        });
      });
    });
    callback(null, {results: results});
  }

  function checkDone() {
    if (++numDone === numDocs) {
      collapseResultsAndFinish();
    }
  }

  function gotResult(docIndex, id, docs) {
    perDocResults[docIndex] = {id: id, docs: docs};
    checkDone();
  }

  var allRequests = [];
  requestsById.forEach(function (value, key) {
    allRequests.push(key);
  });

  var i = 0;

  function nextBatch() {

    if (i >= allRequests.length) {
      return;
    }

    var upTo = Math.min(i + MAX_NUM_CONCURRENT_REQUESTS, allRequests.length);
    var batch = allRequests.slice(i, upTo);
    processBatch(batch, i);
    i += batch.length;
  }

  function processBatch(batch, offset) {
    batch.forEach(function (docId, j) {
      var docIdx = offset + j;
      var docRequests = requestsById.get(docId);

      // just use the first request as the "template"
      // TODO: The _bulk_get API allows for more subtle use cases than this,
      // but for now it is unlikely that there will be a mix of different
      // "atts_since" or "attachments" in the same request, since it's just
      // replicate.js that is using this for the moment.
      // Also, atts_since is aspirational, since we don't support it yet.
      var docOpts = pick(docRequests[0], ['atts_since', 'attachments']);
      docOpts.open_revs = docRequests.map(function (request) {
        // rev is optional, open_revs disallowed
        return request.rev;
      });

      // remove falsey / undefined revisions
      docOpts.open_revs = docOpts.open_revs.filter(identityFunction);

      var formatResult = identityFunction;

      if (docOpts.open_revs.length === 0) {
        delete docOpts.open_revs;

        // when fetching only the "winning" leaf,
        // transform the result so it looks like an open_revs
        // request
        formatResult = formatResultForOpenRevsGet;
      }

      // globally-supplied options
      ['revs', 'attachments', 'binary', 'ajax', 'latest'].forEach(function (param) {
        if (param in opts) {
          docOpts[param] = opts[param];
        }
      });
      db.get(docId, docOpts, function (err, res) {
        var result;
        /* istanbul ignore if */
        if (err) {
          result = [{error: err}];
        } else {
          result = formatResult(res);
        }
        gotResult(docIdx, docId, result);
        nextBatch();
      });
    });
  }

  nextBatch();

}

// in Node of course this is false
function hasLocalStorage() {
  return false;
}

function nextTick(fn) {
  process.nextTick(fn);
}

class Changes extends EE {
  constructor() {
    super();
    
    this._listeners = {};
    
    if (hasLocalStorage()) {
      addEventListener("storage", (e) => {
        this.emit(e.key);
      });
    }
  }

  addListener(dbName, id, db, opts) {
    if (this._listeners[id]) {
      return;
    }
    var inprogress = false;
    var self = this;
    function eventFunction() {
      if (!self._listeners[id]) {
        return;
      }
      if (inprogress) {
        inprogress = 'waiting';
        return;
      }
      inprogress = true;
      var changesOpts = pick(opts, [
        'style', 'include_docs', 'attachments', 'conflicts', 'filter',
        'doc_ids', 'view', 'since', 'query_params', 'binary', 'return_docs'
      ]);
  
      function onError() {
        inprogress = false;
      }
  
      db.changes(changesOpts).on('change', function (c) {
        if (c.seq > opts.since && !opts.cancelled) {
          opts.since = c.seq;
          opts.onChange(c);
        }
      }).on('complete', function () {
        if (inprogress === 'waiting') {
          nextTick(eventFunction);
        }
        inprogress = false;
      }).on('error', onError);
    }
    this._listeners[id] = eventFunction;
    this.on(dbName, eventFunction);
  }
  
  removeListener(dbName, id) {
    if (!(id in this._listeners)) {
      return;
    }
    super.removeListener(dbName, this._listeners[id]);
    delete this._listeners[id];
  }
  
  notifyLocalWindows(dbName) {
    //do a useless change on a storage thing
    //in order to get other windows's listeners to activate
    if (hasLocalStorage()) {
      localStorage[dbName] = (localStorage[dbName] === "a") ? "b" : "a";
    }
  }
  
  notify(dbName) {
    this.emit(dbName);
    this.notifyLocalWindows(dbName);
  }
}

function guardedConsole(method) {
  /* istanbul ignore else */
  if (typeof console !== 'undefined' && typeof console[method] === 'function') {
    var args = Array.prototype.slice.call(arguments, 1);
    console[method].apply(console, args);
  }
}

function randomNumber(min, max) {
  var maxTimeout = 600000; // Hard-coded default of 10 minutes
  min = parseInt(min, 10) || 0;
  max = parseInt(max, 10);
  if (max !== max || max <= min) {
    max = (min || 1) << 1; //doubling
  } else {
    max = max + 1;
  }
  // In order to not exceed maxTimeout, pick a random value between half of maxTimeout and maxTimeout
  if (max > maxTimeout) {
    min = maxTimeout >> 1; // divide by two
    max = maxTimeout;
  }
  var ratio = Math.random();
  var range = max - min;

  return ~~(range * ratio + min); // ~~ coerces to an int, but fast.
}

function defaultBackOff(min) {
  var max = 0;
  if (!min) {
    max = 2000;
  }
  return randomNumber(min, max);
}

// We assume Node users don't need to see this warning
var res = function () {};

var assign;
{
  if (typeof Object.assign === 'function') {
    assign = Object.assign;
  } else {
    // lite Object.assign polyfill based on
    // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign
    assign = function (target) {
      var to = Object(target);

      for (var index = 1; index < arguments.length; index++) {
        var nextSource = arguments[index];

        if (nextSource != null) { // Skip over if undefined or null
          for (var nextKey in nextSource) {
            // Avoid bugs when hasOwnProperty is shadowed
            if (Object.prototype.hasOwnProperty.call(nextSource, nextKey)) {
              to[nextKey] = nextSource[nextKey];
            }
          }
        }
      }
      return to;
    };
  }
}

var $inject_Object_assign = assign;

class PouchError extends Error {
  constructor(status, error, reason) {
    super();
    this.status = status;
    this.name = error;
    this.message = reason;
    this.error = true;
  }

  toString() {
    return JSON.stringify({
      status: this.status,
      name: this.name,
      message: this.message,
      reason: this.reason
    });
  }
}

var UNAUTHORIZED = new PouchError(401, 'unauthorized', "Name or password is incorrect.");
var MISSING_BULK_DOCS = new PouchError(400, 'bad_request', "Missing JSON list of 'docs'");
var MISSING_DOC = new PouchError(404, 'not_found', 'missing');
var REV_CONFLICT = new PouchError(409, 'conflict', 'Document update conflict');
var INVALID_ID = new PouchError(400, 'bad_request', '_id field must contain a string');
var MISSING_ID = new PouchError(412, 'missing_id', '_id is required for puts');
var RESERVED_ID = new PouchError(400, 'bad_request', 'Only reserved document ids may start with underscore.');
var NOT_OPEN = new PouchError(412, 'precondition_failed', 'Database not open');
var UNKNOWN_ERROR = new PouchError(500, 'unknown_error', 'Database encountered an unknown error');
var BAD_ARG = new PouchError(500, 'badarg', 'Some query argument is invalid');
var INVALID_REQUEST = new PouchError(400, 'invalid_request', 'Request was invalid');
var QUERY_PARSE_ERROR = new PouchError(400, 'query_parse_error', 'Some query parameter is invalid');
var DOC_VALIDATION = new PouchError(500, 'doc_validation', 'Bad special document member');
var BAD_REQUEST = new PouchError(400, 'bad_request', 'Something wrong with the request');
var NOT_AN_OBJECT = new PouchError(400, 'bad_request', 'Document must be a JSON object');
var DB_MISSING = new PouchError(404, 'not_found', 'Database not found');
var IDB_ERROR = new PouchError(500, 'indexed_db_went_bad', 'unknown');
var WSQ_ERROR = new PouchError(500, 'web_sql_went_bad', 'unknown');
var LDB_ERROR = new PouchError(500, 'levelDB_went_went_bad', 'unknown');
var FORBIDDEN = new PouchError(403, 'forbidden', 'Forbidden by design doc validate_doc_update function');
var INVALID_REV = new PouchError(400, 'bad_request', 'Invalid rev format');
var FILE_EXISTS = new PouchError(412, 'file_exists', 'The database could not be created, the file already exists.');
var MISSING_STUB = new PouchError(412, 'missing_stub', 'A pre-existing attachment stub wasn\'t found');
var INVALID_URL = new PouchError(413, 'invalid_url', 'Provided URL is invalid');

function createError(error, reason) {
  function CustomPouchError(reason) {
    // inherit error properties from our parent error manually
    // so as to allow proper JSON parsing.
    /* jshint ignore:start */
    var names = Object.getOwnPropertyNames(error);
    for (var i = 0, len = names.length; i < len; i++) {
      if (typeof error[names[i]] !== 'function') {
        this[names[i]] = error[names[i]];
      }
    }

    if (this.stack === undefined) {
      this.stack = (new Error()).stack;
    }

    /* jshint ignore:end */
    if (reason !== undefined) {
      this.reason = reason;
    }
  }
  CustomPouchError.prototype = PouchError.prototype;
  return new CustomPouchError(reason);
}

function generateErrorFromResponse(err) {

  if (typeof err !== 'object') {
    var data = err;
    err = UNKNOWN_ERROR;
    err.data = data;
  }

  if ('error' in err && err.error === 'conflict') {
    err.name = 'conflict';
    err.status = 409;
  }

  if (!('name' in err)) {
    err.name = err.error || 'unknown';
  }

  if (!('status' in err)) {
    err.status = 500;
  }

  if (!('message' in err)) {
    err.message = err.message || err.reason;
  }

  if (!('stack' in err)) {
    err.stack = (new Error()).stack;
  }

  return err;
}

function tryFilter(filter, doc, req) {
  try {
    return !filter(doc, req);
  } catch (err) {
    var msg = 'Filter function threw: ' + err.toString();
    return createError(BAD_REQUEST, msg);
  }
}

function filterChange(opts) {
  var req = {};
  var hasFilter = opts.filter && typeof opts.filter === 'function';
  req.query = opts.query_params;

  return function filter(change) {
    if (!change.doc) {
      // CSG sends events on the changes feed that don't have documents,
      // this hack makes a whole lot of existing code robust.
      change.doc = {};
    }

    var filterReturn = hasFilter && tryFilter(opts.filter, change.doc, req);

    if (typeof filterReturn === 'object') {
      return filterReturn;
    }

    if (filterReturn) {
      return false;
    }

    if (!opts.include_docs) {
      delete change.doc;
    } else if (!opts.attachments) {
      for (var att in change.doc._attachments) {
        /* istanbul ignore else */
        if (Object.prototype.hasOwnProperty.call(change.doc._attachments, att)) {
          change.doc._attachments[att].stub = true;
        }
      }
    }
    return true;
  };
}

function flatten(arrs) {
  var res = [];
  for (var i = 0, len = arrs.length; i < len; i++) {
    res = res.concat(arrs[i]);
  }
  return res;
}

// shim for Function.prototype.name,
// for browsers that don't support it like IE

/* istanbul ignore next */
function f() {}

var hasName = f.name;
var res$1;

// We dont run coverage in IE
/* istanbul ignore else */
if (hasName) {
  res$1 = function (fun) {
    return fun.name;
  };
} else {
  res$1 = function (fun) {
    var match = fun.toString().match(/^\s*function\s*(?:(\S+)\s*)?\(/);
    if (match && match[1]) {
      return match[1];
    }
    else {
      return '';
    }
  };
}

var functionName = res$1;

// Determine id an ID is valid
//   - invalid IDs begin with an underescore that does not begin '_design' or
//     '_local'
//   - any other string value is a valid id
// Returns the specific error object for each case
function invalidIdError(id) {
  var err;
  if (!id) {
    err = createError(MISSING_ID);
  } else if (typeof id !== 'string') {
    err = createError(INVALID_ID);
  } else if (/^_/.test(id) && !(/^_(design|local)/).test(id)) {
    err = createError(RESERVED_ID);
  }
  if (err) {
    throw err;
  }
}

// Checks if a PouchDB object is "remote" or not. This is

function isRemote(db) {
  if (typeof db._remote === 'boolean') {
    return db._remote;
  }
  /* istanbul ignore next */
  if (typeof db.type === 'function') {
    guardedConsole('warn',
      'db.type() is deprecated and will be removed in ' +
      'a future version of PouchDB');
    return db.type() === 'http';
  }
  /* istanbul ignore next */
  return false;
}

function listenerCount(ee, type) {
  return 'listenerCount' in ee ? ee.listenerCount(type) :
                                 EE.listenerCount(ee, type);
}

function parseDesignDocFunctionName(s) {
  if (!s) {
    return null;
  }
  var parts = s.split('/');
  if (parts.length === 2) {
    return parts;
  }
  if (parts.length === 1) {
    return [s, s];
  }
  return null;
}

function normalizeDesignDocFunctionName(s) {
  var normalized = parseDesignDocFunctionName(s);
  return normalized ? normalized.join('/') : null;
}

// originally parseUri 1.2.2, now patched by us
// (c) Steven Levithan <stevenlevithan.com>
// MIT License
var keys = ["source", "protocol", "authority", "userInfo", "user", "password",
    "host", "port", "relative", "path", "directory", "file", "query", "anchor"];
var qName ="queryKey";
var qParser = /(?:^|&)([^&=]*)=?([^&]*)/g;

// use the "loose" parser
/* eslint no-useless-escape: 0 */
var parser = /^(?:(?![^:@]+:[^:@\/]*@)([^:\/?#.]+):)?(?:\/\/)?((?:(([^:@]*)(?::([^:@]*))?)?@)?([^:\/?#]*)(?::(\d*))?)(((\/(?:[^?#](?![^?#\/]*\.[^?#\/.]+(?:[?#]|$)))*\/?)?([^?#\/]*))(?:\?([^#]*))?(?:#(.*))?)/;

function parseUri(str) {
  var m = parser.exec(str);
  var uri = {};
  var i = 14;

  while (i--) {
    var key = keys[i];
    var value = m[i] || "";
    var encoded = ['user', 'password'].indexOf(key) !== -1;
    uri[key] = encoded ? decodeURIComponent(value) : value;
  }

  uri[qName] = {};
  uri[keys[12]].replace(qParser, function ($0, $1, $2) {
    if ($1) {
      uri[qName][$1] = $2;
    }
  });

  return uri;
}

// Based on https://github.com/alexdavid/scope-eval v0.0.3

// this is essentially the "update sugar" function from daleharvey/pouchdb#1388
// the diffFun tells us what delta to apply to the doc.  it either returns
// the doc, or false if it doesn't need to do an update after all
function upsert(db, docId, diffFun) {
  return db.get(docId)
    .catch(function (err) {
      /* istanbul ignore next */
      if (err.status !== 404) {
        throw err;
      }
      return {};
    })
    .then(function (doc) {
      // the user might change the _rev, so save it for posterity
      var docRev = doc._rev;
      var newDoc = diffFun(doc);

      if (!newDoc) {
        // if the diffFun returns falsy, we short-circuit as
        // an optimization
        return {updated: false, rev: docRev};
      }

      // users aren't allowed to modify these values,
      // so reset them here
      newDoc._id = docId;
      newDoc._rev = docRev;
      return tryAndPut(db, newDoc, diffFun);
    });
}

function tryAndPut(db, doc, diffFun) {
  return db.put(doc).then(function (res) {
    return {
      updated: true,
      rev: res.rev
    };
  }, function (err) {
    /* istanbul ignore next */
    if (err.status !== 409) {
      throw err;
    }
    return upsert(db, doc._id, diffFun);
  });
}

function binaryMd5(data, callback) {
  var base64 = crypto.createHash('md5').update(data, 'binary').digest('base64');
  callback(base64);
}

function stringMd5(string) {
  return crypto.createHash('md5').update(string, 'binary').digest('hex');
}

/**
 * Creates a new revision string that does NOT include the revision height
 * For example '56649f1b0506c6ca9fda0746eb0cacdf'
 */
function rev$$1(doc, deterministic_revs) {
  if (!deterministic_revs) {
    return uuid.v4().replace(/-/g, '').toLowerCase();
  }

  var mutateableDoc = $inject_Object_assign({}, doc);
  delete mutateableDoc._rev_tree;
  return stringMd5(JSON.stringify(mutateableDoc));
}

var uuid$1 = uuid.v4; // mimic old import, only v4 is ever used elsewhere

// We fetch all leafs of the revision tree, and sort them based on tree length
// and whether they were deleted, undeleted documents with the longest revision
// tree (most edits) win
// The final sort algorithm is slightly documented in a sidebar here:
// http://guide.couchdb.org/draft/conflicts.html
function winningRev(metadata) {
  var winningId;
  var winningPos;
  var winningDeleted;
  var toVisit = metadata.rev_tree.slice();
  var node;
  while ((node = toVisit.pop())) {
    var tree = node.ids;
    var branches = tree[2];
    var pos = node.pos;
    if (branches.length) { // non-leaf
      for (var i = 0, len = branches.length; i < len; i++) {
        toVisit.push({pos: pos + 1, ids: branches[i]});
      }
      continue;
    }
    var deleted = !!tree[1].deleted;
    var id = tree[0];
    // sort by deleted, then pos, then id
    if (!winningId || (winningDeleted !== deleted ? winningDeleted :
        winningPos !== pos ? winningPos < pos : winningId < id)) {
      winningId = id;
      winningPos = pos;
      winningDeleted = deleted;
    }
  }

  return winningPos + '-' + winningId;
}

// Pretty much all below can be combined into a higher order function to
// traverse revisions
// The return value from the callback will be passed as context to all
// children of that node
function traverseRevTree(revs, callback) {
  var toVisit = revs.slice();

  var node;
  while ((node = toVisit.pop())) {
    var pos = node.pos;
    var tree = node.ids;
    var branches = tree[2];
    var newCtx =
      callback(branches.length === 0, pos, tree[0], node.ctx, tree[1]);
    for (var i = 0, len = branches.length; i < len; i++) {
      toVisit.push({pos: pos + 1, ids: branches[i], ctx: newCtx});
    }
  }
}

function sortByPos(a, b) {
  return a.pos - b.pos;
}

function collectLeaves(revs) {
  var leaves = [];
  traverseRevTree(revs, function (isLeaf, pos, id, acc, opts) {
    if (isLeaf) {
      leaves.push({rev: pos + "-" + id, pos: pos, opts: opts});
    }
  });
  leaves.sort(sortByPos).reverse();
  for (var i = 0, len = leaves.length; i < len; i++) {
    delete leaves[i].pos;
  }
  return leaves;
}

// returns revs of all conflicts that is leaves such that
// 1. are not deleted and
// 2. are different than winning revision
function collectConflicts(metadata) {
  var win = winningRev(metadata);
  var leaves = collectLeaves(metadata.rev_tree);
  var conflicts = [];
  for (var i = 0, len = leaves.length; i < len; i++) {
    var leaf = leaves[i];
    if (leaf.rev !== win && !leaf.opts.deleted) {
      conflicts.push(leaf.rev);
    }
  }
  return conflicts;
}

// compact a tree by marking its non-leafs as missing,
// and return a list of revs to delete
function compactTree(metadata) {
  var revs = [];
  traverseRevTree(metadata.rev_tree, function (isLeaf, pos,
                                               revHash, ctx, opts) {
    if (opts.status === 'available' && !isLeaf) {
      revs.push(pos + '-' + revHash);
      opts.status = 'missing';
    }
  });
  return revs;
}

// `findPathToLeaf()` returns an array of revs that goes from the specified
// leaf rev to the root of that leaf’s branch.
//
// eg. for this rev tree:
// 1-9692 ▶ 2-37aa ▶ 3-df22 ▶ 4-6e94 ▶ 5-df4a ▶ 6-6a3a ▶ 7-57e5
//          ┃                 ┗━━━━━━▶ 5-8d8c ▶ 6-65e0
//          ┗━━━━━━▶ 3-43f6 ▶ 4-a3b4
//
// For a `targetRev` of '7-57e5', `findPathToLeaf()` would return ['7-57e5', '6-6a3a', '5-df4a']
// The `revs` arument has the same structure as what `revs_tree` has on e.g.
// the IndexedDB representation of the rev tree datastructure. Please refer to
// tests/unit/test.purge.js for examples of what these look like.
//
// This function will throw an error if:
// - The requested revision does not exist
// - The requested revision is not a leaf
function findPathToLeaf(revs, targetRev) {
  let path$$1 = [];
  const toVisit = revs.slice();

  let node;
  while ((node = toVisit.pop())) {
    const { pos, ids: tree } = node;
    const rev = `${pos}-${tree[0]}`;
    const branches = tree[2];

    // just assuming we're already working on the path up towards our desired leaf.
    path$$1.push(rev);

    // we've reached the leaf of our dreams, so return the computed path.
    if (rev === targetRev) {
      //…unleeeeess
      if (branches.length !== 0) {
        throw new Error('The requested revision is not a leaf');
      }
      return path$$1.reverse();
    }

    // this is based on the assumption that after we have a leaf (`branches.length == 0`), we handle the next
    // branch. this is true for all branches other than the path leading to the winning rev (which is 7-57e5 in
    // the example above. i've added a reset condition for branching nodes (`branches.length > 1`) as well.
    if (branches.length === 0 || branches.length > 1) {
      path$$1 = [];
    }

    // as a next step, we push the branches of this node to `toVisit` for visiting it during the next iteration
    for (let i = 0, len = branches.length; i < len; i++) {
      toVisit.push({ pos: pos + 1, ids: branches[i] });
    }
  }
  if (path$$1.length === 0) {
    throw new Error('The requested revision does not exist');
  }
  return path$$1.reverse();
}

// build up a list of all the paths to the leafs in this revision tree
function rootToLeaf(revs) {
  var paths = [];
  var toVisit = revs.slice();
  var node;
  while ((node = toVisit.pop())) {
    var pos = node.pos;
    var tree = node.ids;
    var id = tree[0];
    var opts = tree[1];
    var branches = tree[2];
    var isLeaf = branches.length === 0;

    var history = node.history ? node.history.slice() : [];
    history.push({id: id, opts: opts});
    if (isLeaf) {
      paths.push({pos: (pos + 1 - history.length), ids: history});
    }
    for (var i = 0, len = branches.length; i < len; i++) {
      toVisit.push({pos: pos + 1, ids: branches[i], history: history});
    }
  }
  return paths.reverse();
}

// for a better overview of what this is doing, read:

function sortByPos$1(a, b) {
  return a.pos - b.pos;
}

// classic binary search
function binarySearch(arr, item, comparator) {
  var low = 0;
  var high = arr.length;
  var mid;
  while (low < high) {
    mid = (low + high) >>> 1;
    if (comparator(arr[mid], item) < 0) {
      low = mid + 1;
    } else {
      high = mid;
    }
  }
  return low;
}

// assuming the arr is sorted, insert the item in the proper place
function insertSorted(arr, item, comparator) {
  var idx = binarySearch(arr, item, comparator);
  arr.splice(idx, 0, item);
}

// Turn a path as a flat array into a tree with a single branch.
// If any should be stemmed from the beginning of the array, that's passed
// in as the second argument
function pathToTree(path$$1, numStemmed) {
  var root;
  var leaf;
  for (var i = numStemmed, len = path$$1.length; i < len; i++) {
    var node = path$$1[i];
    var currentLeaf = [node.id, node.opts, []];
    if (leaf) {
      leaf[2].push(currentLeaf);
      leaf = currentLeaf;
    } else {
      root = leaf = currentLeaf;
    }
  }
  return root;
}

// compare the IDs of two trees
function compareTree(a, b) {
  return a[0] < b[0] ? -1 : 1;
}

// Merge two trees together
// The roots of tree1 and tree2 must be the same revision
function mergeTree(in_tree1, in_tree2) {
  var queue = [{tree1: in_tree1, tree2: in_tree2}];
  var conflicts = false;
  while (queue.length > 0) {
    var item = queue.pop();
    var tree1 = item.tree1;
    var tree2 = item.tree2;

    if (tree1[1].status || tree2[1].status) {
      tree1[1].status =
        (tree1[1].status ===  'available' ||
        tree2[1].status === 'available') ? 'available' : 'missing';
    }

    for (var i = 0; i < tree2[2].length; i++) {
      if (!tree1[2][0]) {
        conflicts = 'new_leaf';
        tree1[2][0] = tree2[2][i];
        continue;
      }

      var merged = false;
      for (var j = 0; j < tree1[2].length; j++) {
        if (tree1[2][j][0] === tree2[2][i][0]) {
          queue.push({tree1: tree1[2][j], tree2: tree2[2][i]});
          merged = true;
        }
      }
      if (!merged) {
        conflicts = 'new_branch';
        insertSorted(tree1[2], tree2[2][i], compareTree);
      }
    }
  }
  return {conflicts: conflicts, tree: in_tree1};
}

function doMerge(tree, path$$1, dontExpand) {
  var restree = [];
  var conflicts = false;
  var merged = false;
  var res;

  if (!tree.length) {
    return {tree: [path$$1], conflicts: 'new_leaf'};
  }

  for (var i = 0, len = tree.length; i < len; i++) {
    var branch = tree[i];
    if (branch.pos === path$$1.pos && branch.ids[0] === path$$1.ids[0]) {
      // Paths start at the same position and have the same root, so they need
      // merged
      res = mergeTree(branch.ids, path$$1.ids);
      restree.push({pos: branch.pos, ids: res.tree});
      conflicts = conflicts || res.conflicts;
      merged = true;
    } else if (dontExpand !== true) {
      // The paths start at a different position, take the earliest path and
      // traverse up until it as at the same point from root as the path we
      // want to merge.  If the keys match we return the longer path with the
      // other merged After stemming we dont want to expand the trees

      var t1 = branch.pos < path$$1.pos ? branch : path$$1;
      var t2 = branch.pos < path$$1.pos ? path$$1 : branch;
      var diff = t2.pos - t1.pos;

      var candidateParents = [];

      var trees = [];
      trees.push({ids: t1.ids, diff: diff, parent: null, parentIdx: null});
      while (trees.length > 0) {
        var item = trees.pop();
        if (item.diff === 0) {
          if (item.ids[0] === t2.ids[0]) {
            candidateParents.push(item);
          }
          continue;
        }
        var elements = item.ids[2];
        for (var j = 0, elementsLen = elements.length; j < elementsLen; j++) {
          trees.push({
            ids: elements[j],
            diff: item.diff - 1,
            parent: item.ids,
            parentIdx: j
          });
        }
      }

      var el = candidateParents[0];

      if (!el) {
        restree.push(branch);
      } else {
        res = mergeTree(el.ids, t2.ids);
        el.parent[2][el.parentIdx] = res.tree;
        restree.push({pos: t1.pos, ids: t1.ids});
        conflicts = conflicts || res.conflicts;
        merged = true;
      }
    } else {
      restree.push(branch);
    }
  }

  // We didnt find
  if (!merged) {
    restree.push(path$$1);
  }

  restree.sort(sortByPos$1);

  return {
    tree: restree,
    conflicts: conflicts || 'internal_node'
  };
}

// To ensure we dont grow the revision tree infinitely, we stem old revisions
function stem(tree, depth) {
  // First we break out the tree into a complete list of root to leaf paths
  var paths = rootToLeaf(tree);
  var stemmedRevs;

  var result;
  for (var i = 0, len = paths.length; i < len; i++) {
    // Then for each path, we cut off the start of the path based on the
    // `depth` to stem to, and generate a new set of flat trees
    var path$$1 = paths[i];
    var stemmed = path$$1.ids;
    var node;
    if (stemmed.length > depth) {
      // only do the stemming work if we actually need to stem
      if (!stemmedRevs) {
        stemmedRevs = {}; // avoid allocating this object unnecessarily
      }
      var numStemmed = stemmed.length - depth;
      node = {
        pos: path$$1.pos + numStemmed,
        ids: pathToTree(stemmed, numStemmed)
      };

      for (var s = 0; s < numStemmed; s++) {
        var rev = (path$$1.pos + s) + '-' + stemmed[s].id;
        stemmedRevs[rev] = true;
      }
    } else { // no need to actually stem
      node = {
        pos: path$$1.pos,
        ids: pathToTree(stemmed, 0)
      };
    }

    // Then we remerge all those flat trees together, ensuring that we dont
    // connect trees that would go beyond the depth limit
    if (result) {
      result = doMerge(result, node, true).tree;
    } else {
      result = [node];
    }
  }

  // this is memory-heavy per Chrome profiler, avoid unless we actually stemmed
  if (stemmedRevs) {
    traverseRevTree(result, function (isLeaf, pos, revHash) {
      // some revisions may have been removed in a branch but not in another
      delete stemmedRevs[pos + '-' + revHash];
    });
  }

  return {
    tree: result,
    revs: stemmedRevs ? Object.keys(stemmedRevs) : []
  };
}

function merge(tree, path$$1, depth) {
  var newTree = doMerge(tree, path$$1);
  var stemmed = stem(newTree.tree, depth);
  return {
    tree: stemmed.tree,
    stemmedRevs: stemmed.revs,
    conflicts: newTree.conflicts
  };
}

// return true if a rev exists in the rev tree, false otherwise
function revExists(revs, rev) {
  var toVisit = revs.slice();
  var splitRev = rev.split('-');
  var targetPos = parseInt(splitRev[0], 10);
  var targetId = splitRev[1];

  var node;
  while ((node = toVisit.pop())) {
    if (node.pos === targetPos && node.ids[0] === targetId) {
      return true;
    }
    var branches = node.ids[2];
    for (var i = 0, len = branches.length; i < len; i++) {
      toVisit.push({pos: node.pos + 1, ids: branches[i]});
    }
  }
  return false;
}

function getTrees(node) {
  return node.ids;
}

// check if a specific revision of a doc has been deleted
//  - metadata: the metadata object from the doc store
//  - rev: (optional) the revision to check. defaults to winning revision
function isDeleted(metadata, rev) {
  if (!rev) {
    rev = winningRev(metadata);
  }
  var id = rev.substring(rev.indexOf('-') + 1);
  var toVisit = metadata.rev_tree.map(getTrees);

  var tree;
  while ((tree = toVisit.pop())) {
    if (tree[0] === id) {
      return !!tree[1].deleted;
    }
    toVisit = toVisit.concat(tree[2]);
  }
}

function isLocalId(id) {
  return (/^_local/).test(id);
}

// returns the current leaf node for a given revision
function latest(rev, metadata) {
  var toVisit = metadata.rev_tree.slice();
  var node;
  while ((node = toVisit.pop())) {
    var pos = node.pos;
    var tree = node.ids;
    var id = tree[0];
    var opts = tree[1];
    var branches = tree[2];
    var isLeaf = branches.length === 0;

    var history = node.history ? node.history.slice() : [];
    history.push({id: id, pos: pos, opts: opts});

    if (isLeaf) {
      for (var i = 0, len = history.length; i < len; i++) {
        var historyNode = history[i];
        var historyRev = historyNode.pos + '-' + historyNode.id;

        if (historyRev === rev) {
          // return the rev of this leaf
          return pos + '-' + id;
        }
      }
    }

    for (var j = 0, l = branches.length; j < l; j++) {
      toVisit.push({pos: pos + 1, ids: branches[j], history: history});
    }
  }

  /* istanbul ignore next */
  throw new Error('Unable to resolve latest revision for id ' + metadata.id + ', rev ' + rev);
}

function tryCatchInChangeListener(self, change, pending, lastSeq) {
  // isolate try/catches to avoid V8 deoptimizations
  try {
    self.emit('change', change, pending, lastSeq);
  } catch (e) {
    guardedConsole('error', 'Error in .on("change", function):', e);
  }
}

function processChange(doc, metadata, opts) {
  var changeList = [{rev: doc._rev}];
  if (opts.style === 'all_docs') {
    changeList = collectLeaves(metadata.rev_tree)
    .map(function (x) { return {rev: x.rev}; });
  }
  var change = {
    id: metadata.id,
    changes: changeList,
    doc: doc
  };

  if (isDeleted(metadata, doc._rev)) {
    change.deleted = true;
  }
  if (opts.conflicts) {
    change.doc._conflicts = collectConflicts(metadata);
    if (!change.doc._conflicts.length) {
      delete change.doc._conflicts;
    }
  }
  return change;
}

class Changes$1 extends EE {
  constructor(db, opts, callback) {
    super();
    this.db = db;
    opts = opts ? clone(opts) : {};
    var complete = opts.complete = once((err, resp) => {
      if (err) {
        if (listenerCount(this, 'error') > 0) {
          this.emit('error', err);
        }
      } else {
        this.emit('complete', resp);
      }
      this.removeAllListeners();
      db.removeListener('destroyed', onDestroy);
    });
    if (callback) {
      this.on('complete', function (resp) {
        callback(null, resp);
      });
      this.on('error', callback);
    }
    const onDestroy = () => {
      this.cancel();
    };
    db.once('destroyed', onDestroy);
  
    opts.onChange = (change, pending, lastSeq) => {
      /* istanbul ignore if */
      if (this.isCancelled) {
        return;
      }
      tryCatchInChangeListener(this, change, pending, lastSeq);
    };
  
    var promise = new Promise(function (fulfill, reject) {
      opts.complete = function (err, res$$1) {
        if (err) {
          reject(err);
        } else {
          fulfill(res$$1);
        }
      };
    });
    this.once('cancel', function () {
      db.removeListener('destroyed', onDestroy);
      opts.complete(null, {status: 'cancelled'});
    });
    this.then = promise.then.bind(promise);
    this['catch'] = promise['catch'].bind(promise);
    this.then(function (result) {
      complete(null, result);
    }, complete);
  
  
  
    if (!db.taskqueue.isReady) {
      db.taskqueue.addTask((failed) => {
        if (failed) {
          opts.complete(failed);
        } else if (this.isCancelled) {
          this.emit('cancel');
        } else {
          this.validateChanges(opts);
        }
      });
    } else {
      this.validateChanges(opts);
    }
  }

  cancel() {
    this.isCancelled = true;
    if (this.db.taskqueue.isReady) {
      this.emit('cancel');
    }
  }

  validateChanges(opts) {
    var callback = opts.complete;
  
    /* istanbul ignore else */
    if (PouchDB._changesFilterPlugin) {
      PouchDB._changesFilterPlugin.validate(opts, (err) => {
        if (err) {
          return callback(err);
        }
        this.doChanges(opts);
      });
    } else {
      this.doChanges(opts);
    }
  }

  doChanges(opts) {
    var callback = opts.complete;
  
    opts = clone(opts);
    if ('live' in opts && !('continuous' in opts)) {
      opts.continuous = opts.live;
    }
    opts.processChange = processChange;
  
    if (opts.since === 'latest') {
      opts.since = 'now';
    }
    if (!opts.since) {
      opts.since = 0;
    }
    if (opts.since === 'now') {
      this.db.info().then((info) => {
        /* istanbul ignore if */
        if (this.isCancelled) {
          callback(null, {status: 'cancelled'});
          return;
        }
        opts.since = info.update_seq;
        this.doChanges(opts);
      }, callback);
      return;
    }
  
    /* istanbul ignore else */
    if (PouchDB._changesFilterPlugin) {
      PouchDB._changesFilterPlugin.normalize(opts);
      if (PouchDB._changesFilterPlugin.shouldFilter(this, opts)) {
        return PouchDB._changesFilterPlugin.filter(this, opts);
      }
    } else {
      ['doc_ids', 'filter', 'selector', 'view'].forEach(function (key) {
        if (key in opts) {
          guardedConsole('warn',
            'The "' + key + '" option was passed in to changes/replicate, ' +
            'but pouchdb-changes-filter plugin is not installed, so it ' +
            'was ignored. Please install the plugin to enable filtering.'
          );
        }
      });
    }
  
    if (!('descending' in opts)) {
      opts.descending = false;
    }
  
    // 0 and 1 should return 1 document
    opts.limit = opts.limit === 0 ? 1 : opts.limit;
    opts.complete = callback;
    var newPromise = this.db._changes(opts);
    /* istanbul ignore else */
    if (newPromise && typeof newPromise.cancel === 'function') {
      const cancel = this.cancel;
      this.cancel = (...args) => {
        newPromise.cancel();
        cancel.apply(this, args);
      };
    }
  }
}

/*
 * A generic pouch adapter
 */

function compare(left, right) {
  return left < right ? -1 : left > right ? 1 : 0;
}

// Wrapper for functions that call the bulkdocs api with a single doc,
// if the first result is an error, return an error
function yankError(callback, docId) {
  return function (err, results) {
    if (err || (results[0] && results[0].error)) {
      err = err || results[0];
      err.docId = docId;
      callback(err);
    } else {
      callback(null, results.length ? results[0]  : results);
    }
  };
}

// clean docs given to us by the user
function cleanDocs(docs) {
  for (var i = 0; i < docs.length; i++) {
    var doc = docs[i];
    if (doc._deleted) {
      delete doc._attachments; // ignore atts for deleted docs
    } else if (doc._attachments) {
      // filter out extraneous keys from _attachments
      var atts = Object.keys(doc._attachments);
      for (var j = 0; j < atts.length; j++) {
        var att = atts[j];
        doc._attachments[att] = pick(doc._attachments[att],
          ['data', 'digest', 'content_type', 'length', 'revpos', 'stub']);
      }
    }
  }
}

// compare two docs, first by _id then by _rev
function compareByIdThenRev(a, b) {
  var idCompare = compare(a._id, b._id);
  if (idCompare !== 0) {
    return idCompare;
  }
  var aStart = a._revisions ? a._revisions.start : 0;
  var bStart = b._revisions ? b._revisions.start : 0;
  return compare(aStart, bStart);
}

// for every node in a revision tree computes its distance from the closest
// leaf
function computeHeight(revs) {
  var height = {};
  var edges = [];
  traverseRevTree(revs, function (isLeaf, pos, id, prnt) {
    var rev = pos + "-" + id;
    if (isLeaf) {
      height[rev] = 0;
    }
    if (prnt !== undefined) {
      edges.push({from: prnt, to: rev});
    }
    return rev;
  });

  edges.reverse();
  edges.forEach(function (edge) {
    if (height[edge.from] === undefined) {
      height[edge.from] = 1 + height[edge.to];
    } else {
      height[edge.from] = Math.min(height[edge.from], 1 + height[edge.to]);
    }
  });
  return height;
}

function allDocsKeysParse(opts) {
  var keys =  ('limit' in opts) ?
    opts.keys.slice(opts.skip, opts.limit + opts.skip) :
    (opts.skip > 0) ? opts.keys.slice(opts.skip) : opts.keys;
  opts.keys = keys;
  opts.skip = 0;
  delete opts.limit;
  if (opts.descending) {
    keys.reverse();
    opts.descending = false;
  }
}

// all compaction is done in a queue, to avoid attaching
// too many listeners at once
function doNextCompaction(self) {
  var task = self._compactionQueue[0];
  var opts = task.opts;
  var callback = task.callback;
  self.get('_local/compaction').catch(function () {
    return false;
  }).then(function (doc) {
    if (doc && doc.last_seq) {
      opts.last_seq = doc.last_seq;
    }
    self._compact(opts, function (err, res$$1) {
      /* istanbul ignore if */
      if (err) {
        callback(err);
      } else {
        callback(null, res$$1);
      }
      nextTick(function () {
        self._compactionQueue.shift();
        if (self._compactionQueue.length) {
          doNextCompaction(self);
        }
      });
    });
  });
}

function appendPurgeSeq(db, docId, rev) {
  return db.get('_local/purges').then(function (doc) {
    const purgeSeq = doc.purgeSeq + 1;
    doc.purges.push({
      docId,
      rev,
      purgeSeq,
    });
    if (doc.purges.length > self.purged_infos_limit) {
      doc.purges.splice(0, doc.purges.length - self.purged_infos_limit);
    }
    doc.purgeSeq = purgeSeq;
    return doc;
  }).catch(function (err) {
    if (err.status !== 404) {
      throw err;
    }
    return {
      _id: '_local/purges',
      purges: [{
        docId,
        rev,
        purgeSeq: 0,
      }],
      purgeSeq: 0,
    };
  }).then(function (doc) {
    return db.put(doc);
  });
}

function attachmentNameError(name) {
  if (name.charAt(0) === '_') {
    return name + ' is not a valid attachment name, attachment ' +
      'names cannot start with \'_\'';
  }
  return false;
}

class AbstractPouchDB extends EE {
  _setup() {
    this.post = adapterFun('post', function (doc, opts, callback) {
      if (typeof opts === 'function') {
        callback = opts;
        opts = {};
      }
      if (typeof doc !== 'object' || Array.isArray(doc)) {
        return callback(createError(NOT_AN_OBJECT));
      }
      this.bulkDocs({docs: [doc]}, opts, yankError(callback, doc._id));
    }).bind(this);

    this.put = adapterFun('put', function (doc, opts, cb) {
      if (typeof opts === 'function') {
        cb = opts;
        opts = {};
      }
      if (typeof doc !== 'object' || Array.isArray(doc)) {
        return cb(createError(NOT_AN_OBJECT));
      }
      invalidIdError(doc._id);
      if (isLocalId(doc._id) && typeof this._putLocal === 'function') {
        if (doc._deleted) {
          return this._removeLocal(doc, cb);
        } else {
          return this._putLocal(doc, cb);
        }
      }

      const putDoc = (next) => {
        if (typeof this._put === 'function' && opts.new_edits !== false) {
          this._put(doc, opts, next);
        } else {
          this.bulkDocs({docs: [doc]}, opts, yankError(next, doc._id));
        }
      };

      if (opts.force && doc._rev) {
        transformForceOptionToNewEditsOption();
        putDoc(function (err) {
          var result = err ? null : {ok: true, id: doc._id, rev: doc._rev};
          cb(err, result);
        });
      } else {
        putDoc(cb);
      }

      function transformForceOptionToNewEditsOption() {
        var parts = doc._rev.split('-');
        var oldRevId = parts[1];
        var oldRevNum = parseInt(parts[0], 10);

        var newRevNum = oldRevNum + 1;
        var newRevId = rev$$1();

        doc._revisions = {
          start: newRevNum,
          ids: [newRevId, oldRevId]
        };
        doc._rev = newRevNum + '-' + newRevId;
        opts.new_edits = false;
      }
    }).bind(this);

    this.putAttachment = adapterFun('putAttachment', function (docId, attachmentId, rev, blob, type) {
      var api = this;
      if (typeof type === 'function') {
        type = blob;
        blob = rev;
        rev = null;
      }
      // Lets fix in https://github.com/pouchdb/pouchdb/issues/3267
      /* istanbul ignore if */
      if (typeof type === 'undefined') {
        type = blob;
        blob = rev;
        rev = null;
      }
      if (!type) {
        guardedConsole('warn', 'Attachment', attachmentId, 'on document', docId, 'is missing content_type');
      }

      function createAttachment(doc) {
        var prevrevpos = '_rev' in doc ? parseInt(doc._rev, 10) : 0;
        doc._attachments = doc._attachments || {};
        doc._attachments[attachmentId] = {
          content_type: type,
          data: blob,
          revpos: ++prevrevpos
        };
        return api.put(doc);
      }

      return api.get(docId).then(function (doc) {
        if (doc._rev !== rev) {
          throw createError(REV_CONFLICT);
        }

        return createAttachment(doc);
      }, function (err) {
        // create new doc
        /* istanbul ignore else */
        if (err.reason === MISSING_DOC.message) {
          return createAttachment({_id: docId});
        } else {
          throw err;
        }
      });
    }).bind(this);

    this.removeAttachment = adapterFun('removeAttachment', function (docId, attachmentId, rev, callback) {
      this.get(docId, (err, obj) => {
        /* istanbul ignore if */
        if (err) {
          callback(err);
          return;
        }
        if (obj._rev !== rev) {
          callback(createError(REV_CONFLICT));
          return;
        }
        /* istanbul ignore if */
        if (!obj._attachments) {
          return callback();
        }
        delete obj._attachments[attachmentId];
        if (Object.keys(obj._attachments).length === 0) {
          delete obj._attachments;
        }
        this.put(obj, callback);
      });
    }).bind(this);

    this.remove = adapterFun('remove', function (docOrId, optsOrRev, opts, callback) {
      var doc;
      if (typeof optsOrRev === 'string') {
        // id, rev, opts, callback style
        doc = {
          _id: docOrId,
          _rev: optsOrRev
        };
        if (typeof opts === 'function') {
          callback = opts;
          opts = {};
        }
      } else {
        // doc, opts, callback style
        doc = docOrId;
        if (typeof optsOrRev === 'function') {
          callback = optsOrRev;
          opts = {};
        } else {
          callback = opts;
          opts = optsOrRev;
        }
      }
      opts = opts || {};
      opts.was_delete = true;
      var newDoc = {_id: doc._id, _rev: (doc._rev || opts.rev)};
      newDoc._deleted = true;
      if (isLocalId(newDoc._id) && typeof this._removeLocal === 'function') {
        return this._removeLocal(doc, callback);
      }
      this.bulkDocs({docs: [newDoc]}, opts, yankError(callback, newDoc._id));
    }).bind(this);

    this.revsDiff = adapterFun('revsDiff', function (req, opts, callback) {
      if (typeof opts === 'function') {
        callback = opts;
        opts = {};
      }
      var ids = Object.keys(req);

      if (!ids.length) {
        return callback(null, {});
      }

      var count = 0;
      var missing = new ExportedMap();

      function addToMissing(id, revId) {
        if (!missing.has(id)) {
          missing.set(id, {missing: []});
        }
        missing.get(id).missing.push(revId);
      }

      function processDoc(id, rev_tree) {
        // Is this fast enough? Maybe we should switch to a set simulated by a map
        var missingForId = req[id].slice(0);
        traverseRevTree(rev_tree, function (isLeaf, pos, revHash, ctx,
          opts) {
            var rev = pos + '-' + revHash;
            var idx = missingForId.indexOf(rev);
            if (idx === -1) {
              return;
            }

            missingForId.splice(idx, 1);
            /* istanbul ignore if */
            if (opts.status !== 'available') {
              addToMissing(id, rev);
            }
          });

        // Traversing the tree is synchronous, so now `missingForId` contains
        // revisions that were not found in the tree
        missingForId.forEach(function (rev) {
          addToMissing(id, rev);
        });
      }

      ids.map(function (id) {
        this._getRevisionTree(id, function (err, rev_tree) {
          if (err && err.status === 404 && err.message === 'missing') {
            missing.set(id, {missing: req[id]});
          } else if (err) {
            /* istanbul ignore next */
            return callback(err);
          } else {
            processDoc(id, rev_tree);
          }

          if (++count === ids.length) {
            // convert LazyMap to object
            var missingObj = {};
            missing.forEach(function (value, key) {
              missingObj[key] = value;
            });
            return callback(null, missingObj);
          }
        });
      }, this);
    }).bind(this);

    // _bulk_get API for faster replication, as described in
    // https://github.com/apache/couchdb-chttpd/pull/33
    // At the "abstract" level, it will just run multiple get()s in
    // parallel, because this isn't much of a performance cost
    // for local databases (except the cost of multiple transactions, which is
    // small). The http adapter overrides this in order
    // to do a more efficient single HTTP request.
    this.bulkGet = adapterFun('bulkGet', function (opts, callback) {
      bulkGet(this, opts, callback);
    }).bind(this);

    // compact one document and fire callback
    // by compacting we mean removing all revisions which
    // are further from the leaf in revision tree than max_height
    this.compactDocument = adapterFun('compactDocument', function (docId, maxHeight, callback) {
      this._getRevisionTree(docId, (err, revTree) => {
        /* istanbul ignore if */
        if (err) {
          return callback(err);
        }
        var height = computeHeight(revTree);
        var candidates = [];
        var revs = [];
        Object.keys(height).forEach(function (rev) {
          if (height[rev] > maxHeight) {
            candidates.push(rev);
          }
        });

        traverseRevTree(revTree, function (isLeaf, pos, revHash, ctx, opts) {
          var rev = pos + '-' + revHash;
          if (opts.status === 'available' && candidates.indexOf(rev) !== -1) {
            revs.push(rev);
          }
        });
        this._doCompaction(docId, revs, callback);
      });
    }).bind(this);

    // compact the whole database using single document
    // compaction
    this.compact = adapterFun('compact', function (opts, callback) {
      if (typeof opts === 'function') {
        callback = opts;
        opts = {};
      }

      opts = opts || {};

      this._compactionQueue = this._compactionQueue || [];
      this._compactionQueue.push({opts: opts, callback: callback});
      if (this._compactionQueue.length === 1) {
        doNextCompaction(this);
      }
    }).bind(this);

    /* Begin api wrappers. Specific functionality to storage belongs in the _[method] */
    this.get = adapterFun('get', function (id, opts, cb) {
      if (typeof opts === 'function') {
        cb = opts;
        opts = {};
      }
      if (typeof id !== 'string') {
        return cb(createError(INVALID_ID));
      }
      if (isLocalId(id) && typeof this._getLocal === 'function') {
        return this._getLocal(id, cb);
      }
      var leaves = [];

      const finishOpenRevs = () => {
        var result = [];
        var count = leaves.length;
        /* istanbul ignore if */
        if (!count) {
          return cb(null, result);
        }

        // order with open_revs is unspecified
        leaves.forEach((leaf) => {
          this.get(id, {
            rev: leaf,
            revs: opts.revs,
            latest: opts.latest,
            attachments: opts.attachments,
            binary: opts.binary
          }, function (err, doc) {
            if (!err) {
              // using latest=true can produce duplicates
              var existing;
              for (var i = 0, l = result.length; i < l; i++) {
                if (result[i].ok && result[i].ok._rev === doc._rev) {
                  existing = true;
                  break;
                }
              }
              if (!existing) {
                result.push({ok: doc});
              }
            } else {
              result.push({missing: leaf});
            }
            count--;
            if (!count) {
              cb(null, result);
            }
          });
        });
      };

      if (opts.open_revs) {
        if (opts.open_revs === "all") {
          this._getRevisionTree(id, function (err, rev_tree) {
            /* istanbul ignore if */
            if (err) {
              return cb(err);
            }
            leaves = collectLeaves(rev_tree).map(function (leaf) {
              return leaf.rev;
            });
            finishOpenRevs();
          });
        } else {
          if (Array.isArray(opts.open_revs)) {
            leaves = opts.open_revs;
            for (var i = 0; i < leaves.length; i++) {
              var l = leaves[i];
              // looks like it's the only thing couchdb checks
              if (!(typeof (l) === "string" && /^\d+-/.test(l))) {
                return cb(createError(INVALID_REV));
              }
            }
            finishOpenRevs();
          } else {
            return cb(createError(UNKNOWN_ERROR, 'function_clause'));
          }
        }
        return; // open_revs does not like other options
      }

      return this._get(id, opts, (err, result) => {
        if (err) {
          err.docId = id;
          return cb(err);
        }

        var doc = result.doc;
        var metadata = result.metadata;
        var ctx = result.ctx;

        if (opts.conflicts) {
          var conflicts = collectConflicts(metadata);
          if (conflicts.length) {
            doc._conflicts = conflicts;
          }
        }

        if (isDeleted(metadata, doc._rev)) {
          doc._deleted = true;
        }

        if (opts.revs || opts.revs_info) {
          var splittedRev = doc._rev.split('-');
          var revNo       = parseInt(splittedRev[0], 10);
          var revHash     = splittedRev[1];

          var paths = rootToLeaf(metadata.rev_tree);
          var path$$1 = null;

          for (var i = 0; i < paths.length; i++) {
            var currentPath = paths[i];
            var hashIndex = currentPath.ids.map(function (x) { return x.id; })
              .indexOf(revHash);
            var hashFoundAtRevPos = hashIndex === (revNo - 1);

            if (hashFoundAtRevPos || (!path$$1 && hashIndex !== -1)) {
              path$$1 = currentPath;
            }
          }

          /* istanbul ignore if */
          if (!path$$1) {
            err = new Error('invalid rev tree');
            err.docId = id;
            return cb(err);
          }

          var indexOfRev = path$$1.ids.map(function (x) { return x.id; })
            .indexOf(doc._rev.split('-')[1]) + 1;
          var howMany = path$$1.ids.length - indexOfRev;
          path$$1.ids.splice(indexOfRev, howMany);
          path$$1.ids.reverse();

          if (opts.revs) {
            doc._revisions = {
              start: (path$$1.pos + path$$1.ids.length) - 1,
              ids: path$$1.ids.map(function (rev) {
                return rev.id;
              })
            };
          }
          if (opts.revs_info) {
            var pos =  path$$1.pos + path$$1.ids.length;
            doc._revs_info = path$$1.ids.map(function (rev) {
              pos--;
              return {
                rev: pos + '-' + rev.id,
                status: rev.opts.status
              };
            });
          }
        }

        if (opts.attachments && doc._attachments) {
          var attachments = doc._attachments;
          var count = Object.keys(attachments).length;
          if (count === 0) {
            return cb(null, doc);
          }
          Object.keys(attachments).forEach((key) => {
            this._getAttachment(doc._id, key, attachments[key], {
              // Previously the revision handling was done in adapter.js
              // getAttachment, however since idb-next doesnt we need to
              // pass the rev through
              rev: doc._rev,
              binary: opts.binary,
              ctx: ctx
            }, function (err, data) {
              var att = doc._attachments[key];
              att.data = data;
              delete att.stub;
              delete att.length;
              if (!--count) {
                cb(null, doc);
              }
            });
          });
        } else {
          if (doc._attachments) {
            for (var key in doc._attachments) {
              /* istanbul ignore else */
              if (Object.prototype.hasOwnProperty.call(doc._attachments, key)) {
                doc._attachments[key].stub = true;
              }
            }
          }
          cb(null, doc);
        }
      });
    }).bind(this);

    // TODO: I dont like this, it forces an extra read for every
    // attachment read and enforces a confusing api between
    // adapter.js and the adapter implementation
    this.getAttachment = adapterFun('getAttachment', function (docId, attachmentId, opts, callback) {
      if (opts instanceof Function) {
        callback = opts;
        opts = {};
      }
      this._get(docId, opts, (err, res$$1) => {
        if (err) {
          return callback(err);
        }
        if (res$$1.doc._attachments && res$$1.doc._attachments[attachmentId]) {
          opts.ctx = res$$1.ctx;
          opts.binary = true;
          this._getAttachment(docId, attachmentId,
                              res$$1.doc._attachments[attachmentId], opts, callback);
        } else {
          return callback(createError(MISSING_DOC));
        }
      });
    }).bind(this);

    this.allDocs = adapterFun('allDocs', function (opts, callback) {
      if (typeof opts === 'function') {
        callback = opts;
        opts = {};
      }
      opts.skip = typeof opts.skip !== 'undefined' ? opts.skip : 0;
      if (opts.start_key) {
        opts.startkey = opts.start_key;
      }
      if (opts.end_key) {
        opts.endkey = opts.end_key;
      }
      if ('keys' in opts) {
        if (!Array.isArray(opts.keys)) {
          return callback(new TypeError('options.keys must be an array'));
        }
        var incompatibleOpt =
          ['startkey', 'endkey', 'key'].filter(function (incompatibleOpt) {
          return incompatibleOpt in opts;
        })[0];
        if (incompatibleOpt) {
          callback(createError(QUERY_PARSE_ERROR,
            'Query parameter `' + incompatibleOpt +
            '` is not compatible with multi-get'
          ));
          return;
        }
        if (!isRemote(this)) {
          allDocsKeysParse(opts);
          if (opts.keys.length === 0) {
            return this._allDocs({limit: 0}, callback);
          }
        }
      }

      return this._allDocs(opts, callback);
    }).bind(this);

    this.close = adapterFun('close', function (callback) {
      this._closed = true;
      this.emit('closed');
      return this._close(callback);
    }).bind(this);

    this.info = adapterFun('info', function (callback) {
      this._info((err, info) => {
        if (err) {
          return callback(err);
        }
        // assume we know better than the adapter, unless it informs us
        info.db_name = info.db_name || this.name;
        info.auto_compaction = !!(this.auto_compaction && !isRemote(this));
        info.adapter = this.adapter;
        callback(null, info);
      });
    }).bind(this);

    this.id = adapterFun('id', function (callback) {
      return this._id(callback);
    }).bind(this);

    this.bulkDocs = adapterFun('bulkDocs', function (req, opts, callback) {
      if (typeof opts === 'function') {
        callback = opts;
        opts = {};
      }

      opts = opts || {};

      if (Array.isArray(req)) {
        req = {
          docs: req
        };
      }

      if (!req || !req.docs || !Array.isArray(req.docs)) {
        return callback(createError(MISSING_BULK_DOCS));
      }

      for (var i = 0; i < req.docs.length; ++i) {
        if (typeof req.docs[i] !== 'object' || Array.isArray(req.docs[i])) {
          return callback(createError(NOT_AN_OBJECT));
        }
      }

      var attachmentError;
      req.docs.forEach(function (doc) {
        if (doc._attachments) {
          Object.keys(doc._attachments).forEach(function (name) {
            attachmentError = attachmentError || attachmentNameError(name);
            if (!doc._attachments[name].content_type) {
              guardedConsole('warn', 'Attachment', name, 'on document', doc._id, 'is missing content_type');
            }
          });
        }
      });

      if (attachmentError) {
        return callback(createError(BAD_REQUEST, attachmentError));
      }

      if (!('new_edits' in opts)) {
        if ('new_edits' in req) {
          opts.new_edits = req.new_edits;
        } else {
          opts.new_edits = true;
        }
      }

      var adapter = this;
      if (!opts.new_edits && !isRemote(adapter)) {
        // ensure revisions of the same doc are sorted, so that
        // the local adapter processes them correctly (#2935)
        req.docs.sort(compareByIdThenRev);
      }

      cleanDocs(req.docs);

      // in the case of conflicts, we want to return the _ids to the user
      // however, the underlying adapter may destroy the docs array, so
      // create a copy here
      var ids = req.docs.map(function (doc) {
        return doc._id;
      });

      this._bulkDocs(req, opts, function (err, res$$1) {
        if (err) {
          return callback(err);
        }
        if (!opts.new_edits) {
          // this is what couch does when new_edits is false
          res$$1 = res$$1.filter(function (x) {
            return x.error;
          });
        }
        // add ids for error/conflict responses (not required for CouchDB)
        if (!isRemote(adapter)) {
          for (var i = 0, l = res$$1.length; i < l; i++) {
            res$$1[i].id = res$$1[i].id || ids[i];
          }
        }

        callback(null, res$$1);
      });
    }).bind(this);

    this.registerDependentDatabase = adapterFun('registerDependentDatabase', function (dependentDb, callback) {
      var dbOptions = clone(this.__opts);
      if (this.__opts.view_adapter) {
        dbOptions.adapter = this.__opts.view_adapter;
      }

      var depDB = new this.constructor(dependentDb, dbOptions);

      function diffFun(doc) {
        doc.dependentDbs = doc.dependentDbs || {};
        if (doc.dependentDbs[dependentDb]) {
          return false; // no update required
        }
        doc.dependentDbs[dependentDb] = true;
        return doc;
      }
      upsert(this, '_local/_pouch_dependentDbs', diffFun).then(function () {
        callback(null, {db: depDB});
      }).catch(callback);
    }).bind(this);

    this.destroy = adapterFun('destroy', function (opts, callback) {

      if (typeof opts === 'function') {
        callback = opts;
        opts = {};
      }

      var usePrefix = 'use_prefix' in this ? this.use_prefix : true;

      const destroyDb = () => {
        // call destroy method of the particular adaptor
        this._destroy(opts, (err, resp) => {
          if (err) {
            return callback(err);
          }
          this._destroyed = true;
          this.emit('destroyed');
          callback(null, resp || { 'ok': true });
        });
      };

      if (isRemote(this)) {
        // no need to check for dependent DBs if it's a remote DB
        return destroyDb();
      }

      this.get('_local/_pouch_dependentDbs', (err, localDoc) => {
        if (err) {
          /* istanbul ignore if */
          if (err.status !== 404) {
            return callback(err);
          } else { // no dependencies
            return destroyDb();
          }
        }
        var dependentDbs = localDoc.dependentDbs;
        var PouchDB = this.constructor;
        var deletedMap = Object.keys(dependentDbs).map((name) => {
          // use_prefix is only false in the browser
          /* istanbul ignore next */
          var trueName = usePrefix ?
            name.replace(new RegExp('^' + PouchDB.prefix), '') : name;
          return new PouchDB(trueName, this.__opts).destroy();
        });
        Promise.all(deletedMap).then(destroyDb, callback);
      });
    }).bind(this);
  }

  _compact(opts, callback) {
    var changesOpts = {
      return_docs: false,
      last_seq: opts.last_seq || 0
    };
    var promises = [];

    var taskId;
    var compactedDocs = 0;

    const onChange = (row) => {
      this.activeTasks.update(taskId, {
        completed_items: ++compactedDocs
      });
      promises.push(this.compactDocument(row.id, 0));
    };
    const onError = (err) => {
      this.activeTasks.remove(taskId, err);
      callback(err);
    };
    const onComplete = (resp) => {
      var lastSeq = resp.last_seq;
      Promise.all(promises).then(() => {
        return upsert(this, '_local/compaction', (doc) => {
          if (!doc.last_seq || doc.last_seq < lastSeq) {
            doc.last_seq = lastSeq;
            return doc;
          }
          return false; // somebody else got here first, don't update
        });
      }).then(() => {
        this.activeTasks.remove(taskId);
        callback(null, {ok: true});
      }).catch(onError);
    };

    this.info().then((info) => {
      taskId = this.activeTasks.add({
        name: 'database_compaction',
        total_items: info.update_seq - changesOpts.last_seq,
      });

      this.changes(changesOpts)
        .on('change', onChange)
        .on('complete', onComplete)
        .on('error', onError);
    });
  }

  changes(opts, callback) {
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }

    opts = opts || {};

    // By default set return_docs to false if the caller has opts.live = true,
    // this will prevent us from collecting the set of changes indefinitely
    // resulting in growing memory
    opts.return_docs = ('return_docs' in opts) ? opts.return_docs : !opts.live;

    return new Changes$1(this, opts, callback);
  }

  type() {
    return (typeof this._type === 'function') ? this._type() : this.adapter;
  }
}

// The abstract purge implementation expects a doc id and the rev of a leaf node in that doc.
// It will return errors if the rev doesn’t exist or isn’t a leaf.
AbstractPouchDB.prototype.purge = adapterFun('_purge', function (docId, rev, callback) {
  if (typeof this._purge === 'undefined') {
    return callback(createError(UNKNOWN_ERROR, 'Purge is not implemented in the ' + this.adapter + ' adapter.'));
  }
  var self = this;

  self._getRevisionTree(docId, (error, revs) => {
    if (error) {
      return callback(error);
    }
    if (!revs) {
      return callback(createError(MISSING_DOC));
    }
    let path$$1;
    try {
      path$$1 = findPathToLeaf(revs, rev);
    } catch (error) {
      return callback(error.message || error);
    }
    self._purge(docId, path$$1, (error, result) => {
      if (error) {
        return callback(error);
      } else {
        appendPurgeSeq(self, docId, rev).then(function () {
          return callback(null, result);
        });
      }
    });
  });
});

class TaskQueue {
  constructor() {
    this.isReady = false;
    this.failed = false;
    this.queue = [];
  }

  execute() {
    var fun;
    if (this.failed) {
      while ((fun = this.queue.shift())) {
        fun(this.failed);
      }
    } else {
      while ((fun = this.queue.shift())) {
        fun();
      }
    }
  }

  fail(err) {
    this.failed = err;
    this.execute();
  }

  ready(db) {
    this.isReady = true;
    this.db = db;
    this.execute();
  }

  addTask(fun) {
    this.queue.push(fun);
    if (this.failed) {
      this.execute();
    }
  }
}

function parseAdapter(name, opts) {
  var match = name.match(/([a-z-]*):\/\/(.*)/);
  if (match) {
    // the http adapter expects the fully qualified name
    return {
      name: /https?/.test(match[1]) ? match[1] + '://' + match[2] : match[2],
      adapter: match[1]
    };
  }

  var adapters = PouchDB.adapters;
  var preferredAdapters = PouchDB.preferredAdapters;
  var prefix = PouchDB.prefix;
  var adapterName = opts.adapter;

  if (!adapterName) { // automatically determine adapter
    for (var i = 0; i < preferredAdapters.length; ++i) {
      adapterName = preferredAdapters[i];
      // check for browsers that have been upgraded from websql-only to websql+idb
      /* istanbul ignore if */
      if (adapterName === 'idb' && 'websql' in adapters &&
          hasLocalStorage() && localStorage['_pouch__websqldb_' + prefix + name]) {
        // log it, because this can be confusing during development
        guardedConsole('log', 'PouchDB is downgrading "' + name + '" to WebSQL to' +
          ' avoid data loss, because it was already opened with WebSQL.');
        continue; // keep using websql to avoid user data loss
      }
      break;
    }
  }

  var adapter = adapters[adapterName];

  // if adapter is invalid, then an error will be thrown later
  var usePrefix = (adapter && 'use_prefix' in adapter) ?
    adapter.use_prefix : true;

  return {
    name: usePrefix ? (prefix + name) : name,
    adapter: adapterName
  };
}

function inherits(A, B) {
  A.prototype = Object.create(B.prototype, {
    constructor: { value: A }
  });
}

function createClass(parent, init) {
  let klass = function (...args) {
    if (!(this instanceof klass)) {
      return new klass(...args);
    }
    init.apply(this, args);
  };
  inherits(klass, parent);
  return klass;
}

// OK, so here's the deal. Consider this code:
//     var db1 = new PouchDB('foo');
//     var db2 = new PouchDB('foo');
//     db1.destroy();
// ^ these two both need to emit 'destroyed' events,
// as well as the PouchDB constructor itself.
// So we have one db object (whichever one got destroy() called on it)
// responsible for emitting the initial event, which then gets emitted
// by the constructor, which then broadcasts it to any other dbs
// that may have been created with the same name.
function prepareForDestruction(self) {

  function onDestroyed(from_constructor) {
    self.removeListener('closed', onClosed);
    if (!from_constructor) {
      self.constructor.emit('destroyed', self.name);
    }
  }

  function onClosed() {
    self.removeListener('destroyed', onDestroyed);
    self.constructor.emit('unref', self);
  }

  self.once('destroyed', onDestroyed);
  self.once('closed', onClosed);
  self.constructor.emit('ref', self);
}

class PouchInternal extends AbstractPouchDB {
  constructor(name, opts) {
    super();
    this._setup(name, opts);
  }

  _setup(name, opts) {
    super._setup();
    opts = opts || {};

    if (name && typeof name === 'object') {
      opts = name;
      name = opts.name;
      delete opts.name;
    }

    if (opts.deterministic_revs === undefined) {
      opts.deterministic_revs = true;
    }

    this.__opts = opts = clone(opts);

    this.auto_compaction = opts.auto_compaction;
    this.purged_infos_limit = opts.purged_infos_limit || 1000;
    this.prefix = PouchDB.prefix;

    if (typeof name !== 'string') {
      throw new Error('Missing/invalid DB name');
    }

    var prefixedName = (opts.prefix || '') + name;
    var backend = parseAdapter(prefixedName, opts);

    opts.name = backend.name;
    opts.adapter = opts.adapter || backend.adapter;

    this.name = name;
    this._adapter = opts.adapter;
    PouchDB.emit('debug', ['adapter', 'Picked adapter: ', opts.adapter]);

    if (!PouchDB.adapters[opts.adapter] ||
        !PouchDB.adapters[opts.adapter].valid()) {
      throw new Error('Invalid Adapter: ' + opts.adapter);
    }

    if (opts.view_adapter) {
      if (!PouchDB.adapters[opts.view_adapter] ||
          !PouchDB.adapters[opts.view_adapter].valid()) {
        throw new Error('Invalid View Adapter: ' + opts.view_adapter);
      }
    }

    this.taskqueue = new TaskQueue();

    this.adapter = opts.adapter;

    PouchDB.adapters[opts.adapter].call(this, opts, (err) => {
      if (err) {
        return this.taskqueue.fail(err);
      }
      prepareForDestruction(this);

      this.emit('created', this);
      PouchDB.emit('created', this.name);
      this.taskqueue.ready(this);
    });
  }
}

const PouchDB = createClass(PouchInternal, function (name, opts) {
  PouchInternal.prototype._setup.call(this, name, opts);
});

var fetch = fetchCookie(nodeFetch__default);

class ActiveTasks {
  constructor() {
    this.tasks = {};
  }

  list() {
    return Object.values(this.tasks);
  }

  add(task) {
    const id = uuid.v4();
    this.tasks[id] = {
      id,
      name: task.name,
      total_items: task.total_items,
      created_at: new Date().toJSON()
    };
    return id;
  }

  get(id) {
    return this.tasks[id];
  }

  /* eslint-disable no-unused-vars */
  remove(id, reason) {
    delete this.tasks[id];
    return this.tasks;
  }

  update(id, updatedTask) {
    const task = this.tasks[id];
    if (typeof task !== 'undefined') {
      const mergedTask = {
        id: task.id,
        name: task.name,
        created_at: task.created_at,
        total_items: updatedTask.total_items || task.total_items,
        completed_items: updatedTask.completed_items || task.completed_items,
        updated_at: new Date().toJSON()
      };
      this.tasks[id] = mergedTask;
    }
    return this.tasks;
  }
}

PouchDB.adapters = {};
PouchDB.preferredAdapters = [];

PouchDB.prefix = '_pouch_';

var eventEmitter = new EE();

function setUpEventEmitter(Pouch) {
  Object.keys(EE.prototype).forEach(function (key) {
    if (typeof EE.prototype[key] === 'function') {
      Pouch[key] = eventEmitter[key].bind(eventEmitter);
    }
  });

  // these are created in constructor.js, and allow us to notify each DB with
  // the same name that it was destroyed, via the constructor object
  var destructListeners = Pouch._destructionListeners = new ExportedMap();

  Pouch.on('ref', function onConstructorRef(db) {
    if (!destructListeners.has(db.name)) {
      destructListeners.set(db.name, []);
    }
    destructListeners.get(db.name).push(db);
  });

  Pouch.on('unref', function onConstructorUnref(db) {
    if (!destructListeners.has(db.name)) {
      return;
    }
    var dbList = destructListeners.get(db.name);
    var pos = dbList.indexOf(db);
    if (pos < 0) {
      /* istanbul ignore next */
      return;
    }
    dbList.splice(pos, 1);
    if (dbList.length > 1) {
      /* istanbul ignore next */
      destructListeners.set(db.name, dbList);
    } else {
      destructListeners.delete(db.name);
    }
  });

  Pouch.on('destroyed', function onConstructorDestroyed(name) {
    if (!destructListeners.has(name)) {
      return;
    }
    var dbList = destructListeners.get(name);
    destructListeners.delete(name);
    dbList.forEach(function (db) {
      db.emit('destroyed',true);
    });
  });
}

setUpEventEmitter(PouchDB);

PouchDB.adapter = function (id, obj, addToPreferredAdapters) {
  /* istanbul ignore else */
  if (obj.valid()) {
    PouchDB.adapters[id] = obj;
    if (addToPreferredAdapters) {
      PouchDB.preferredAdapters.push(id);
    }
  }
};

PouchDB.plugin = function (obj) {
  if (typeof obj === 'function') { // function style for plugins
    obj(PouchDB);
  } else if (typeof obj !== 'object' || Object.keys(obj).length === 0) {
    throw new Error('Invalid plugin: got "' + obj + '", expected an object or a function');
  } else {
    Object.keys(obj).forEach(function (id) { // object style for plugins
      PouchDB.prototype[id] = obj[id];
    });
  }
  if (this.__defaults) {
    PouchDB.__defaults = $inject_Object_assign({}, this.__defaults);
  }
  return PouchDB;
};

PouchDB.defaults = function (defaultOpts) {
  let PouchWithDefaults = createClass(PouchDB, function (name, opts) {
    opts = opts || {};

    if (name && typeof name === 'object') {
      opts = name;
      name = opts.name;
      delete opts.name;
    }

    opts = $inject_Object_assign({}, PouchWithDefaults.__defaults, opts);
    PouchDB.call(this, name, opts);
  });

  PouchWithDefaults.preferredAdapters = PouchDB.preferredAdapters.slice();
  Object.keys(PouchDB).forEach(function (key) {
    if (!(key in PouchWithDefaults)) {
      PouchWithDefaults[key] = PouchDB[key];
    }
  });

  // make default options transitive
  // https://github.com/pouchdb/pouchdb/issues/5922
  PouchWithDefaults.__defaults = $inject_Object_assign({}, this.__defaults, defaultOpts);

  return PouchWithDefaults;
};

PouchDB.fetch = function (url, opts) {
  return fetch(url, opts);
};

PouchDB.prototype.activeTasks = PouchDB.activeTasks = new ActiveTasks();

// managed automatically by set-version.js
var version = "8.0.1";

// this would just be "return doc[field]", but fields
// can be "deep" due to dot notation
function getFieldFromDoc(doc, parsedField) {
  var value = doc;
  for (var i = 0, len = parsedField.length; i < len; i++) {
    var key = parsedField[i];
    value = value[key];
    if (!value) {
      break;
    }
  }
  return value;
}

function compare$1(left, right) {
  return left < right ? -1 : left > right ? 1 : 0;
}

// Converts a string in dot notation to an array of its components, with backslash escaping
function parseField(fieldName) {
  // fields may be deep (e.g. "foo.bar.baz"), so parse
  var fields = [];
  var current = '';
  for (var i = 0, len = fieldName.length; i < len; i++) {
    var ch = fieldName[i];
    if (i > 0 && fieldName[i - 1] === '\\' && (ch === '$' || ch === '.')) {
      // escaped delimiter
      current = current.substring(0, current.length - 1) + ch;
    } else if (ch === '.') {
      // When `.` is not escaped (above), it is a field delimiter
      fields.push(current);
      current = '';
    } else { // normal character
      current += ch;
    }
  }
  fields.push(current);
  return fields;
}

var combinationFields = ['$or', '$nor', '$not'];
function isCombinationalField(field) {
  return combinationFields.indexOf(field) > -1;
}

function getKey(obj) {
  return Object.keys(obj)[0];
}

function getValue(obj) {
  return obj[getKey(obj)];
}


// flatten an array of selectors joined by an $and operator
function mergeAndedSelectors(selectors) {

  // sort to ensure that e.g. if the user specified
  // $and: [{$gt: 'a'}, {$gt: 'b'}], then it's collapsed into
  // just {$gt: 'b'}
  var res$$1 = {};
  var first = {$or: true, $nor: true};

  selectors.forEach(function (selector) {
    Object.keys(selector).forEach(function (field) {
      var matcher = selector[field];
      if (typeof matcher !== 'object') {
        matcher = {$eq: matcher};
      }

      if (isCombinationalField(field)) {
        // or, nor
        if (matcher instanceof Array) {
          if (first[field]) {
            first[field] = false;
            res$$1[field] = matcher;
            return;
          }

          var entries = [];
          res$$1[field].forEach(function (existing) {
            Object.keys(matcher).forEach(function (key) {
              var m = matcher[key];
              var longest = Math.max(Object.keys(existing).length, Object.keys(m).length);
              var merged = mergeAndedSelectors([existing, m]);
              if (Object.keys(merged).length <= longest) {
                // we have a situation like: (a :{$eq :1} || ...) && (a {$eq: 2} || ...)
                // merging would produce a $eq 2 when actually we shouldn't ever match against these merged conditions
                // merged should always contain more values to be valid
                return;
              }
              entries.push(merged);
            });
          });
          res$$1[field] = entries;
        } else {
          // not
          res$$1[field] = mergeAndedSelectors([matcher]);
        }
      } else {
        var fieldMatchers = res$$1[field] = res$$1[field] || {};
        Object.keys(matcher).forEach(function (operator) {
          var value = matcher[operator];

          if (operator === '$gt' || operator === '$gte') {
            return mergeGtGte(operator, value, fieldMatchers);
          } else if (operator === '$lt' || operator === '$lte') {
            return mergeLtLte(operator, value, fieldMatchers);
          } else if (operator === '$ne') {
            return mergeNe(value, fieldMatchers);
          } else if (operator === '$eq') {
            return mergeEq(value, fieldMatchers);
          } else if (operator === "$regex") {
            return mergeRegex(value, fieldMatchers);
          }
          fieldMatchers[operator] = value;
        });
      }
    });
  });

  return res$$1;
}



// collapse logically equivalent gt/gte values
function mergeGtGte(operator, value, fieldMatchers) {
  if (typeof fieldMatchers.$eq !== 'undefined') {
    return; // do nothing
  }
  if (typeof fieldMatchers.$gte !== 'undefined') {
    if (operator === '$gte') {
      if (value > fieldMatchers.$gte) { // more specificity
        fieldMatchers.$gte = value;
      }
    } else { // operator === '$gt'
      if (value >= fieldMatchers.$gte) { // more specificity
        delete fieldMatchers.$gte;
        fieldMatchers.$gt = value;
      }
    }
  } else if (typeof fieldMatchers.$gt !== 'undefined') {
    if (operator === '$gte') {
      if (value > fieldMatchers.$gt) { // more specificity
        delete fieldMatchers.$gt;
        fieldMatchers.$gte = value;
      }
    } else { // operator === '$gt'
      if (value > fieldMatchers.$gt) { // more specificity
        fieldMatchers.$gt = value;
      }
    }
  } else {
    fieldMatchers[operator] = value;
  }
}

// collapse logically equivalent lt/lte values
function mergeLtLte(operator, value, fieldMatchers) {
  if (typeof fieldMatchers.$eq !== 'undefined') {
    return; // do nothing
  }
  if (typeof fieldMatchers.$lte !== 'undefined') {
    if (operator === '$lte') {
      if (value < fieldMatchers.$lte) { // more specificity
        fieldMatchers.$lte = value;
      }
    } else { // operator === '$gt'
      if (value <= fieldMatchers.$lte) { // more specificity
        delete fieldMatchers.$lte;
        fieldMatchers.$lt = value;
      }
    }
  } else if (typeof fieldMatchers.$lt !== 'undefined') {
    if (operator === '$lte') {
      if (value < fieldMatchers.$lt) { // more specificity
        delete fieldMatchers.$lt;
        fieldMatchers.$lte = value;
      }
    } else { // operator === '$gt'
      if (value < fieldMatchers.$lt) { // more specificity
        fieldMatchers.$lt = value;
      }
    }
  } else {
    fieldMatchers[operator] = value;
  }
}

// combine $ne values into one array
function mergeNe(value, fieldMatchers) {
  if ('$ne' in fieldMatchers) {
    // there are many things this could "not" be
    fieldMatchers.$ne.push(value);
  } else { // doesn't exist yet
    fieldMatchers.$ne = [value];
  }
}

// add $eq into the mix
function mergeEq(value, fieldMatchers) {
  // these all have less specificity than the $eq
  // TODO: check for user errors here
  delete fieldMatchers.$gt;
  delete fieldMatchers.$gte;
  delete fieldMatchers.$lt;
  delete fieldMatchers.$lte;
  delete fieldMatchers.$ne;
  fieldMatchers.$eq = value;
}

// combine $regex values into one array
function mergeRegex(value, fieldMatchers) {
  if ('$regex' in fieldMatchers) {
    // a value could match multiple regexes
    fieldMatchers.$regex.push(value);
  } else { // doesn't exist yet
    fieldMatchers.$regex = [value];
  }
}

//#7458: execute function mergeAndedSelectors on nested $and
function mergeAndedSelectorsNested(obj) {
    for (var prop in obj) {
        if (Array.isArray(obj)) {
            for (var i in obj) {
                if (obj[i]['$and']) {
                    obj[i] = mergeAndedSelectors(obj[i]['$and']);
                }
            }
        }
        var value = obj[prop];
        if (typeof value === 'object') {
            mergeAndedSelectorsNested(value); // <- recursive call
        }
    }
    return obj;
}

//#7458: determine id $and is present in selector (at any level)
function isAndInSelector(obj, isAnd) {
    for (var prop in obj) {
        if (prop === '$and') {
            isAnd = true;
        }
        var value = obj[prop];
        if (typeof value === 'object') {
            isAnd = isAndInSelector(value, isAnd); // <- recursive call
        }
    }
    return isAnd;
}

//
// normalize the selector
//
function massageSelector(input) {
  var result = clone(input);

  //#7458: if $and is present in selector (at any level) merge nested $and
  if (isAndInSelector(result, false)) {
    result = mergeAndedSelectorsNested(result);
    if ('$and' in result) {
      result = mergeAndedSelectors(result['$and']);
    }
  }

  ['$or', '$nor'].forEach(function (orOrNor) {
    if (orOrNor in result) {
      // message each individual selector
      // e.g. {foo: 'bar'} becomes {foo: {$eq: 'bar'}}
      result[orOrNor].forEach(function (subSelector) {
        var fields = Object.keys(subSelector);
        for (var i = 0; i < fields.length; i++) {
          var field = fields[i];
          var matcher = subSelector[field];
          if (typeof matcher !== 'object' || matcher === null) {
            subSelector[field] = {$eq: matcher};
          }
        }
      });
    }
  });

  if ('$not' in result) {
    //This feels a little like forcing, but it will work for now,
    //I would like to come back to this and make the merging of selectors a little more generic
    result['$not'] = mergeAndedSelectors([result['$not']]);
  }

  var fields = Object.keys(result);

  for (var i = 0; i < fields.length; i++) {
    var field = fields[i];
    var matcher = result[field];

    if (typeof matcher !== 'object' || matcher === null) {
      matcher = {$eq: matcher};
    }
    result[field] = matcher;
  }

  normalizeArrayOperators(result);

  return result;
}

//
// The $ne and $regex values must be placed in an array because these operators can be used multiple times on the same field.
// When $and is used, mergeAndedSelectors takes care of putting some of them into arrays, otherwise it's done here.
//
function normalizeArrayOperators(selector) {
  Object.keys(selector).forEach(function (field) {
    var matcher = selector[field];

    if (Array.isArray(matcher)) {
      matcher.forEach(function (matcherItem) {
        if (matcherItem && typeof matcherItem === 'object') {
          normalizeArrayOperators(matcherItem);
        }
      });
    } else if (field === '$ne') {
      selector.$ne = [matcher];
    } else if (field === '$regex') {
      selector.$regex = [matcher];
    } else if (matcher && typeof matcher === 'object') {
      normalizeArrayOperators(matcher);
    }
  });
}

function pad(str, padWith, upToLength) {
  var padding = '';
  var targetLength = upToLength - str.length;
  /* istanbul ignore next */
  while (padding.length < targetLength) {
    padding += padWith;
  }
  return padding;
}

function padLeft(str, padWith, upToLength) {
  var padding = pad(str, padWith, upToLength);
  return padding + str;
}

var MIN_MAGNITUDE = -324; // verified by -Number.MIN_VALUE
var MAGNITUDE_DIGITS = 3; // ditto
var SEP = ''; // set to '_' for easier debugging

function collate(a, b) {

  if (a === b) {
    return 0;
  }

  a = normalizeKey(a);
  b = normalizeKey(b);

  var ai = collationIndex(a);
  var bi = collationIndex(b);
  if ((ai - bi) !== 0) {
    return ai - bi;
  }
  switch (typeof a) {
    case 'number':
      return a - b;
    case 'boolean':
      return a < b ? -1 : 1;
    case 'string':
      return stringCollate(a, b);
  }
  return Array.isArray(a) ? arrayCollate(a, b) : objectCollate(a, b);
}

// couch considers null/NaN/Infinity/-Infinity === undefined,
// for the purposes of mapreduce indexes. also, dates get stringified.
function normalizeKey(key) {
  switch (typeof key) {
    case 'undefined':
      return null;
    case 'number':
      if (key === Infinity || key === -Infinity || isNaN(key)) {
        return null;
      }
      return key;
    case 'object':
      var origKey = key;
      if (Array.isArray(key)) {
        var len = key.length;
        key = new Array(len);
        for (var i = 0; i < len; i++) {
          key[i] = normalizeKey(origKey[i]);
        }
      /* istanbul ignore next */
      } else if (key instanceof Date) {
        return key.toJSON();
      } else if (key !== null) { // generic object
        key = {};
        for (var k in origKey) {
          if (Object.prototype.hasOwnProperty.call(origKey, k)) {
            var val = origKey[k];
            if (typeof val !== 'undefined') {
              key[k] = normalizeKey(val);
            }
          }
        }
      }
  }
  return key;
}

function indexify(key) {
  if (key !== null) {
    switch (typeof key) {
      case 'boolean':
        return key ? 1 : 0;
      case 'number':
        return numToIndexableString(key);
      case 'string':
        // We've to be sure that key does not contain \u0000
        // Do order-preserving replacements:
        // 0 -> 1, 1
        // 1 -> 1, 2
        // 2 -> 2, 2
        /* eslint-disable no-control-regex */
        return key
          .replace(/\u0002/g, '\u0002\u0002')
          .replace(/\u0001/g, '\u0001\u0002')
          .replace(/\u0000/g, '\u0001\u0001');
        /* eslint-enable no-control-regex */
      case 'object':
        var isArray = Array.isArray(key);
        var arr = isArray ? key : Object.keys(key);
        var i = -1;
        var len = arr.length;
        var result = '';
        if (isArray) {
          while (++i < len) {
            result += toIndexableString(arr[i]);
          }
        } else {
          while (++i < len) {
            var objKey = arr[i];
            result += toIndexableString(objKey) +
                toIndexableString(key[objKey]);
          }
        }
        return result;
    }
  }
  return '';
}

// convert the given key to a string that would be appropriate
// for lexical sorting, e.g. within a database, where the
// sorting is the same given by the collate() function.
function toIndexableString(key) {
  var zero = '\u0000';
  key = normalizeKey(key);
  return collationIndex(key) + SEP + indexify(key) + zero;
}

function parseNumber(str, i) {
  var originalIdx = i;
  var num;
  var zero = str[i] === '1';
  if (zero) {
    num = 0;
    i++;
  } else {
    var neg = str[i] === '0';
    i++;
    var numAsString = '';
    var magAsString = str.substring(i, i + MAGNITUDE_DIGITS);
    var magnitude = parseInt(magAsString, 10) + MIN_MAGNITUDE;
    /* istanbul ignore next */
    if (neg) {
      magnitude = -magnitude;
    }
    i += MAGNITUDE_DIGITS;
    while (true) {
      var ch = str[i];
      if (ch === '\u0000') {
        break;
      } else {
        numAsString += ch;
      }
      i++;
    }
    numAsString = numAsString.split('.');
    if (numAsString.length === 1) {
      num = parseInt(numAsString, 10);
    } else {
      /* istanbul ignore next */
      num = parseFloat(numAsString[0] + '.' + numAsString[1]);
    }
    /* istanbul ignore next */
    if (neg) {
      num = num - 10;
    }
    /* istanbul ignore next */
    if (magnitude !== 0) {
      // parseFloat is more reliable than pow due to rounding errors
      // e.g. Number.MAX_VALUE would return Infinity if we did
      // num * Math.pow(10, magnitude);
      num = parseFloat(num + 'e' + magnitude);
    }
  }
  return {num: num, length : i - originalIdx};
}

// move up the stack while parsing
// this function moved outside of parseIndexableString for performance
function pop(stack, metaStack) {
  var obj = stack.pop();

  if (metaStack.length) {
    var lastMetaElement = metaStack[metaStack.length - 1];
    if (obj === lastMetaElement.element) {
      // popping a meta-element, e.g. an object whose value is another object
      metaStack.pop();
      lastMetaElement = metaStack[metaStack.length - 1];
    }
    var element = lastMetaElement.element;
    var lastElementIndex = lastMetaElement.index;
    if (Array.isArray(element)) {
      element.push(obj);
    } else if (lastElementIndex === stack.length - 2) { // obj with key+value
      var key = stack.pop();
      element[key] = obj;
    } else {
      stack.push(obj); // obj with key only
    }
  }
}

function parseIndexableString(str) {
  var stack = [];
  var metaStack = []; // stack for arrays and objects
  var i = 0;

  /*eslint no-constant-condition: ["error", { "checkLoops": false }]*/
  while (true) {
    var collationIndex = str[i++];
    if (collationIndex === '\u0000') {
      if (stack.length === 1) {
        return stack.pop();
      } else {
        pop(stack, metaStack);
        continue;
      }
    }
    switch (collationIndex) {
      case '1':
        stack.push(null);
        break;
      case '2':
        stack.push(str[i] === '1');
        i++;
        break;
      case '3':
        var parsedNum = parseNumber(str, i);
        stack.push(parsedNum.num);
        i += parsedNum.length;
        break;
      case '4':
        var parsedStr = '';
        /*eslint no-constant-condition: ["error", { "checkLoops": false }]*/
        while (true) {
          var ch = str[i];
          if (ch === '\u0000') {
            break;
          }
          parsedStr += ch;
          i++;
        }
        // perform the reverse of the order-preserving replacement
        // algorithm (see above)
        /* eslint-disable no-control-regex */
        parsedStr = parsedStr.replace(/\u0001\u0001/g, '\u0000')
          .replace(/\u0001\u0002/g, '\u0001')
          .replace(/\u0002\u0002/g, '\u0002');
        /* eslint-enable no-control-regex */
        stack.push(parsedStr);
        break;
      case '5':
        var arrayElement = { element: [], index: stack.length };
        stack.push(arrayElement.element);
        metaStack.push(arrayElement);
        break;
      case '6':
        var objElement = { element: {}, index: stack.length };
        stack.push(objElement.element);
        metaStack.push(objElement);
        break;
      /* istanbul ignore next */
      default:
        throw new Error(
          'bad collationIndex or unexpectedly reached end of input: ' +
            collationIndex);
    }
  }
}

function arrayCollate(a, b) {
  var len = Math.min(a.length, b.length);
  for (var i = 0; i < len; i++) {
    var sort = collate(a[i], b[i]);
    if (sort !== 0) {
      return sort;
    }
  }
  return (a.length === b.length) ? 0 :
    (a.length > b.length) ? 1 : -1;
}
function stringCollate(a, b) {
  // See: https://github.com/daleharvey/pouchdb/issues/40
  // This is incompatible with the CouchDB implementation, but its the
  // best we can do for now
  return (a === b) ? 0 : ((a > b) ? 1 : -1);
}
function objectCollate(a, b) {
  var ak = Object.keys(a), bk = Object.keys(b);
  var len = Math.min(ak.length, bk.length);
  for (var i = 0; i < len; i++) {
    // First sort the keys
    var sort = collate(ak[i], bk[i]);
    if (sort !== 0) {
      return sort;
    }
    // if the keys are equal sort the values
    sort = collate(a[ak[i]], b[bk[i]]);
    if (sort !== 0) {
      return sort;
    }

  }
  return (ak.length === bk.length) ? 0 :
    (ak.length > bk.length) ? 1 : -1;
}
// The collation is defined by erlangs ordered terms
// the atoms null, true, false come first, then numbers, strings,
// arrays, then objects
// null/undefined/NaN/Infinity/-Infinity are all considered null
function collationIndex(x) {
  var id = ['boolean', 'number', 'string', 'object'];
  var idx = id.indexOf(typeof x);
  //false if -1 otherwise true, but fast!!!!1
  if (~idx) {
    if (x === null) {
      return 1;
    }
    if (Array.isArray(x)) {
      return 5;
    }
    return idx < 3 ? (idx + 2) : (idx + 3);
  }
  /* istanbul ignore next */
  if (Array.isArray(x)) {
    return 5;
  }
}

// conversion:
// x yyy zz...zz
// x = 0 for negative, 1 for 0, 2 for positive
// y = exponent (for negative numbers negated) moved so that it's >= 0
// z = mantisse
function numToIndexableString(num) {

  if (num === 0) {
    return '1';
  }

  // convert number to exponential format for easier and
  // more succinct string sorting
  var expFormat = num.toExponential().split(/e\+?/);
  var magnitude = parseInt(expFormat[1], 10);

  var neg = num < 0;

  var result = neg ? '0' : '2';

  // first sort by magnitude
  // it's easier if all magnitudes are positive
  var magForComparison = ((neg ? -magnitude : magnitude) - MIN_MAGNITUDE);
  var magString = padLeft((magForComparison).toString(), '0', MAGNITUDE_DIGITS);

  result += SEP + magString;

  // then sort by the factor
  var factor = Math.abs(parseFloat(expFormat[0])); // [1..10)
  /* istanbul ignore next */
  if (neg) { // for negative reverse ordering
    factor = 10 - factor;
  }

  var factorStr = factor.toFixed(20);

  // strip zeros from the end
  factorStr = factorStr.replace(/\.?0+$/, '');

  result += SEP + factorStr;

  return result;
}

// create a comparator based on the sort object
function createFieldSorter(sort) {

  function getFieldValuesAsArray(doc) {
    return sort.map(function (sorting) {
      var fieldName = getKey(sorting);
      var parsedField = parseField(fieldName);
      var docFieldValue = getFieldFromDoc(doc, parsedField);
      return docFieldValue;
    });
  }

  return function (aRow, bRow) {
    var aFieldValues = getFieldValuesAsArray(aRow.doc);
    var bFieldValues = getFieldValuesAsArray(bRow.doc);
    var collation = collate(aFieldValues, bFieldValues);
    if (collation !== 0) {
      return collation;
    }
    // this is what mango seems to do
    return compare$1(aRow.doc._id, bRow.doc._id);
  };
}

function filterInMemoryFields(rows, requestDef, inMemoryFields) {
  rows = rows.filter(function (row) {
    return rowFilter(row.doc, requestDef.selector, inMemoryFields);
  });

  if (requestDef.sort) {
    // in-memory sort
    var fieldSorter = createFieldSorter(requestDef.sort);
    rows = rows.sort(fieldSorter);
    if (typeof requestDef.sort[0] !== 'string' &&
        getValue(requestDef.sort[0]) === 'desc') {
      rows = rows.reverse();
    }
  }

  if ('limit' in requestDef || 'skip' in requestDef) {
    // have to do the limit in-memory
    var skip = requestDef.skip || 0;
    var limit = ('limit' in requestDef ? requestDef.limit : rows.length) + skip;
    rows = rows.slice(skip, limit);
  }
  return rows;
}

function rowFilter(doc, selector, inMemoryFields) {
  return inMemoryFields.every(function (field) {
    var matcher = selector[field];
    var parsedField = parseField(field);
    var docFieldValue = getFieldFromDoc(doc, parsedField);
    if (isCombinationalField(field)) {
      return matchCominationalSelector(field, matcher, doc);
    }

    return matchSelector(matcher, doc, parsedField, docFieldValue);
  });
}

function matchSelector(matcher, doc, parsedField, docFieldValue) {
  if (!matcher) {
    // no filtering necessary; this field is just needed for sorting
    return true;
  }

  // is matcher an object, if so continue recursion
  if (typeof matcher === 'object') {
    return Object.keys(matcher).every(function (maybeUserOperator) {
      var userValue = matcher[ maybeUserOperator ];
      // explicit operator
      if (maybeUserOperator.indexOf("$") === 0) {
        return match(maybeUserOperator, doc, userValue, parsedField, docFieldValue);
      } else {
        var subParsedField = parseField(maybeUserOperator);

        if (
          docFieldValue === undefined &&
          typeof userValue !== "object" &&
          subParsedField.length > 0
        ) {
          // the field does not exist, return or getFieldFromDoc will throw
          return false;
        }

        var subDocFieldValue = getFieldFromDoc(docFieldValue, subParsedField);

        if (typeof userValue === "object") {
          // field value is an object that might contain more operators
          return matchSelector(userValue, doc, parsedField, subDocFieldValue);
        }

        // implicit operator
        return match("$eq", doc, userValue, subParsedField, subDocFieldValue);
      }
    });
  }

  // no more depth, No need to recurse further
  return matcher === docFieldValue;
}

function matchCominationalSelector(field, matcher, doc) {

  if (field === '$or') {
    return matcher.some(function (orMatchers) {
      return rowFilter(doc, orMatchers, Object.keys(orMatchers));
    });
  }

  if (field === '$not') {
    return !rowFilter(doc, matcher, Object.keys(matcher));
  }

  //`$nor`
  return !matcher.find(function (orMatchers) {
    return rowFilter(doc, orMatchers, Object.keys(orMatchers));
  });

}

function match(userOperator, doc, userValue, parsedField, docFieldValue) {
  if (!matchers[userOperator]) {
    /* istanbul ignore next */
    throw new Error('unknown operator "' + userOperator +
      '" - should be one of $eq, $lte, $lt, $gt, $gte, $exists, $ne, $in, ' +
      '$nin, $size, $mod, $regex, $elemMatch, $type, $allMatch or $all');
  }
  return matchers[userOperator](doc, userValue, parsedField, docFieldValue);
}

function fieldExists(docFieldValue) {
  return typeof docFieldValue !== 'undefined' && docFieldValue !== null;
}

function fieldIsNotUndefined(docFieldValue) {
  return typeof docFieldValue !== 'undefined';
}

function modField(docFieldValue, userValue) {
  if (typeof docFieldValue !== "number" ||
    parseInt(docFieldValue, 10) !== docFieldValue) {
    return false;
  }

  var divisor = userValue[0];
  var mod = userValue[1];

  return docFieldValue % divisor === mod;
}

function arrayContainsValue(docFieldValue, userValue) {
  return userValue.some(function (val) {
    if (docFieldValue instanceof Array) {
      return docFieldValue.some(function (docFieldValueItem) {
        return collate(val, docFieldValueItem) === 0;
      });
    }

    return collate(val, docFieldValue) === 0;
  });
}

function arrayContainsAllValues(docFieldValue, userValue) {
  return userValue.every(function (val) {
    return docFieldValue.some(function (docFieldValueItem) {
      return collate(val, docFieldValueItem) === 0;
    });
  });
}

function arraySize(docFieldValue, userValue) {
  return docFieldValue.length === userValue;
}

function regexMatch(docFieldValue, userValue) {
  var re = new RegExp(userValue);

  return re.test(docFieldValue);
}

function typeMatch(docFieldValue, userValue) {

  switch (userValue) {
    case 'null':
      return docFieldValue === null;
    case 'boolean':
      return typeof (docFieldValue) === 'boolean';
    case 'number':
      return typeof (docFieldValue) === 'number';
    case 'string':
      return typeof (docFieldValue) === 'string';
    case 'array':
      return docFieldValue instanceof Array;
    case 'object':
      return ({}).toString.call(docFieldValue) === '[object Object]';
  }
}

var matchers = {

  '$elemMatch': function (doc, userValue, parsedField, docFieldValue) {
    if (!Array.isArray(docFieldValue)) {
      return false;
    }

    if (docFieldValue.length === 0) {
      return false;
    }

    if (typeof docFieldValue[0] === 'object' &&  docFieldValue[0] !== null) {
      return docFieldValue.some(function (val) {
        return rowFilter(val, userValue, Object.keys(userValue));
      });
    }

    return docFieldValue.some(function (val) {
      return matchSelector(userValue, doc, parsedField, val);
    });
  },

  '$allMatch': function (doc, userValue, parsedField, docFieldValue) {
    if (!Array.isArray(docFieldValue)) {
      return false;
    }

    /* istanbul ignore next */
    if (docFieldValue.length === 0) {
      return false;
    }

    if (typeof docFieldValue[0] === 'object' &&  docFieldValue[0] !== null) {
      return docFieldValue.every(function (val) {
        return rowFilter(val, userValue, Object.keys(userValue));
      });
    }

    return docFieldValue.every(function (val) {
      return matchSelector(userValue, doc, parsedField, val);
    });
  },

  '$eq': function (doc, userValue, parsedField, docFieldValue) {
    return fieldIsNotUndefined(docFieldValue) && collate(docFieldValue, userValue) === 0;
  },

  '$gte': function (doc, userValue, parsedField, docFieldValue) {
    return fieldIsNotUndefined(docFieldValue) && collate(docFieldValue, userValue) >= 0;
  },

  '$gt': function (doc, userValue, parsedField, docFieldValue) {
    return fieldIsNotUndefined(docFieldValue) && collate(docFieldValue, userValue) > 0;
  },

  '$lte': function (doc, userValue, parsedField, docFieldValue) {
    return fieldIsNotUndefined(docFieldValue) && collate(docFieldValue, userValue) <= 0;
  },

  '$lt': function (doc, userValue, parsedField, docFieldValue) {
    return fieldIsNotUndefined(docFieldValue) && collate(docFieldValue, userValue) < 0;
  },

  '$exists': function (doc, userValue, parsedField, docFieldValue) {
    //a field that is null is still considered to exist
    if (userValue) {
      return fieldIsNotUndefined(docFieldValue);
    }

    return !fieldIsNotUndefined(docFieldValue);
  },

  '$mod': function (doc, userValue, parsedField, docFieldValue) {
    return fieldExists(docFieldValue) && modField(docFieldValue, userValue);
  },

  '$ne': function (doc, userValue, parsedField, docFieldValue) {
    return userValue.every(function (neValue) {
      return collate(docFieldValue, neValue) !== 0;
    });
  },
  '$in': function (doc, userValue, parsedField, docFieldValue) {
    return fieldExists(docFieldValue) && arrayContainsValue(docFieldValue, userValue);
  },

  '$nin': function (doc, userValue, parsedField, docFieldValue) {
    return fieldExists(docFieldValue) && !arrayContainsValue(docFieldValue, userValue);
  },

  '$size': function (doc, userValue, parsedField, docFieldValue) {
    return fieldExists(docFieldValue) &&
      Array.isArray(docFieldValue) &&
      arraySize(docFieldValue, userValue);
  },

  '$all': function (doc, userValue, parsedField, docFieldValue) {
    return Array.isArray(docFieldValue) && arrayContainsAllValues(docFieldValue, userValue);
  },

  '$regex': function (doc, userValue, parsedField, docFieldValue) {
    return fieldExists(docFieldValue) &&
      typeof docFieldValue == "string" &&
      userValue.every(function (regexValue) {
        return regexMatch(docFieldValue, regexValue);
      });
  },

  '$type': function (doc, userValue, parsedField, docFieldValue) {
    return typeMatch(docFieldValue, userValue);
  }
};

// return true if the given doc matches the supplied selector
function matchesSelector(doc, selector) {
  /* istanbul ignore if */
  if (typeof selector !== 'object') {
    // match the CouchDB error message
    throw new Error('Selector error: expected a JSON object');
  }

  selector = massageSelector(selector);
  var row = {
    'doc': doc
  };

  var rowsMatched = filterInMemoryFields([row], { 'selector': selector }, Object.keys(selector));
  return rowsMatched && rowsMatched.length === 1;
}

function evalFilter(input) {
  var code = '(function() {\n"use strict";\nreturn ' + input + '\n})()';

  return vm.runInNewContext(code);
}

function evalView(input) {
  var code = [
    '"use strict";',
    'var emitted = false;',
    'var emit = function (a, b) {',
    '  emitted = true;',
    '};',
    'var view = ' + input + ';',
    'view(doc);',
    'if (emitted) {',
    '  return true;',
    '}'
  ].join('\n');

  return vm.runInNewContext('(function(doc) {\n' + code + '\n})');
}

function validate(opts, callback) {
  if (opts.selector) {
    if (opts.filter && opts.filter !== '_selector') {
      var filterName = typeof opts.filter === 'string' ?
        opts.filter : 'function';
      return callback(new Error('selector invalid for filter "' + filterName + '"'));
    }
  }
  callback();
}

function normalize(opts) {
  if (opts.view && !opts.filter) {
    opts.filter = '_view';
  }

  if (opts.selector && !opts.filter) {
    opts.filter = '_selector';
  }

  if (opts.filter && typeof opts.filter === 'string') {
    if (opts.filter === '_view') {
      opts.view = normalizeDesignDocFunctionName(opts.view);
    } else {
      opts.filter = normalizeDesignDocFunctionName(opts.filter);
    }
  }
}

function shouldFilter(changesHandler, opts) {
  return opts.filter && typeof opts.filter === 'string' &&
    !opts.doc_ids && !isRemote(changesHandler.db);
}

function filter(changesHandler, opts) {
  var callback = opts.complete;
  if (opts.filter === '_view') {
    if (!opts.view || typeof opts.view !== 'string') {
      var err = createError(BAD_REQUEST,
        '`view` filter parameter not found or invalid.');
      return callback(err);
    }
    // fetch a view from a design doc, make it behave like a filter
    var viewName = parseDesignDocFunctionName(opts.view);
    changesHandler.db.get('_design/' + viewName[0], function (err, ddoc) {
      /* istanbul ignore if */
      if (changesHandler.isCancelled) {
        return callback(null, {status: 'cancelled'});
      }
      /* istanbul ignore next */
      if (err) {
        return callback(generateErrorFromResponse(err));
      }
      var mapFun = ddoc && ddoc.views && ddoc.views[viewName[1]] &&
        ddoc.views[viewName[1]].map;
      if (!mapFun) {
        return callback(createError(MISSING_DOC,
          (ddoc.views ? 'missing json key: ' + viewName[1] :
            'missing json key: views')));
      }
      opts.filter = evalView(mapFun);
      changesHandler.doChanges(opts);
    });
  } else if (opts.selector) {
    opts.filter = function (doc) {
      return matchesSelector(doc, opts.selector);
    };
    changesHandler.doChanges(opts);
  } else {
    // fetch a filter from a design doc
    var filterName = parseDesignDocFunctionName(opts.filter);
    changesHandler.db.get('_design/' + filterName[0], function (err, ddoc) {
      /* istanbul ignore if */
      if (changesHandler.isCancelled) {
        return callback(null, {status: 'cancelled'});
      }
      /* istanbul ignore next */
      if (err) {
        return callback(generateErrorFromResponse(err));
      }
      var filterFun = ddoc && ddoc.filters && ddoc.filters[filterName[1]];
      if (!filterFun) {
        return callback(createError(MISSING_DOC,
          ((ddoc && ddoc.filters) ? 'missing json key: ' + filterName[1]
            : 'missing json key: filters')));
      }
      opts.filter = evalFilter(filterFun);
      changesHandler.doChanges(opts);
    });
  }
}

function applyChangesFilterPlugin(PouchDB) {
  PouchDB._changesFilterPlugin = {
    validate: validate,
    normalize: normalize,
    shouldFilter: shouldFilter,
    filter: filter
  };
}

// TODO: remove from pouchdb-core (breaking)
PouchDB.plugin(applyChangesFilterPlugin);

PouchDB.version = version;

function isFunction(f) {
  return 'function' === typeof f;
}

function getPrefix(db) {
  if (isFunction(db.prefix)) {
    return db.prefix();
  }
  return db;
}

function clone$1(_obj) {
  var obj = {};
  for (var k in _obj) {
    obj[k] = _obj[k];
  }
  return obj;
}

function nut(db, precodec, codec) {
  function encodePrefix(prefix, key, opts1, opts2) {
    return precodec.encode([ prefix, codec.encodeKey(key, opts1, opts2 ) ]);
  }

  function addEncodings(op, prefix) {
    if (prefix && prefix.options) {
      op.keyEncoding =
        op.keyEncoding || prefix.options.keyEncoding;
      op.valueEncoding =
        op.valueEncoding || prefix.options.valueEncoding;
    }
    return op;
  }

  db.open(function () { /* no-op */});

  return {
    apply: function (ops, opts, cb) {
      opts = opts || {};

      var batch = [];
      var i = -1;
      var len = ops.length;

      while (++i < len) {
        var op = ops[i];
        addEncodings(op, op.prefix);
        op.prefix = getPrefix(op.prefix);
        batch.push({
          key: encodePrefix(op.prefix, op.key, opts, op),
          value: op.type !== 'del' && codec.encodeValue(op.value, opts, op),
          type: op.type
        });
      }
      db.db.batch(batch, opts, cb);
    },
    get: function (key, prefix, opts, cb) {
      opts.asBuffer = codec.valueAsBuffer(opts);
      return db.db.get(
        encodePrefix(prefix, key, opts),
        opts,
        function (err, value) {
          if (err) {
            cb(err);
          } else {
            cb(null, codec.decodeValue(value, opts));
          }
        }
      );
    },
    createDecoder: function (opts) {
      return function (key, value) {
        return {
          key: codec.decodeKey(precodec.decode(key)[1], opts),
          value: codec.decodeValue(value, opts)
        };
      };
    },
    isClosed: function isClosed() {
      return db.isClosed();
    },
    close: function close(cb) {
      return db.close(cb);
    },
    iterator: function (_opts) {
      var opts = clone$1(_opts || {});
      var prefix = _opts.prefix || [];

      function encodeKey(key) {
        return encodePrefix(prefix, key, opts, {});
      }

      ltgt.toLtgt(_opts, opts, encodeKey, precodec.lowerBound, precodec.upperBound);

      // if these legacy values are in the options, remove them

      opts.prefix = null;

      //************************************************
      //hard coded defaults, for now...
      //TODO: pull defaults and encoding out of levelup.
      opts.keyAsBuffer = opts.valueAsBuffer = false;
      //************************************************


      //this is vital, otherwise limit: undefined will
      //create an empty stream.
      /* istanbul ignore next */
      if ('number' !== typeof opts.limit) {
        opts.limit = -1;
      }

      opts.keyAsBuffer = precodec.buffer;
      opts.valueAsBuffer = codec.valueAsBuffer(opts);

      function wrapIterator(iterator) {
        return {
          next: function (cb) {
            return iterator.next(cb);
          },
          end: function (cb) {
            iterator.end(cb);
          }
        };
      }

      return wrapIterator(db.db.iterator(opts));
    }
  };
}

class NotFoundError extends Error {
  constructor() {
    super();
    this.name = 'NotFoundError';
  }
}

var EventEmitter = EE.EventEmitter;
var version$1 = "6.5.4";

var NOT_FOUND_ERROR = new NotFoundError();

var sublevel = function (nut, prefix, createStream, options) {
  var emitter = new EventEmitter();
  emitter.sublevels = {};
  emitter.options = options;

  emitter.version = version$1;

  emitter.methods = {};
  prefix = prefix || [];

  function mergeOpts(opts) {
    var o = {};
    var k;
    if (options) {
      for (k in options) {
        if (typeof options[k] !== 'undefined') {
          o[k] = options[k];
        }
      }
    }
    if (opts) {
      for (k in opts) {
        if (typeof opts[k] !== 'undefined') {
          o[k] = opts[k];
        }
      }
    }
    return o;
  }

  emitter.put = function (key, value, opts, cb) {
    if ('function' === typeof opts) {
      cb = opts;
      opts = {};
    }

    nut.apply([{
      key: key, value: value,
      prefix: prefix.slice(), type: 'put'
    }], mergeOpts(opts), function (err) {
      /* istanbul ignore next */
      if (err) {
        return cb(err);
      }
      emitter.emit('put', key, value);
      cb(null);
    });
  };

  emitter.prefix = function () {
    return prefix.slice();
  };

  emitter.batch = function (ops, opts, cb) {
    if ('function' === typeof opts) {
      cb = opts;
      opts = {};
    }

    ops = ops.map(function (op) {
      return {
        key: op.key,
        value: op.value,
        prefix: op.prefix || prefix,
        keyEncoding: op.keyEncoding,    // *
        valueEncoding: op.valueEncoding,  // * (TODO: encodings on sublevel)
        type: op.type
      };
    });

    nut.apply(ops, mergeOpts(opts), function (err) {
      /* istanbul ignore next */
      if (err) {
        return cb(err);
      }
      emitter.emit('batch', ops);
      cb(null);
    });
  };

  emitter.get = function (key, opts, cb) {
    /* istanbul ignore else */
    if ('function' === typeof opts) {
      cb = opts;
      opts = {};
    }
    nut.get(key, prefix, mergeOpts(opts), function (err, value) {
      if (err) {
        cb(NOT_FOUND_ERROR);
      } else {
        cb(null, value);
      }
    });
  };

  emitter.sublevel = function (name, opts) {
    return emitter.sublevels[name] =
      emitter.sublevels[name] || sublevel(nut, prefix.concat(name), createStream, mergeOpts(opts));
  };

  emitter.readStream = emitter.createReadStream = function (opts) {
    opts = mergeOpts(opts);
    opts.prefix = prefix;
    var stream;
    var it = nut.iterator(opts);

    stream = createStream(opts, nut.createDecoder(opts));
    stream.setIterator(it);

    return stream;
  };

  emitter.close = function (cb) {
    nut.close(cb);
  };

  emitter.isOpen = nut.isOpen;
  emitter.isClosed = nut.isClosed;

  return emitter;
};

/* Copyright (c) 2012-2014 LevelUP contributors
 * See list at <https://github.com/rvagg/node-levelup#contributing>
 * MIT License <https://github.com/rvagg/node-levelup/blob/master/LICENSE.md>
 */

var Readable = ReadableStreamCore.Readable;

function createClass$1(parent, init) {
  let klass = function (...args) {
    if (!(this instanceof klass)) {
      return new klass(...args);
    }
    init.apply(this, args);
  };
  klass.prototype = Object.create(parent.prototype, {
    constructor: { value: klass }
  });
  return klass;
}

class ReadStreamInternal extends Readable {
  constructor(options, makeData) {
    super({ objectMode: true, highWaterMark: options.highWaterMark });
    this._setup(options, makeData);
  }

  _setup(options, makeData) {
    super.constructor({ objectMode: true, highWaterMark: options.highWaterMark });

    // purely to keep `db` around until we're done so it's not GCed if the user doesn't keep a ref
    this._waiting = false;
    this._options = options;
    this._makeData = makeData;
  }

  setIterator(it) {
    this._iterator = it;
    /* istanbul ignore if */
    if (this._destroyed) {
      return it.end(function () {});
    }
    /* istanbul ignore if */
    if (this._waiting) {
      this._waiting = false;
      return this._read();
    }
    return this;
  }

  _cleanup(err) {
    if (this._destroyed) {
      return;
    }
  
    this._destroyed = true;
  
    var self = this;
    /* istanbul ignore if */
    if (err && err.message !== 'iterator has ended') {
      self.emit('error', err);
    }
  
    /* istanbul ignore else */
    if (self._iterator) {
      self._iterator.end(function () {
        self._iterator = null;
        self.emit('close');
      });
    } else {
      self.emit('close');
    }
  }

  destroy() {
    this._cleanup();
  }
  
  _read() {
    var self = this;
    /* istanbul ignore if */
    if (self._destroyed) {
      return;
    }
    /* istanbul ignore if */
    if (!self._iterator) {
      return this._waiting = true;
    }
  
    self._iterator.next(function (err, key, value) {
      if (err || (key === undefined && value === undefined)) {
        if (!err && !self._destroyed) {
          self.push(null);
        }
        return self._cleanup(err);
      }
  
  
      value = self._makeData(key, value);
      if (!self._destroyed) {
        self.push(value);
      }
    });
  }
}

const ReadStream = createClass$1(ReadStreamInternal, function (options, makeData) {
  ReadStreamInternal.prototype._setup.call(this, options, makeData);
});

var precodec = {
  encode: function (decodedKey) {
    return '\xff' + decodedKey[0] + '\xff' + decodedKey[1];
  },
  decode: function (encodedKeyAsBuffer) {
    var str = encodedKeyAsBuffer.toString();
    var idx = str.indexOf('\xff', 1);
    return [str.substring(1, idx), str.substring(idx + 1)];
  },
  lowerBound: '\x00',
  upperBound: '\xff'
};

var codec = new Codec();

function sublevelPouch(db) {
  return sublevel(nut(db, precodec, codec), [], ReadStream, db.options);
}

function allDocsKeysQuery(api, opts) {
  var keys = opts.keys;
  var finalResults = {
    offset: opts.skip
  };
  return Promise.all(keys.map(function (key) {
    var subOpts = $inject_Object_assign({key: key, deleted: 'ok'}, opts);
    ['limit', 'skip', 'keys'].forEach(function (optKey) {
      delete subOpts[optKey];
    });
    return new Promise(function (resolve, reject) {
      api._allDocs(subOpts, function (err, res$$1) {
        /* istanbul ignore if */
        if (err) {
          return reject(err);
        }
        /* istanbul ignore if */
        if (opts.update_seq && res$$1.update_seq !== undefined) {
          finalResults.update_seq = res$$1.update_seq;
        }
        finalResults.total_rows = res$$1.total_rows;
        resolve(res$$1.rows[0] || {key: key, error: 'not_found'});
      });
    });
  })).then(function (results) {
    finalResults.rows = results;
    return finalResults;
  });
}

function toObject(array) {
  return array.reduce(function (obj, item) {
    obj[item] = true;
    return obj;
  }, {});
}
// List of top level reserved words for doc
var reservedWords = toObject([
  '_id',
  '_rev',
  '_access',
  '_attachments',
  '_deleted',
  '_revisions',
  '_revs_info',
  '_conflicts',
  '_deleted_conflicts',
  '_local_seq',
  '_rev_tree',
  // replication documents
  '_replication_id',
  '_replication_state',
  '_replication_state_time',
  '_replication_state_reason',
  '_replication_stats',
  // Specific to Couchbase Sync Gateway
  '_removed'
]);

// List of reserved words that should end up in the document
var dataWords = toObject([
  '_access',
  '_attachments',
  // replication documents
  '_replication_id',
  '_replication_state',
  '_replication_state_time',
  '_replication_state_reason',
  '_replication_stats'
]);

function parseRevisionInfo(rev) {
  if (!/^\d+-/.test(rev)) {
    return createError(INVALID_REV);
  }
  var idx = rev.indexOf('-');
  var left = rev.substring(0, idx);
  var right = rev.substring(idx + 1);
  return {
    prefix: parseInt(left, 10),
    id: right
  };
}

function makeRevTreeFromRevisions(revisions, opts) {
  var pos = revisions.start - revisions.ids.length + 1;

  var revisionIds = revisions.ids;
  var ids = [revisionIds[0], opts, []];

  for (var i = 1, len = revisionIds.length; i < len; i++) {
    ids = [revisionIds[i], {status: 'missing'}, [ids]];
  }

  return [{
    pos: pos,
    ids: ids
  }];
}

// Preprocess documents, parse their revisions, assign an id and a
// revision for new writes that are missing them, etc
function parseDoc(doc, newEdits, dbOpts) {
  if (!dbOpts) {
    dbOpts = {
      deterministic_revs: true
    };
  }

  var nRevNum;
  var newRevId;
  var revInfo;
  var opts = {status: 'available'};
  if (doc._deleted) {
    opts.deleted = true;
  }

  if (newEdits) {
    if (!doc._id) {
      doc._id = uuid$1();
    }
    newRevId = rev$$1(doc, dbOpts.deterministic_revs);
    if (doc._rev) {
      revInfo = parseRevisionInfo(doc._rev);
      if (revInfo.error) {
        return revInfo;
      }
      doc._rev_tree = [{
        pos: revInfo.prefix,
        ids: [revInfo.id, {status: 'missing'}, [[newRevId, opts, []]]]
      }];
      nRevNum = revInfo.prefix + 1;
    } else {
      doc._rev_tree = [{
        pos: 1,
        ids : [newRevId, opts, []]
      }];
      nRevNum = 1;
    }
  } else {
    if (doc._revisions) {
      doc._rev_tree = makeRevTreeFromRevisions(doc._revisions, opts);
      nRevNum = doc._revisions.start;
      newRevId = doc._revisions.ids[0];
    }
    if (!doc._rev_tree) {
      revInfo = parseRevisionInfo(doc._rev);
      if (revInfo.error) {
        return revInfo;
      }
      nRevNum = revInfo.prefix;
      newRevId = revInfo.id;
      doc._rev_tree = [{
        pos: nRevNum,
        ids: [newRevId, opts, []]
      }];
    }
  }

  invalidIdError(doc._id);

  doc._rev = nRevNum + '-' + newRevId;

  var result = {metadata : {}, data : {}};
  for (var key in doc) {
    /* istanbul ignore else */
    if (Object.prototype.hasOwnProperty.call(doc, key)) {
      var specialKey = key[0] === '_';
      if (specialKey && !reservedWords[key]) {
        var error = createError(DOC_VALIDATION, key);
        error.message = DOC_VALIDATION.message + ': ' + key;
        throw error;
      } else if (specialKey && !dataWords[key]) {
        result.metadata[key.slice(1)] = doc[key];
      } else {
        result.data[key] = doc[key];
      }
    }
  }
  return result;
}

function thisAtob(str) {
  var base64 = new Buffer(str, 'base64');
  // Node.js will just skip the characters it can't decode instead of
  // throwing an exception
  if (base64.toString('base64') !== str) {
    throw new Error("attachment is not a valid base64 string");
  }
  return base64.toString('binary');
}

function thisBtoa(str) {
  return bufferFrom(str, 'binary').toString('base64');
}

function typedBuffer(binString, buffType, type) {
  // buffType is either 'binary' or 'base64'
  var buff = bufferFrom(binString, buffType);
  buff.type = type; // non-standard, but used for consistency with the browser
  return buff;
}

function b64ToBluffer(b64, type) {
  return typedBuffer(b64, 'base64', type);
}

// From http://stackoverflow.com/questions/14967647/ (continues on next line)

function binStringToBluffer(binString, type) {
  return typedBuffer(binString, 'binary', type);
}

// This function is unused in Node

function blobToBase64(blobOrBuffer, callback) {
  callback(blobOrBuffer.toString('base64'));
}

// not used in Node, but here for completeness

// simplified API. universal browser support is assumed

//Can't find original post, but this is close

function updateDoc(revLimit, prev, docInfo, results,
                   i, cb, writeDoc, newEdits) {

  if (revExists(prev.rev_tree, docInfo.metadata.rev) && !newEdits) {
    results[i] = docInfo;
    return cb();
  }

  // sometimes this is pre-calculated. historically not always
  var previousWinningRev = prev.winningRev || winningRev(prev);
  var previouslyDeleted = 'deleted' in prev ? prev.deleted :
    isDeleted(prev, previousWinningRev);
  var deleted = 'deleted' in docInfo.metadata ? docInfo.metadata.deleted :
    isDeleted(docInfo.metadata);
  var isRoot = /^1-/.test(docInfo.metadata.rev);

  if (previouslyDeleted && !deleted && newEdits && isRoot) {
    var newDoc = docInfo.data;
    newDoc._rev = previousWinningRev;
    newDoc._id = docInfo.metadata.id;
    docInfo = parseDoc(newDoc, newEdits);
  }

  var merged = merge(prev.rev_tree, docInfo.metadata.rev_tree[0], revLimit);

  var inConflict = newEdits && ((
    (previouslyDeleted && deleted && merged.conflicts !== 'new_leaf') ||
    (!previouslyDeleted && merged.conflicts !== 'new_leaf') ||
    (previouslyDeleted && !deleted && merged.conflicts === 'new_branch')));

  if (inConflict) {
    var err = createError(REV_CONFLICT);
    results[i] = err;
    return cb();
  }

  var newRev = docInfo.metadata.rev;
  docInfo.metadata.rev_tree = merged.tree;
  docInfo.stemmedRevs = merged.stemmedRevs || [];
  /* istanbul ignore else */
  if (prev.rev_map) {
    docInfo.metadata.rev_map = prev.rev_map; // used only by leveldb
  }

  // recalculate
  var winningRev$$1 = winningRev(docInfo.metadata);
  var winningRevIsDeleted = isDeleted(docInfo.metadata, winningRev$$1);

  // calculate the total number of documents that were added/removed,
  // from the perspective of total_rows/doc_count
  var delta = (previouslyDeleted === winningRevIsDeleted) ? 0 :
    previouslyDeleted < winningRevIsDeleted ? -1 : 1;

  var newRevIsDeleted;
  if (newRev === winningRev$$1) {
    // if the new rev is the same as the winning rev, we can reuse that value
    newRevIsDeleted = winningRevIsDeleted;
  } else {
    // if they're not the same, then we need to recalculate
    newRevIsDeleted = isDeleted(docInfo.metadata, newRev);
  }

  writeDoc(docInfo, winningRev$$1, winningRevIsDeleted, newRevIsDeleted,
    true, delta, i, cb);
}

function rootIsMissing(docInfo) {
  return docInfo.metadata.rev_tree[0].ids[1].status === 'missing';
}

function processDocs(revLimit, docInfos, api, fetchedDocs, tx, results,
                     writeDoc, opts, overallCallback) {

  // Default to 1000 locally
  revLimit = revLimit || 1000;

  function insertDoc(docInfo, resultsIdx, callback) {
    // Cant insert new deleted documents
    var winningRev$$1 = winningRev(docInfo.metadata);
    var deleted = isDeleted(docInfo.metadata, winningRev$$1);
    if ('was_delete' in opts && deleted) {
      results[resultsIdx] = createError(MISSING_DOC, 'deleted');
      return callback();
    }

    // 4712 - detect whether a new document was inserted with a _rev
    var inConflict = newEdits && rootIsMissing(docInfo);

    if (inConflict) {
      var err = createError(REV_CONFLICT);
      results[resultsIdx] = err;
      return callback();
    }

    var delta = deleted ? 0 : 1;

    writeDoc(docInfo, winningRev$$1, deleted, deleted, false,
      delta, resultsIdx, callback);
  }

  var newEdits = opts.new_edits;
  var idsToDocs = new ExportedMap();

  var docsDone = 0;
  var docsToDo = docInfos.length;

  function checkAllDocsDone() {
    if (++docsDone === docsToDo && overallCallback) {
      overallCallback();
    }
  }

  docInfos.forEach(function (currentDoc, resultsIdx) {

    if (currentDoc._id && isLocalId(currentDoc._id)) {
      var fun = currentDoc._deleted ? '_removeLocal' : '_putLocal';
      api[fun](currentDoc, {ctx: tx}, function (err, res) {
        results[resultsIdx] = err || res;
        checkAllDocsDone();
      });
      return;
    }

    var id = currentDoc.metadata.id;
    if (idsToDocs.has(id)) {
      docsToDo--; // duplicate
      idsToDocs.get(id).push([currentDoc, resultsIdx]);
    } else {
      idsToDocs.set(id, [[currentDoc, resultsIdx]]);
    }
  });

  // in the case of new_edits, the user can provide multiple docs
  // with the same id. these need to be processed sequentially
  idsToDocs.forEach(function (docs, id) {
    var numDone = 0;

    function docWritten() {
      if (++numDone < docs.length) {
        nextDoc();
      } else {
        checkAllDocsDone();
      }
    }
    function nextDoc() {
      var value = docs[numDone];
      var currentDoc = value[0];
      var resultsIdx = value[1];

      if (fetchedDocs.has(id)) {
        updateDoc(revLimit, fetchedDocs.get(id), currentDoc, results,
          resultsIdx, docWritten, writeDoc, newEdits);
      } else {
        // Ensure stemming applies to new writes as well
        var merged = merge([], currentDoc.metadata.rev_tree[0], revLimit);
        currentDoc.metadata.rev_tree = merged.tree;
        currentDoc.stemmedRevs = merged.stemmedRevs || [];
        insertDoc(currentDoc, resultsIdx, docWritten);
      }
    }
    nextDoc();
  });
}

function safeJsonParse(str) {
  // This try/catch guards against stack overflow errors.
  // JSON.parse() is faster than vuvuzela.parse() but vuvuzela
  // cannot overflow.
  try {
    return JSON.parse(str);
  } catch (e) {
    /* istanbul ignore next */
    return vuvuzela.parse(str);
  }
}

function safeJsonStringify(json) {
  try {
    return JSON.stringify(json);
  } catch (e) {
    /* istanbul ignore next */
    return vuvuzela.stringify(json);
  }
}

function readAsBlobOrBuffer(storedObject, type) {
  // In Node, we've stored a buffer
  storedObject.type = type; // non-standard, but used for consistency
  return storedObject;
}

// in Node, we store the buffer directly
function prepareAttachmentForStorage(attData, cb) {
  cb(attData);
}

function createEmptyBlobOrBuffer(type) {
  return typedBuffer('', 'binary', type);
}

function getCacheFor(transaction, store) {
  var prefix = store.prefix()[0];
  var cache = transaction._cache;
  var subCache = cache.get(prefix);
  if (!subCache) {
    subCache = new ExportedMap();
    cache.set(prefix, subCache);
  }
  return subCache;
}

class LevelTransaction {
  constructor() {
    this._batch = [];
    this._cache = new ExportedMap();
  }

  get(store, key, callback) {
    var cache = getCacheFor(this, store);
    var exists = cache.get(key);
    if (exists) {
      return nextTick(function () {
        callback(null, exists);
      });
    } else if (exists === null) { // deleted marker
      /* istanbul ignore next */
      return nextTick(function () {
        callback({name: 'NotFoundError'});
      });
    }
    store.get(key, function (err, res$$1) {
      if (err) {
        /* istanbul ignore else */
        if (err.name === 'NotFoundError') {
          cache.set(key, null);
        }
        return callback(err);
      }
      cache.set(key, res$$1);
      callback(null, res$$1);
    });
  }

  batch(batch) {
    for (var i = 0, len = batch.length; i < len; i++) {
      var operation = batch[i];

      var cache = getCacheFor(this, operation.prefix);

      if (operation.type === 'put') {
        cache.set(operation.key, operation.value);
      } else {
        cache.set(operation.key, null);
      }
    }
    this._batch = this._batch.concat(batch);
  }

  execute(db, callback) {
    var keys = new ExportedSet();
    var uniqBatches = [];

    // remove duplicates; last one wins
    for (var i = this._batch.length - 1; i >= 0; i--) {
      var operation = this._batch[i];
      var lookupKey = operation.prefix.prefix()[0] + '\xff' + operation.key;
      if (keys.has(lookupKey)) {
        continue;
      }
      keys.add(lookupKey);
      uniqBatches.push(operation);
    }

    db.batch(uniqBatches, callback);
  }
}

var DOC_STORE = 'document-store';
var BY_SEQ_STORE = 'by-sequence';
var ATTACHMENT_STORE = 'attach-store';
var BINARY_STORE = 'attach-binary-store';
var LOCAL_STORE = 'local-store';
var META_STORE = 'meta-store';

// leveldb barks if we try to open a db multiple times
// so we cache opened connections here for initstore()
var dbStores = new ExportedMap();

// store the value of update_seq in the by-sequence store the key name will
// never conflict, since the keys in the by-sequence store are integers
var UPDATE_SEQ_KEY = '_local_last_update_seq';
var DOC_COUNT_KEY = '_local_doc_count';
var UUID_KEY = '_local_uuid';

var MD5_PREFIX = 'md5-';

var safeJsonEncoding = {
  encode: safeJsonStringify,
  decode: safeJsonParse,
  buffer: false,
  type: 'cheap-json'
};

var levelChanges = new Changes();

// winningRev and deleted are performance-killers, but
// in newer versions of PouchDB, they are cached on the metadata
function getWinningRev(metadata) {
  return 'winningRev' in metadata ?
    metadata.winningRev : winningRev(metadata);
}

function getIsDeleted(metadata, winningRev$$1) {
  return 'deleted' in metadata ?
    metadata.deleted : isDeleted(metadata, winningRev$$1);
}

function fetchAttachment(att, stores, opts) {
  var type = att.content_type;
  return new Promise(function (resolve, reject) {
    stores.binaryStore.get(att.digest, function (err, buffer) {
      var data;
      if (err) {
        /* istanbul ignore if */
        if (err.name !== 'NotFoundError') {
          return reject(err);
        } else {
          // empty
          if (!opts.binary) {
            data = '';
          } else {
            data = binStringToBluffer('', type);
          }
        }
      } else { // non-empty
        if (opts.binary) {
          data = readAsBlobOrBuffer(buffer, type);
        } else {
          data = buffer.toString('base64');
        }
      }
      delete att.stub;
      delete att.length;
      att.data = data;
      resolve();
    });
  });
}

function fetchAttachments(results, stores, opts) {
  var atts = [];
  results.forEach(function (row) {
    if (!(row.doc && row.doc._attachments)) {
      return;
    }
    var attNames = Object.keys(row.doc._attachments);
    attNames.forEach(function (attName) {
      var att = row.doc._attachments[attName];
      if (!('data' in att)) {
        atts.push(att);
      }
    });
  });

  return Promise.all(atts.map(function (att) {
    return fetchAttachment(att, stores, opts);
  }));
}

function LevelPouch(opts, callback) {
  opts = clone(opts);
  var api = this;
  var instanceId;
  var stores = {};
  var revLimit = opts.revs_limit;
  var db;
  var name = opts.name;
  // TODO: this is undocumented and unused probably
  /* istanbul ignore else */
  if (typeof opts.createIfMissing === 'undefined') {
    opts.createIfMissing = true;
  }

  var leveldown = opts.db;

  var dbStore;
  var leveldownName = functionName(leveldown);
  if (dbStores.has(leveldownName)) {
    dbStore = dbStores.get(leveldownName);
  } else {
    dbStore = new ExportedMap();
    dbStores.set(leveldownName, dbStore);
  }
  if (dbStore.has(name)) {
    db = dbStore.get(name);
    afterDBCreated();
  } else {
    dbStore.set(name, sublevelPouch(levelup(leveldown(name), opts, function (err) {
      /* istanbul ignore if */
      if (err) {
        dbStore.delete(name);
        return callback(err);
      }
      db = dbStore.get(name);
      db._docCount  = -1;
      db._queue = new Deque();
      /* istanbul ignore else */
      if (typeof opts.migrate === 'object') { // migration for leveldown
        opts.migrate.doMigrationOne(name, db, afterDBCreated);
      } else {
        afterDBCreated();
      }
    })));
  }

  function afterDBCreated() {
    stores.docStore = db.sublevel(DOC_STORE, {valueEncoding: safeJsonEncoding});
    stores.bySeqStore = db.sublevel(BY_SEQ_STORE, {valueEncoding: 'json'});
    stores.attachmentStore =
      db.sublevel(ATTACHMENT_STORE, {valueEncoding: 'json'});
    stores.binaryStore = db.sublevel(BINARY_STORE, {valueEncoding: 'binary'});
    stores.localStore = db.sublevel(LOCAL_STORE, {valueEncoding: 'json'});
    stores.metaStore = db.sublevel(META_STORE, {valueEncoding: 'json'});
    /* istanbul ignore else */
    if (typeof opts.migrate === 'object') { // migration for leveldown
      opts.migrate.doMigrationTwo(db, stores, afterLastMigration);
    } else {
      afterLastMigration();
    }
  }

  function afterLastMigration() {
    stores.metaStore.get(UPDATE_SEQ_KEY, function (err, value) {
      if (typeof db._updateSeq === 'undefined') {
        db._updateSeq = value || 0;
      }
      stores.metaStore.get(DOC_COUNT_KEY, function (err, value) {
        db._docCount = !err ? value : 0;
        stores.metaStore.get(UUID_KEY, function (err, value) {
          instanceId = !err ? value : uuid$1();
          stores.metaStore.put(UUID_KEY, instanceId, function () {
            nextTick(function () {
              callback(null, api);
            });
          });
        });
      });
    });
  }

  function countDocs(callback) {
    /* istanbul ignore if */
    if (db.isClosed()) {
      return callback(new Error('database is closed'));
    }
    return callback(null, db._docCount); // use cached value
  }

  api._remote = false;
  /* istanbul ignore next */
  api.type = function () {
    return 'leveldb';
  };

  api._id = function (callback) {
    callback(null, instanceId);
  };

  api._info = function (callback) {
    var res$$1 = {
      doc_count: db._docCount,
      update_seq: db._updateSeq,
      backend_adapter: functionName(leveldown)
    };
    return nextTick(function () {
      callback(null, res$$1);
    });
  };

  function tryCode(fun, args) {
    try {
      fun.apply(null, args);
    } catch (err) {
      args[args.length - 1](err);
    }
  }

  function executeNext() {
    var firstTask = db._queue.peekFront();

    if (firstTask.type === 'read') {
      runReadOperation(firstTask);
    } else { // write, only do one at a time
      runWriteOperation(firstTask);
    }
  }

  function runReadOperation(firstTask) {
    // do multiple reads at once simultaneously, because it's safe

    var readTasks = [firstTask];
    var i = 1;
    var nextTask = db._queue.get(i);
    while (typeof nextTask !== 'undefined' && nextTask.type === 'read') {
      readTasks.push(nextTask);
      i++;
      nextTask = db._queue.get(i);
    }

    var numDone = 0;

    readTasks.forEach(function (readTask) {
      var args = readTask.args;
      var callback = args[args.length - 1];
      args[args.length - 1] = function (...cbArgs) {
        callback.apply(null, cbArgs);
        if (++numDone === readTasks.length) {
          nextTick(function () {
            // all read tasks have finished
            readTasks.forEach(function () {
              db._queue.shift();
            });
            if (db._queue.length) {
              executeNext();
            }
          });
        }
      };
      tryCode(readTask.fun, args);
    });
  }

  function runWriteOperation(firstTask) {
    var args = firstTask.args;
    var callback = args[args.length - 1];
    args[args.length - 1] = function (...cbArgs) {
      callback.apply(null, cbArgs);
      nextTick(function () {
        db._queue.shift();
        if (db._queue.length) {
          executeNext();
        }
      });
    };
    tryCode(firstTask.fun, args);
  }

  // all read/write operations to the database are done in a queue,
  // similar to how websql/idb works. this avoids problems such
  // as e.g. compaction needing to have a lock on the database while
  // it updates stuff. in the future we can revisit this.
  function writeLock(fun) {
    return function (...args) {
      db._queue.push({
        fun: fun,
        args: args,
        type: 'write'
      });

      if (db._queue.length === 1) {
        nextTick(executeNext);
      }
    };
  }

  // same as the writelock, but multiple can run at once
  function readLock(fun) {
    return function (...args) {
      db._queue.push({
        fun: fun,
        args: args,
        type: 'read'
      });

      if (db._queue.length === 1) {
        nextTick(executeNext);
      }
    };
  }

  function formatSeq(n) {
    return ('0000000000000000' + n).slice(-16);
  }

  function parseSeq(s) {
    return parseInt(s, 10);
  }

  api._get = readLock(function (id, opts, callback) {
    opts = clone(opts);

    stores.docStore.get(id, function (err, metadata) {

      if (err || !metadata) {
        return callback(createError(MISSING_DOC, 'missing'));
      }

      var rev;
      if (!opts.rev) {
        rev = getWinningRev(metadata);
        var deleted = getIsDeleted(metadata, rev);
        if (deleted) {
          return callback(createError(MISSING_DOC, "deleted"));
        }
      } else {
        rev = opts.latest ? latest(opts.rev, metadata) : opts.rev;
      }

      var seq = metadata.rev_map[rev];

      stores.bySeqStore.get(formatSeq(seq), function (err, doc) {
        if (!doc) {
          return callback(createError(MISSING_DOC));
        }
        /* istanbul ignore if */
        if ('_id' in doc && doc._id !== metadata.id) {
          // this failing implies something very wrong
          return callback(new Error('wrong doc returned'));
        }
        doc._id = metadata.id;
        if ('_rev' in doc) {
          /* istanbul ignore if */
          if (doc._rev !== rev) {
            // this failing implies something very wrong
            return callback(new Error('wrong doc returned'));
          }
        } else {
          // we didn't always store this
          doc._rev = rev;
        }
        return callback(null, {doc: doc, metadata: metadata});
      });
    });
  });

  // not technically part of the spec, but if putAttachment has its own
  // method...
  api._getAttachment = function (docId, attachId, attachment, opts, callback) {
    var digest = attachment.digest;
    var type = attachment.content_type;

    stores.binaryStore.get(digest, function (err, attach) {
      if (err) {
        /* istanbul ignore if */
        if (err.name !== 'NotFoundError') {
          return callback(err);
        }
        // Empty attachment
        return callback(null, opts.binary ? createEmptyBlobOrBuffer(type) : '');
      }

      if (opts.binary) {
        callback(null, readAsBlobOrBuffer(attach, type));
      } else {
        callback(null, attach.toString('base64'));
      }
    });
  };

  api._bulkDocs = writeLock(function (req, opts, callback) {
    var newEdits = opts.new_edits;
    var results = new Array(req.docs.length);
    var fetchedDocs = new ExportedMap();
    var stemmedRevs = new ExportedMap();

    var txn = new LevelTransaction();
    var docCountDelta = 0;
    var newUpdateSeq = db._updateSeq;

    // parse the docs and give each a sequence number
    var userDocs = req.docs;
    var docInfos = userDocs.map(function (doc) {
      if (doc._id && isLocalId(doc._id)) {
        return doc;
      }
      var newDoc = parseDoc(doc, newEdits, api.__opts);

      if (newDoc.metadata && !newDoc.metadata.rev_map) {
        newDoc.metadata.rev_map = {};
      }

      return newDoc;
    });
    var infoErrors = docInfos.filter(function (doc) {
      return doc.error;
    });

    if (infoErrors.length) {
      return callback(infoErrors[0]);
    }

    // verify any stub attachments as a precondition test

    function verifyAttachment(digest, callback) {
      txn.get(stores.attachmentStore, digest, function (levelErr) {
        if (levelErr) {
          var err = createError(MISSING_STUB,
                                'unknown stub attachment with digest ' +
                                digest);
          callback(err);
        } else {
          callback();
        }
      });
    }

    function verifyAttachments(finish) {
      var digests = [];
      userDocs.forEach(function (doc) {
        if (doc && doc._attachments) {
          Object.keys(doc._attachments).forEach(function (filename) {
            var att = doc._attachments[filename];
            if (att.stub) {
              digests.push(att.digest);
            }
          });
        }
      });
      if (!digests.length) {
        return finish();
      }
      var numDone = 0;
      var err;

      digests.forEach(function (digest) {
        verifyAttachment(digest, function (attErr) {
          if (attErr && !err) {
            err = attErr;
          }

          if (++numDone === digests.length) {
            finish(err);
          }
        });
      });
    }

    function fetchExistingDocs(finish) {
      var numDone = 0;
      var overallErr;
      function checkDone() {
        if (++numDone === userDocs.length) {
          return finish(overallErr);
        }
      }

      userDocs.forEach(function (doc) {
        if (doc._id && isLocalId(doc._id)) {
          // skip local docs
          return checkDone();
        }
        txn.get(stores.docStore, doc._id, function (err, info) {
          if (err) {
            /* istanbul ignore if */
            if (err.name !== 'NotFoundError') {
              overallErr = err;
            }
          } else {
            fetchedDocs.set(doc._id, info);
          }
          checkDone();
        });
      });
    }

    function compact(revsMap, callback) {
      var promise = Promise.resolve();
      revsMap.forEach(function (revs, docId) {
        // TODO: parallelize, for now need to be sequential to
        // pass orphaned attachment tests
        promise = promise.then(function () {
          return new Promise(function (resolve, reject) {
            api._doCompactionNoLock(docId, revs, {ctx: txn}, function (err) {
              /* istanbul ignore if */
              if (err) {
                return reject(err);
              }
              resolve();
            });
          });
        });
      });

      promise.then(function () {
        callback();
      }, callback);
    }

    function autoCompact(callback) {
      var revsMap = new ExportedMap();
      fetchedDocs.forEach(function (metadata, docId) {
        revsMap.set(docId, compactTree(metadata));
      });
      compact(revsMap, callback);
    }

    function finish() {
      compact(stemmedRevs, function (error) {
        /* istanbul ignore if */
        if (error) {
          complete(error);
        }
        if (api.auto_compaction) {
          return autoCompact(complete);
        }
        complete();
      });
    }

    function writeDoc(docInfo, winningRev$$1, winningRevIsDeleted, newRevIsDeleted,
                      isUpdate, delta, resultsIdx, callback2) {
      docCountDelta += delta;

      var err = null;
      var recv = 0;

      docInfo.metadata.winningRev = winningRev$$1;
      docInfo.metadata.deleted = winningRevIsDeleted;

      docInfo.data._id = docInfo.metadata.id;
      docInfo.data._rev = docInfo.metadata.rev;

      if (newRevIsDeleted) {
        docInfo.data._deleted = true;
      }

      if (docInfo.stemmedRevs.length) {
        stemmedRevs.set(docInfo.metadata.id, docInfo.stemmedRevs);
      }

      var attachments = docInfo.data._attachments ?
        Object.keys(docInfo.data._attachments) :
        [];

      function attachmentSaved(attachmentErr) {
        recv++;
        if (!err) {
          /* istanbul ignore if */
          if (attachmentErr) {
            err = attachmentErr;
            callback2(err);
          } else if (recv === attachments.length) {
            finish();
          }
        }
      }

      function onMD5Load(doc, key, data, attachmentSaved) {
        return function (result) {
          saveAttachment(doc, MD5_PREFIX + result, key, data, attachmentSaved);
        };
      }

      function doMD5(doc, key, attachmentSaved) {
        return function (data) {
          binaryMd5(data, onMD5Load(doc, key, data, attachmentSaved));
        };
      }

      for (var i = 0; i < attachments.length; i++) {
        var key = attachments[i];
        var att = docInfo.data._attachments[key];

        if (att.stub) {
          // still need to update the refs mapping
          var id = docInfo.data._id;
          var rev = docInfo.data._rev;
          saveAttachmentRefs(id, rev, att.digest, attachmentSaved);
          continue;
        }
        var data;
        if (typeof att.data === 'string') {
          // input is assumed to be a base64 string
          try {
            data = thisAtob(att.data);
          } catch (e) {
            callback(createError(BAD_ARG,
                     'Attachment is not a valid base64 string'));
            return;
          }
          doMD5(docInfo, key, attachmentSaved)(data);
        } else {
          prepareAttachmentForStorage(att.data,
            doMD5(docInfo, key, attachmentSaved));
        }
      }

      function finish() {
        var seq = docInfo.metadata.rev_map[docInfo.metadata.rev];
        /* istanbul ignore if */
        if (seq) {
          // check that there aren't any existing revisions with the same
          // revision id, else we shouldn't do anything
          return callback2();
        }
        seq = ++newUpdateSeq;
        docInfo.metadata.rev_map[docInfo.metadata.rev] =
          docInfo.metadata.seq = seq;
        var seqKey = formatSeq(seq);
        var batch = [{
          key: seqKey,
          value: docInfo.data,
          prefix: stores.bySeqStore,
          type: 'put'
        }, {
          key: docInfo.metadata.id,
          value: docInfo.metadata,
          prefix: stores.docStore,
          type: 'put'
        }];
        txn.batch(batch);
        results[resultsIdx] = {
          ok: true,
          id: docInfo.metadata.id,
          rev: docInfo.metadata.rev
        };
        fetchedDocs.set(docInfo.metadata.id, docInfo.metadata);
        callback2();
      }

      if (!attachments.length) {
        finish();
      }
    }

    // attachments are queued per-digest, otherwise the refs could be
    // overwritten by concurrent writes in the same bulkDocs session
    var attachmentQueues = {};

    function saveAttachmentRefs(id, rev, digest, callback) {

      function fetchAtt() {
        return new Promise(function (resolve, reject) {
          txn.get(stores.attachmentStore, digest, function (err, oldAtt) {
            /* istanbul ignore if */
            if (err && err.name !== 'NotFoundError') {
              return reject(err);
            }
            resolve(oldAtt);
          });
        });
      }

      function saveAtt(oldAtt) {
        var ref = [id, rev].join('@');
        var newAtt = {};

        if (oldAtt) {
          if (oldAtt.refs) {
            // only update references if this attachment already has them
            // since we cannot migrate old style attachments here without
            // doing a full db scan for references
            newAtt.refs = oldAtt.refs;
            newAtt.refs[ref] = true;
          }
        } else {
          newAtt.refs = {};
          newAtt.refs[ref] = true;
        }

        return new Promise(function (resolve) {
          txn.batch([{
            type: 'put',
            prefix: stores.attachmentStore,
            key: digest,
            value: newAtt
          }]);
          resolve(!oldAtt);
        });
      }

      // put attachments in a per-digest queue, to avoid two docs with the same
      // attachment overwriting each other
      var queue = attachmentQueues[digest] || Promise.resolve();
      attachmentQueues[digest] = queue.then(function () {
        return fetchAtt().then(saveAtt).then(function (isNewAttachment) {
          callback(null, isNewAttachment);
        }, callback);
      });
    }

    function saveAttachment(docInfo, digest, key, data, callback) {
      var att = docInfo.data._attachments[key];
      delete att.data;
      att.digest = digest;
      att.length = data.length;
      var id = docInfo.metadata.id;
      var rev = docInfo.metadata.rev;
      att.revpos = parseInt(rev, 10);

      saveAttachmentRefs(id, rev, digest, function (err, isNewAttachment) {
        /* istanbul ignore if */
        if (err) {
          return callback(err);
        }
        // do not try to store empty attachments
        if (data.length === 0) {
          return callback(err);
        }
        if (!isNewAttachment) {
          // small optimization - don't bother writing it again
          return callback(err);
        }
        txn.batch([{
          type: 'put',
          prefix: stores.binaryStore,
          key: digest,
          value: bufferFrom(data, 'binary')
        }]);
        callback();
      });
    }

    function complete(err) {
      /* istanbul ignore if */
      if (err) {
        return nextTick(function () {
          callback(err);
        });
      }
      txn.batch([
        {
          prefix: stores.metaStore,
          type: 'put',
          key: UPDATE_SEQ_KEY,
          value: newUpdateSeq
        },
        {
          prefix: stores.metaStore,
          type: 'put',
          key: DOC_COUNT_KEY,
          value: db._docCount + docCountDelta
        }
      ]);
      txn.execute(db, function (err) {
        /* istanbul ignore if */
        if (err) {
          return callback(err);
        }
        db._docCount += docCountDelta;
        db._updateSeq = newUpdateSeq;
        levelChanges.notify(name);
        nextTick(function () {
          callback(null, results);
        });
      });
    }

    if (!docInfos.length) {
      return callback(null, []);
    }

    verifyAttachments(function (err) {
      if (err) {
        return callback(err);
      }
      fetchExistingDocs(function (err) {
        /* istanbul ignore if */
        if (err) {
          return callback(err);
        }
        processDocs(revLimit, docInfos, api, fetchedDocs, txn, results,
                    writeDoc, opts, finish);
      });
    });
  });
  api._allDocs = function (opts, callback) {
    if ('keys' in opts) {
      return allDocsKeysQuery(this, opts);
    }
    return readLock(function (opts, callback) {
      opts = clone(opts);
      countDocs(function (err, docCount) {
        /* istanbul ignore if */
        if (err) {
          return callback(err);
        }
        var readstreamOpts = {};
        var skip = opts.skip || 0;
        if (opts.startkey) {
          readstreamOpts.gte = opts.startkey;
        }
        if (opts.endkey) {
          readstreamOpts.lte = opts.endkey;
        }
        if (opts.key) {
          readstreamOpts.gte = readstreamOpts.lte = opts.key;
        }
        if (opts.descending) {
          readstreamOpts.reverse = true;
          // switch start and ends
          var tmp = readstreamOpts.lte;
          readstreamOpts.lte = readstreamOpts.gte;
          readstreamOpts.gte = tmp;
        }
        var limit;
        if (typeof opts.limit === 'number') {
          limit = opts.limit;
        }
        if (limit === 0 ||
            ('gte' in readstreamOpts && 'lte' in readstreamOpts &&
            readstreamOpts.gte > readstreamOpts.lte)) {
          // should return 0 results when start is greater than end.
          // normally level would "fix" this for us by reversing the order,
          // so short-circuit instead
          var returnVal = {
            total_rows: docCount,
            offset: opts.skip,
            rows: []
          };
          /* istanbul ignore if */
          if (opts.update_seq) {
            returnVal.update_seq = db._updateSeq;
          }
          return callback(null, returnVal);
        }
        var results = [];
        var docstream = stores.docStore.readStream(readstreamOpts);

        var throughStream = through2.obj(function (entry, _, next) {
          var metadata = entry.value;
          // winningRev and deleted are performance-killers, but
          // in newer versions of PouchDB, they are cached on the metadata
          var winningRev$$1 = getWinningRev(metadata);
          var deleted = getIsDeleted(metadata, winningRev$$1);
          if (!deleted) {
            if (skip-- > 0) {
              next();
              return;
            } else if (typeof limit === 'number' && limit-- <= 0) {
              docstream.unpipe();
              docstream.destroy();
              next();
              return;
            }
          } else if (opts.deleted !== 'ok') {
            next();
            return;
          }
          function allDocsInner(data) {
            var doc = {
              id: metadata.id,
              key: metadata.id,
              value: {
                rev: winningRev$$1
              }
            };
            if (opts.include_docs) {
              doc.doc = data;
              doc.doc._rev = doc.value.rev;
              if (opts.conflicts) {
                var conflicts = collectConflicts(metadata);
                if (conflicts.length) {
                  doc.doc._conflicts = conflicts;
                }
              }
              for (var att in doc.doc._attachments) {
                if (Object.prototype.hasOwnProperty.call(doc.doc._attachments, att)) {
                  doc.doc._attachments[att].stub = true;
                }
              }
            }
            if (opts.inclusive_end === false && metadata.id === opts.endkey) {
              return next();
            } else if (deleted) {
              if (opts.deleted === 'ok') {
                doc.value.deleted = true;
                doc.doc = null;
              } else {
                /* istanbul ignore next */
                return next();
              }
            }
            results.push(doc);
            next();
          }
          if (opts.include_docs) {
            var seq = metadata.rev_map[winningRev$$1];
            stores.bySeqStore.get(formatSeq(seq), function (err, data) {
              allDocsInner(data);
            });
          }
          else {
            allDocsInner();
          }
        }, function (next) {
          Promise.resolve().then(function () {
            if (opts.include_docs && opts.attachments) {
              return fetchAttachments(results, stores, opts);
            }
          }).then(function () {
            var returnVal = {
              total_rows: docCount,
              offset: opts.skip,
              rows: results
            };

            /* istanbul ignore if */
            if (opts.update_seq) {
              returnVal.update_seq = db._updateSeq;
            }
            callback(null, returnVal);
          }, callback);
          next();
        }).on('unpipe', function () {
          throughStream.end();
        });

        docstream.on('error', callback);

        docstream.pipe(throughStream);
      });
    })(opts, callback);
  };

  api._changes = function (opts) {
    opts = clone(opts);

    if (opts.continuous) {
      var id = name + ':' + uuid$1();
      levelChanges.addListener(name, id, api, opts);
      levelChanges.notify(name);
      return {
        cancel: function () {
          levelChanges.removeListener(name, id);
        }
      };
    }

    var descending = opts.descending;
    var results = [];
    var lastSeq = opts.since || 0;
    var called = 0;
    var streamOpts = {
      reverse: descending
    };
    var limit;
    if ('limit' in opts && opts.limit > 0) {
      limit = opts.limit;
    }
    if (!streamOpts.reverse) {
      streamOpts.start = formatSeq(opts.since || 0);
    }

    var docIds = opts.doc_ids && new ExportedSet(opts.doc_ids);
    var filter = filterChange(opts);
    var docIdsToMetadata = new ExportedMap();

    function complete() {
      opts.done = true;
      if (opts.return_docs && opts.limit) {
        /* istanbul ignore if */
        if (opts.limit < results.length) {
          results.length = opts.limit;
        }
      }
      changeStream.unpipe(throughStream);
      changeStream.destroy();
      if (!opts.continuous && !opts.cancelled) {
        if (opts.include_docs && opts.attachments && opts.return_docs) {
          fetchAttachments(results, stores, opts).then(function () {
            opts.complete(null, {results: results, last_seq: lastSeq});
          });
        } else {
          opts.complete(null, {results: results, last_seq: lastSeq});
        }
      }
    }
    var changeStream = stores.bySeqStore.readStream(streamOpts);
    var throughStream = through2.obj(function (data, _, next) {
      if (limit && called >= limit) {
        complete();
        return next();
      }
      if (opts.cancelled || opts.done) {
        return next();
      }

      var seq = parseSeq(data.key);
      var doc = data.value;

      if (seq === opts.since && !descending) {
        // couchdb ignores `since` if descending=true
        return next();
      }

      if (docIds && !docIds.has(doc._id)) {
        return next();
      }

      var metadata;

      function onGetMetadata(metadata) {
        var winningRev$$1 = getWinningRev(metadata);

        function onGetWinningDoc(winningDoc) {

          var change = opts.processChange(winningDoc, metadata, opts);
          change.seq = metadata.seq;

          var filtered = filter(change);
          if (typeof filtered === 'object') {
            return opts.complete(filtered);
          }

          if (filtered) {
            called++;

            if (opts.attachments && opts.include_docs) {
              // fetch attachment immediately for the benefit
              // of live listeners
              fetchAttachments([change], stores, opts).then(function () {
                opts.onChange(change);
              });
            } else {
              opts.onChange(change);
            }

            if (opts.return_docs) {
              results.push(change);
            }
          }
          next();
        }

        if (metadata.seq !== seq) {
          // some other seq is later
          return next();
        }

        lastSeq = seq;

        if (winningRev$$1 === doc._rev) {
          return onGetWinningDoc(doc);
        }

        // fetch the winner

        var winningSeq = metadata.rev_map[winningRev$$1];

        stores.bySeqStore.get(formatSeq(winningSeq), function (err, doc) {
          onGetWinningDoc(doc);
        });
      }

      metadata = docIdsToMetadata.get(doc._id);
      if (metadata) { // cached
        return onGetMetadata(metadata);
      }
      // metadata not cached, have to go fetch it
      stores.docStore.get(doc._id, function (err, metadata) {
        /* istanbul ignore if */
        if (opts.cancelled || opts.done || db.isClosed() ||
          isLocalId(metadata.id)) {
          return next();
        }
        docIdsToMetadata.set(doc._id, metadata);
        onGetMetadata(metadata);
      });
    }, function (next) {
      if (opts.cancelled) {
        return next();
      }
      if (opts.return_docs && opts.limit) {
        /* istanbul ignore if */
        if (opts.limit < results.length) {
          results.length = opts.limit;
        }
      }

      next();
    }).on('unpipe', function () {
      throughStream.end();
      complete();
    });
    changeStream.pipe(throughStream);
    return {
      cancel: function () {
        opts.cancelled = true;
        complete();
      }
    };
  };

  api._close = function (callback) {
    /* istanbul ignore if */
    if (db.isClosed()) {
      return callback(createError(NOT_OPEN));
    }
    db.close(function (err) {
      /* istanbul ignore if */
      if (err) {
        callback(err);
      } else {
        dbStore.delete(name);

        var adapterName = functionName(leveldown);
        var adapterStore = dbStores.get(adapterName);
        var viewNamePrefix = PouchDB.prefix + name + "-mrview-";
        var keys = [...adapterStore.keys()].filter(k => k.includes(viewNamePrefix));
        keys.forEach(key => {
          var eventEmitter = adapterStore.get(key);
          eventEmitter.removeAllListeners();
          eventEmitter.close();
          adapterStore.delete(key);
        });

        callback();
      }
    });
  };

  api._getRevisionTree = function (docId, callback) {
    stores.docStore.get(docId, function (err, metadata) {
      if (err) {
        callback(createError(MISSING_DOC));
      } else {
        callback(null, metadata.rev_tree);
      }
    });
  };

  api._doCompaction = writeLock(function (docId, revs, opts, callback) {
    api._doCompactionNoLock(docId, revs, opts, callback);
  });

  // the NoLock version is for use by bulkDocs
  api._doCompactionNoLock = function (docId, revs, opts, callback) {
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }

    if (!revs.length) {
      return callback();
    }
    var txn = opts.ctx || new LevelTransaction();

    txn.get(stores.docStore, docId, function (err, metadata) {
      /* istanbul ignore if */
      if (err) {
        return callback(err);
      }
      var seqs = revs.map(function (rev) {
        var seq = metadata.rev_map[rev];
        delete metadata.rev_map[rev];
        return seq;
      });
      traverseRevTree(metadata.rev_tree, function (isLeaf, pos,
                                                         revHash, ctx, opts) {
        var rev = pos + '-' + revHash;
        if (revs.indexOf(rev) !== -1) {
          opts.status = 'missing';
        }
      });

      var batch = [];
      batch.push({
        key: metadata.id,
        value: metadata,
        type: 'put',
        prefix: stores.docStore
      });

      var digestMap = {};
      var numDone = 0;
      var overallErr;
      function checkDone(err) {
        /* istanbul ignore if */
        if (err) {
          overallErr = err;
        }
        if (++numDone === revs.length) { // done
          /* istanbul ignore if */
          if (overallErr) {
            return callback(overallErr);
          }
          deleteOrphanedAttachments();
        }
      }

      function finish(err) {
        /* istanbul ignore if */
        if (err) {
          return callback(err);
        }
        txn.batch(batch);
        if (opts.ctx) {
          // don't execute immediately
          return callback();
        }
        txn.execute(db, callback);
      }

      function deleteOrphanedAttachments() {
        var possiblyOrphanedAttachments = Object.keys(digestMap);
        if (!possiblyOrphanedAttachments.length) {
          return finish();
        }
        var numDone = 0;
        var overallErr;
        function checkDone(err) {
          /* istanbul ignore if */
          if (err) {
            overallErr = err;
          }
          if (++numDone === possiblyOrphanedAttachments.length) {
            finish(overallErr);
          }
        }
        var refsToDelete = new ExportedMap();
        revs.forEach(function (rev) {
          refsToDelete.set(docId + '@' + rev, true);
        });
        possiblyOrphanedAttachments.forEach(function (digest) {
          txn.get(stores.attachmentStore, digest, function (err, attData) {
            /* istanbul ignore if */
            if (err) {
              if (err.name === 'NotFoundError') {
                return checkDone();
              } else {
                return checkDone(err);
              }
            }
            var refs = Object.keys(attData.refs || {}).filter(function (ref) {
              return !refsToDelete.has(ref);
            });
            var newRefs = {};
            refs.forEach(function (ref) {
              newRefs[ref] = true;
            });
            if (refs.length) { // not orphaned
              batch.push({
                key: digest,
                type: 'put',
                value: {refs: newRefs},
                prefix: stores.attachmentStore
              });
            } else { // orphaned, can safely delete
              batch = batch.concat([{
                key: digest,
                type: 'del',
                prefix: stores.attachmentStore
              }, {
                key: digest,
                type: 'del',
                prefix: stores.binaryStore
              }]);
            }
            checkDone();
          });
        });
      }

      seqs.forEach(function (seq) {
        batch.push({
          key: formatSeq(seq),
          type: 'del',
          prefix: stores.bySeqStore
        });
        txn.get(stores.bySeqStore, formatSeq(seq), function (err, doc) {
          /* istanbul ignore if */
          if (err) {
            if (err.name === 'NotFoundError') {
              return checkDone();
            } else {
              return checkDone(err);
            }
          }
          var atts = Object.keys(doc._attachments || {});
          atts.forEach(function (attName) {
            var digest = doc._attachments[attName].digest;
            digestMap[digest] = true;
          });
          checkDone();
        });
      });
    });
  };

  api._getLocal = function (id, callback) {
    stores.localStore.get(id, function (err, doc) {
      if (err) {
        callback(createError(MISSING_DOC));
      } else {
        callback(null, doc);
      }
    });
  };

  api._putLocal = function (doc, opts, callback) {
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
    if (opts.ctx) {
      api._putLocalNoLock(doc, opts, callback);
    } else {
      api._putLocalWithLock(doc, opts, callback);
    }
  };

  api._putLocalWithLock = writeLock(function (doc, opts, callback) {
    api._putLocalNoLock(doc, opts, callback);
  });

  // the NoLock version is for use by bulkDocs
  api._putLocalNoLock = function (doc, opts, callback) {
    delete doc._revisions; // ignore this, trust the rev
    var oldRev = doc._rev;
    var id = doc._id;

    var txn = opts.ctx || new LevelTransaction();

    txn.get(stores.localStore, id, function (err, resp) {
      if (err && oldRev) {
        return callback(createError(REV_CONFLICT));
      }
      if (resp && resp._rev !== oldRev) {
        return callback(createError(REV_CONFLICT));
      }
      doc._rev =
          oldRev ? '0-' + (parseInt(oldRev.split('-')[1], 10) + 1) : '0-1';
      var batch = [
        {
          type: 'put',
          prefix: stores.localStore,
          key: id,
          value: doc
        }
      ];

      txn.batch(batch);
      var ret = {ok: true, id: doc._id, rev: doc._rev};

      if (opts.ctx) {
        // don't execute immediately
        return callback(null, ret);
      }
      txn.execute(db, function (err) {
        /* istanbul ignore if */
        if (err) {
          return callback(err);
        }
        callback(null, ret);
      });
    });
  };

  api._removeLocal = function (doc, opts, callback) {
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
    if (opts.ctx) {
      api._removeLocalNoLock(doc, opts, callback);
    } else {
      api._removeLocalWithLock(doc, opts, callback);
    }
  };

  api._removeLocalWithLock = writeLock(function (doc, opts, callback) {
    api._removeLocalNoLock(doc, opts, callback);
  });

  // the NoLock version is for use by bulkDocs
  api._removeLocalNoLock = function (doc, opts, callback) {
    var txn = opts.ctx || new LevelTransaction();
    txn.get(stores.localStore, doc._id, function (err, resp) {
      if (err) {
        /* istanbul ignore if */
        if (err.name !== 'NotFoundError') {
          return callback(err);
        } else {
          return callback(createError(MISSING_DOC));
        }
      }
      if (resp._rev !== doc._rev) {
        return callback(createError(REV_CONFLICT));
      }
      txn.batch([{
        prefix: stores.localStore,
        type: 'del',
        key: doc._id
      }]);
      var ret = {ok: true, id: doc._id, rev: '0-0'};
      if (opts.ctx) {
        // don't execute immediately
        return callback(null, ret);
      }
      txn.execute(db, function (err) {
        /* istanbul ignore if */
        if (err) {
          return callback(err);
        }
        callback(null, ret);
      });
    });
  };

  // close and delete open leveldb stores
  api._destroy = function (opts, callback) {
    var dbStore;
    var leveldownName = functionName(leveldown);
    /* istanbul ignore else */
    if (dbStores.has(leveldownName)) {
      dbStore = dbStores.get(leveldownName);
    } else {
      return callDestroy(name, callback);
    }

    /* istanbul ignore else */
    if (dbStore.has(name)) {
      levelChanges.removeAllListeners(name);

      dbStore.get(name).close(function () {
        dbStore.delete(name);
        callDestroy(name, callback);
      });
    } else {
      callDestroy(name, callback);
    }
  };
  function callDestroy(name, cb) {
    // May not exist if leveldown is backed by memory adapter
    /* istanbul ignore else */
    if ('destroy' in leveldown) {
      leveldown.destroy(name, cb);
    } else {
      cb(null);
    }
  }
}

// require leveldown. provide verbose output on error as it is the default
// nodejs adapter, which we do not provide for the user
/* istanbul ignore next */
var requireLeveldown = function () {
  try {
    return require('leveldown');
  } catch (err) {
    /* eslint no-ex-assign: 0*/
    err = err || 'leveldown import error';
    if (err.code === 'MODULE_NOT_FOUND') {
      // handle leveldown not installed case
      return new Error([
        'the \'leveldown\' package is not available. install it, or,',
        'specify another storage backend using the \'db\' option'
      ].join(' '));
    } else if (err.message && err.message.match('Module version mismatch')) {
      // handle common user enviornment error
      return new Error([
        err.message,
        'This generally implies that leveldown was built with a different',
        'version of node than that which is running now.  You may try',
        'fully removing and reinstalling PouchDB or leveldown to resolve.'
      ].join(' '));
    }
    // handle general internal nodejs require error
    return new Error(err.toString() + ': unable to import leveldown');
  }
};

var stores = [
  'document-store',
  'by-sequence',
  'attach-store',
  'attach-binary-store'
];
function formatSeq(n) {
  return ('0000000000000000' + n).slice(-16);
}
var UPDATE_SEQ_KEY$1 = '_local_last_update_seq';
var DOC_COUNT_KEY$1 = '_local_doc_count';
var UUID_KEY$1 = '_local_uuid';

var doMigrationOne = function (name, db, callback) {
  // local require to prevent crashing if leveldown isn't installed.
  var leveldown = require("leveldown");

  var base = path.resolve(name);
  function move(store, index, cb) {
    var storePath = path.join(base, store);
    var opts;
    if (index === 3) {
      opts = {
        valueEncoding: 'binary'
      };
    } else {
      opts = {
        valueEncoding: 'json'
      };
    }
    var sub = db.sublevel(store, opts);
    var orig = level(storePath, opts);
    var from = orig.createReadStream();
    var writeStream = new LevelWriteStream(sub);
    var to = writeStream();
    from.on('end', function () {
      orig.close(function (err) {
        cb(err, storePath);
      });
    });
    from.pipe(to);
  }
  fs.unlink(base + '.uuid', function (err) {
    if (err) {
      return callback();
    }
    var todo = 4;
    var done = [];
    stores.forEach(function (store, i) {
      move(store, i, function (err, storePath) {
        /* istanbul ignore if */
        if (err) {
          return callback(err);
        }
        done.push(storePath);
        if (!(--todo)) {
          done.forEach(function (item) {
            leveldown.destroy(item, function () {
              if (++todo === done.length) {
                fs.rmdir(base, callback);
              }
            });
          });
        }
      });
    });
  });
};
var doMigrationTwo = function (db, stores, callback) {
  var batches = [];
  stores.bySeqStore.get(UUID_KEY$1, function (err, value) {
    if (err) {
      // no uuid key, so don't need to migrate;
      return callback();
    }
    batches.push({
      key: UUID_KEY$1,
      value: value,
      prefix: stores.metaStore,
      type: 'put',
      valueEncoding: 'json'
    });
    batches.push({
      key: UUID_KEY$1,
      prefix: stores.bySeqStore,
      type: 'del'
    });
    stores.bySeqStore.get(DOC_COUNT_KEY$1, function (err, value) {
      if (value) {
        // if no doc count key,
        // just skip
        // we can live with this
        batches.push({
          key: DOC_COUNT_KEY$1,
          value: value,
          prefix: stores.metaStore,
          type: 'put',
          valueEncoding: 'json'
        });
        batches.push({
          key: DOC_COUNT_KEY$1,
          prefix: stores.bySeqStore,
          type: 'del'
        });
      }
      stores.bySeqStore.get(UPDATE_SEQ_KEY$1, function (err, value) {
        if (value) {
          // if no UPDATE_SEQ_KEY
          // just skip
          // we've gone to far to stop.
          batches.push({
            key: UPDATE_SEQ_KEY$1,
            value: value,
            prefix: stores.metaStore,
            type: 'put',
            valueEncoding: 'json'
          });
          batches.push({
            key: UPDATE_SEQ_KEY$1,
            prefix: stores.bySeqStore,
            type: 'del'
          });
        }
        var deletedSeqs = {};
        stores.docStore.createReadStream({
          startKey: '_',
          endKey: '_\xFF'
        }).pipe(through2.obj(function (ch, _, next) {
          if (!isLocalId(ch.key)) {
            return next();
          }
          batches.push({
            key: ch.key,
            prefix: stores.docStore,
            type: 'del'
          });
          var winner = winningRev(ch.value);
          Object.keys(ch.value.rev_map).forEach(function (key) {
            if (key !== 'winner') {
              this.push(formatSeq(ch.value.rev_map[key]));
            }
          }, this);
          var winningSeq = ch.value.rev_map[winner];
          stores.bySeqStore.get(formatSeq(winningSeq), function (err, value) {
            if (!err) {
              batches.push({
                key: ch.key,
                value: value,
                prefix: stores.localStore,
                type: 'put',
                valueEncoding: 'json'
              });
            }
            next();
          });

        })).pipe(through2.obj(function (seq, _, next) {
          /* istanbul ignore if */
          if (deletedSeqs[seq]) {
            return next();
          }
          deletedSeqs[seq] = true;
          stores.bySeqStore.get(seq, function (err, resp) {
            /* istanbul ignore if */
            if (err || !isLocalId(resp._id)) {
              return next();
            }
            batches.push({
              key: seq,
              prefix: stores.bySeqStore,
              type: 'del'
            });
            next();
          });
        }, function () {
          db.batch(batches, callback);
        }));
      });
    });
  });

};

var migrate = {
  doMigrationOne: doMigrationOne,
  doMigrationTwo: doMigrationTwo
};

function LevelDownPouch(opts, callback) {

  // Users can pass in their own leveldown alternative here, in which case
  // it overrides the default one. (This is in addition to the custom builds.)
  var leveldown = opts.db;

  /* istanbul ignore else */
  if (!leveldown) {
    leveldown = requireLeveldown();

    /* istanbul ignore if */
    if (leveldown instanceof Error) {
      return callback(leveldown);
    }
  }

  var _opts = $inject_Object_assign({
    db: leveldown,
    migrate: migrate
  }, opts);

  LevelPouch.call(this, _opts, callback);
}

// overrides for normal LevelDB behavior on Node
LevelDownPouch.valid = function () {
  return true;
};
LevelDownPouch.use_prefix = false;

function LevelPouch$1 (PouchDB) {
  PouchDB.adapter('leveldb', LevelDownPouch, true);
}

// dead simple promise pool, inspired by https://github.com/timdp/es6-promise-pool
// but much smaller in code size. limits the number of concurrent promises that are executed


function pool(promiseFactories, limit) {
  return new Promise(function (resolve, reject) {
    var running = 0;
    var current = 0;
    var done = 0;
    var len = promiseFactories.length;
    var err;

    function runNext() {
      running++;
      promiseFactories[current++]().then(onSuccess, onError);
    }

    function doNext() {
      if (++done === len) {
        /* istanbul ignore if */
        if (err) {
          reject(err);
        } else {
          resolve();
        }
      } else {
        runNextBatch();
      }
    }

    function onSuccess() {
      running--;
      doNext();
    }

    /* istanbul ignore next */
    function onError(thisErr) {
      running--;
      err = err || thisErr;
      doNext();
    }

    function runNextBatch() {
      while (running < limit && current < len) {
        runNext();
      }
    }

    runNextBatch();
  });
}

const CHANGES_BATCH_SIZE = 25;
const MAX_SIMULTANEOUS_REVS = 50;
const CHANGES_TIMEOUT_BUFFER = 5000;
const DEFAULT_HEARTBEAT = 10000;

let supportsBulkGetMap = {};

function readAttachmentsAsBlobOrBuffer(row) {
  let doc = row.doc || row.ok;
  let atts = doc && doc._attachments;
  if (!atts) {
    return;
  }
  Object.keys(atts).forEach(function (filename) {
    let att = atts[filename];
    att.data = b64ToBluffer(att.data, att.content_type);
  });
}

function encodeDocId(id) {
  if (/^_design/.test(id)) {
    return '_design/' + encodeURIComponent(id.slice(8));
  }
  if (/^_local/.test(id)) {
    return '_local/' + encodeURIComponent(id.slice(7));
  }
  return encodeURIComponent(id);
}

function preprocessAttachments$1(doc) {
  if (!doc._attachments || !Object.keys(doc._attachments)) {
    return Promise.resolve();
  }

  return Promise.all(Object.keys(doc._attachments).map(function (key) {
    let attachment = doc._attachments[key];
    if (attachment.data && typeof attachment.data !== 'string') {
      return new Promise(function (resolve) {
        blobToBase64(attachment.data, resolve);
      }).then(function (b64) {
        attachment.data = b64;
      });
    }
  }));
}

function hasUrlPrefix(opts) {
  if (!opts.prefix) {
    return false;
  }
  let protocol = parseUri(opts.prefix).protocol;
  return protocol === 'http' || protocol === 'https';
}

// Get all the information you possibly can about the URI given by name and
// return it as a suitable object.
function getHost(name, opts) {
  // encode db name if opts.prefix is a url (#5574)
  if (hasUrlPrefix(opts)) {
    let dbName = opts.name.substr(opts.prefix.length);
    // Ensure prefix has a trailing slash
    let prefix = opts.prefix.replace(/\/?$/, '/');
    name = prefix + encodeURIComponent(dbName);
  }

  let uri = parseUri(name);
  if (uri.user || uri.password) {
    uri.auth = {username: uri.user, password: uri.password};
  }

  // Split the path part of the URI into parts using '/' as the delimiter
  // after removing any leading '/' and any trailing '/'
  let parts = uri.path.replace(/(^\/|\/$)/g, '').split('/');

  uri.db = parts.pop();
  // Prevent double encoding of URI component
  if (uri.db.indexOf('%') === -1) {
    uri.db = encodeURIComponent(uri.db);
  }

  uri.path = parts.join('/');

  return uri;
}

// Generate a URL with the host data given by opts and the given path
function genDBUrl(opts, path$$1) {
  return genUrl(opts, opts.db + '/' + path$$1);
}

// Generate a URL with the host data given by opts and the given path
function genUrl(opts, path$$1) {
  // If the host already has a path, then we need to have a path delimiter
  // Otherwise, the path delimiter is the empty string
  let pathDel = !opts.path ? '' : '/';

  // If the host already has a path, then we need to have a path delimiter
  // Otherwise, the path delimiter is the empty string
  return opts.protocol + '://' + opts.host +
         (opts.port ? (':' + opts.port) : '') +
         '/' + opts.path + pathDel + path$$1;
}

function paramsToStr(params) {
  return '?' + Object.keys(params).map(function (k) {
    return k + '=' + encodeURIComponent(params[k]);
  }).join('&');
}

function shouldCacheBust(opts) {
  let ua = (typeof navigator !== 'undefined' && navigator.userAgent) ?
      navigator.userAgent.toLowerCase() : '';
  let isIE = ua.indexOf('msie') !== -1;
  let isTrident = ua.indexOf('trident') !== -1;
  let isEdge = ua.indexOf('edge') !== -1;
  let isGET = !('method' in opts) || opts.method === 'GET';
  return (isIE || isTrident || isEdge) && isGET;
}

// Implements the PouchDB API for dealing with CouchDB instances over HTTP
function HttpPouch(opts, callback) {

  // The functions that will be publicly available for HttpPouch
  let api = this;

  let host = getHost(opts.name, opts);
  let dbUrl = genDBUrl(host, '');

  opts = clone(opts);

  const ourFetch = async function (url, options) {

    options = options || {};
    options.headers = options.headers || new nodeFetch.Headers();

    options.credentials = 'include';

    if (opts.auth || host.auth) {
      let nAuth = opts.auth || host.auth;
      let str = nAuth.username + ':' + nAuth.password;
      let token = thisBtoa(unescape(encodeURIComponent(str)));
      options.headers.set('Authorization', 'Basic ' + token);
    }

    let headers = opts.headers || {};
    Object.keys(headers).forEach(function (key) {
      options.headers.append(key, headers[key]);
    });

    /* istanbul ignore if */
    if (shouldCacheBust(options)) {
      url += (url.indexOf('?') === -1 ? '?' : '&') + '_nonce=' + Date.now();
    }

    let fetchFun = opts.fetch || fetch;
    return await fetchFun(url, options);
  };

  function adapterFun$$1(name, fun) {
    return adapterFun(name, function (...args) {
      setup().then(function () {
        return fun.apply(this, args);
      }).catch(function (e) {
        let callback = args.pop();
        callback(e);
      });
    }).bind(api);
  }

  async function fetchJSON(url, options) {

    let result = {};

    options = options || {};
    options.headers = options.headers || new nodeFetch.Headers();

    if (!options.headers.get('Content-Type')) {
      options.headers.set('Content-Type', 'application/json');
    }
    if (!options.headers.get('Accept')) {
      options.headers.set('Accept', 'application/json');
    }

    const response = await ourFetch(url, options);
    result.ok = response.ok;
    result.status = response.status;
    const json = await response.json();

    result.data = json;
    if (!result.ok) {
      result.data.status = result.status;
      let err = generateErrorFromResponse(result.data);
      throw err;
    }

    if (Array.isArray(result.data)) {
      result.data = result.data.map(function (v) {
        if (v.error || v.missing) {
          return generateErrorFromResponse(v);
        } else {
          return v;
        }
      });
    }

    return result;
  }

  let setupPromise;

  async function setup() {
    if (opts.skip_setup) {
      return Promise.resolve();
    }

    // If there is a setup in process or previous successful setup
    // done then we will use that
    // If previous setups have been rejected we will try again
    if (setupPromise) {
      return setupPromise;
    }

    setupPromise = fetchJSON(dbUrl).catch(function (err) {
      if (err && err.status && err.status === 404) {
        return fetchJSON(dbUrl, {method: 'PUT'});
      } else {
        return Promise.reject(err);
      }
    }).catch(function (err) {
      // If we try to create a database that already exists, skipped in
      // istanbul since its catching a race condition.
      /* istanbul ignore if */
      if (err && err.status && err.status === 412) {
        return true;
      }
      return Promise.reject(err);
    });

    setupPromise.catch(function () {
      setupPromise = null;
    });

    return setupPromise;
  }

  nextTick(function () {
    callback(null, api);
  });

  api._remote = true;

  /* istanbul ignore next */
  api.type = function () {
    return 'http';
  };

  api.id = adapterFun$$1('id', async function (callback) {
    let result;
    try {
      const response = await ourFetch(genUrl(host, ''));
      result = await response.json();
    } catch (err) {
      result = {};
    }

    // Bad response or missing `uuid` should not prevent ID generation.
    let uuid$$1 = (result && result.uuid) ? (result.uuid + host.db) : genDBUrl(host, '');
    callback(null, uuid$$1);
  });

  // Sends a POST request to the host calling the couchdb _compact function
  //    version: The version of CouchDB it is running
  api.compact = adapterFun$$1('compact', async function (opts, callback) {
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
    opts = clone(opts);

    await fetchJSON(genDBUrl(host, '_compact'), {method: 'POST'});

    function ping() {
      api.info(function (err, res$$1) {
        // CouchDB may send a "compact_running:true" if it's
        // already compacting. PouchDB Server doesn't.
        /* istanbul ignore else */
        if (res$$1 && !res$$1.compact_running) {
          callback(null, {ok: true});
        } else {
          setTimeout(ping, opts.interval || 200);
        }
      });
    }
    // Ping the http if it's finished compaction
    ping();
  });

  api.bulkGet = adapterFun('bulkGet', function (opts, callback) {
    let self = this;

    async function doBulkGet(cb) {
      let params = {};
      if (opts.revs) {
        params.revs = true;
      }
      if (opts.attachments) {
        /* istanbul ignore next */
        params.attachments = true;
      }
      if (opts.latest) {
        params.latest = true;
      }
      try {
        const result = await fetchJSON(genDBUrl(host, '_bulk_get' + paramsToStr(params)), {
          method: 'POST',
          body: JSON.stringify({ docs: opts.docs})
        });

        if (opts.attachments && opts.binary) {
          result.data.results.forEach(function (res$$1) {
            res$$1.docs.forEach(readAttachmentsAsBlobOrBuffer);
          });
        }
        cb(null, result.data);
      } catch (error) {
        cb(error);
      }
    }

    /* istanbul ignore next */
    function doBulkGetShim() {
      // avoid "url too long error" by splitting up into multiple requests
      let batchSize = MAX_SIMULTANEOUS_REVS;
      let numBatches = Math.ceil(opts.docs.length / batchSize);
      let numDone = 0;
      let results = new Array(numBatches);

      function onResult(batchNum) {
        return function (err, res$$1) {
          // err is impossible because shim returns a list of errs in that case
          results[batchNum] = res$$1.results;
          if (++numDone === numBatches) {
            callback(null, {results: flatten(results)});
          }
        };
      }

      for (let i = 0; i < numBatches; i++) {
        let subOpts = pick(opts, ['revs', 'attachments', 'binary', 'latest']);
        subOpts.docs = opts.docs.slice(i * batchSize,
          Math.min(opts.docs.length, (i + 1) * batchSize));
        bulkGet(self, subOpts, onResult(i));
      }
    }

    // mark the whole database as either supporting or not supporting _bulk_get
    let dbUrl = genUrl(host, '');
    let supportsBulkGet = supportsBulkGetMap[dbUrl];

    /* istanbul ignore next */
    if (typeof supportsBulkGet !== 'boolean') {
      // check if this database supports _bulk_get
      doBulkGet(function (err, res$$1) {
        if (err) {
          supportsBulkGetMap[dbUrl] = false;
          res(
            err.status,
            'PouchDB is just detecting if the remote ' +
            'supports the _bulk_get API.'
          );
          doBulkGetShim();
        } else {
          supportsBulkGetMap[dbUrl] = true;
          callback(null, res$$1);
        }
      });
    } else if (supportsBulkGet) {
      doBulkGet(callback);
    } else {
      doBulkGetShim();
    }
  });

  // Calls GET on the host, which gets back a JSON string containing
  //    couchdb: A welcome string
  //    version: The version of CouchDB it is running
  api._info = async function (callback) {
    try {
      await setup();
      const response = await ourFetch(genDBUrl(host, ''));
      const info = await response.json();
      info.host = genDBUrl(host, '');
      callback(null, info);
    } catch (err) {
      callback(err);
    }
  };

  api.fetch = async function (path$$1, options) {
    await setup();
    const url = path$$1.substring(0, 1) === '/' ?
    genUrl(host, path$$1.substring(1)) :
    genDBUrl(host, path$$1);
    return ourFetch(url, options);
  };

  // Get the document with the given id from the database given by host.
  // The id could be solely the _id in the database, or it may be a
  // _design/ID or _local/ID path
  api.get = adapterFun$$1('get', async function (id, opts, callback) {
    // If no options were given, set the callback to the second parameter
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
    opts = clone(opts);

    // List of parameters to add to the GET request
    let params = {};

    if (opts.revs) {
      params.revs = true;
    }

    if (opts.revs_info) {
      params.revs_info = true;
    }

    if (opts.latest) {
      params.latest = true;
    }

    if (opts.open_revs) {
      if (opts.open_revs !== "all") {
        opts.open_revs = JSON.stringify(opts.open_revs);
      }
      params.open_revs = opts.open_revs;
    }

    if (opts.rev) {
      params.rev = opts.rev;
    }

    if (opts.conflicts) {
      params.conflicts = opts.conflicts;
    }

    /* istanbul ignore if */
    if (opts.update_seq) {
      params.update_seq = opts.update_seq;
    }

    id = encodeDocId(id);

    function fetchAttachments(doc) {
      let atts = doc._attachments;
      let filenames = atts && Object.keys(atts);
      if (!atts || !filenames.length) {
        return;
      }
      // we fetch these manually in separate XHRs, because
      // Sync Gateway would normally send it back as multipart/mixed,
      // which we cannot parse. Also, this is more efficient than
      // receiving attachments as base64-encoded strings.
      async function fetchData(filename) {
        const att = atts[filename];
        const path$$1 = encodeDocId(doc._id) + '/' + encodeAttachmentId(filename) +
            '?rev=' + doc._rev;

        const response = await ourFetch(genDBUrl(host, path$$1));

        let blob;
        if ('buffer' in response) {
          blob = await response.buffer();
        } else {
          /* istanbul ignore next */
          blob = await response.blob();
        }

        let data;
        if (opts.binary) {
          let typeFieldDescriptor = Object.getOwnPropertyDescriptor(blob.__proto__, 'type');
          if (!typeFieldDescriptor || typeFieldDescriptor.set) {
            blob.type = att.content_type;
          }
          data = blob;
        } else {
          data = await new Promise(function (resolve) {
            blobToBase64(blob, resolve);
          });
        }

        delete att.stub;
        delete att.length;
        att.data = data;
      }

      let promiseFactories = filenames.map(function (filename) {
        return function () {
          return fetchData(filename);
        };
      });

      // This limits the number of parallel xhr requests to 5 any time
      // to avoid issues with maximum browser request limits
      return pool(promiseFactories, 5);
    }

    function fetchAllAttachments(docOrDocs) {
      if (Array.isArray(docOrDocs)) {
        return Promise.all(docOrDocs.map(function (doc) {
          if (doc.ok) {
            return fetchAttachments(doc.ok);
          }
        }));
      }
      return fetchAttachments(docOrDocs);
    }

    const url = genDBUrl(host, id + paramsToStr(params));
    try {
      const res$$1 = await fetchJSON(url);
      if (opts.attachments) {
        await fetchAllAttachments(res$$1.data);
      }
      callback(null, res$$1.data);
    } catch (error) {
      error.docId = id;
      callback(error);
    }
  });


  // Delete the document given by doc from the database given by host.
  api.remove = adapterFun$$1('remove', async function (docOrId, optsOrRev, opts, cb) {
    let doc;
    if (typeof optsOrRev === 'string') {
      // id, rev, opts, callback style
      doc = {
        _id: docOrId,
        _rev: optsOrRev
      };
      if (typeof opts === 'function') {
        cb = opts;
        opts = {};
      }
    } else {
      // doc, opts, callback style
      doc = docOrId;
      if (typeof optsOrRev === 'function') {
        cb = optsOrRev;
        opts = {};
      } else {
        cb = opts;
        opts = optsOrRev;
      }
    }

    const rev = (doc._rev || opts.rev);
    const url = genDBUrl(host, encodeDocId(doc._id)) + '?rev=' + rev;

    try {
      const result = await fetchJSON(url, {method: 'DELETE'});
      cb(null, result.data);
    } catch (error) {
      cb(error);
    }
  });

  function encodeAttachmentId(attachmentId) {
    return attachmentId.split("/").map(encodeURIComponent).join("/");
  }

  // Get the attachment
  api.getAttachment = adapterFun$$1('getAttachment', async function (docId, attachmentId,
                                                            opts, callback) {
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
    const params = opts.rev ? ('?rev=' + opts.rev) : '';
    const url = genDBUrl(host, encodeDocId(docId)) + '/' +
        encodeAttachmentId(attachmentId) + params;
    let contentType;
    try {
      const response = await ourFetch(url, {method: 'GET'});

      if (!response.ok) {
        throw response;
      }

      contentType = response.headers.get('content-type');
      let blob;
      if (typeof process !== 'undefined' && !process.browser && typeof response.buffer === 'function') {
        blob = await response.buffer();
      } else {
        /* istanbul ignore next */
        blob = await response.blob();
      }

      // TODO: also remove
      if (typeof process !== 'undefined' && !process.browser) {
        var typeFieldDescriptor = Object.getOwnPropertyDescriptor(blob.__proto__, 'type');
        if (!typeFieldDescriptor || typeFieldDescriptor.set) {
          blob.type = contentType;
        }
      }
      callback(null, blob);
    } catch (err) {
      callback(err);
    }
  });

  // Remove the attachment given by the id and rev
  api.removeAttachment =  adapterFun$$1('removeAttachment', async function (
    docId,
    attachmentId,
    rev,
    callback,
  ) {
    const url = genDBUrl(host, encodeDocId(docId) + '/' + encodeAttachmentId(attachmentId)) + '?rev=' + rev;

    try {
      const result = await fetchJSON(url, {method: 'DELETE'});
      callback(null, result.data);
    } catch (error) {
      callback(error);
    }
  });

  // Add the attachment given by blob and its contentType property
  // to the document with the given id, the revision given by rev, and
  // add it to the database given by host.
  api.putAttachment = adapterFun$$1('putAttachment', async function (
    docId,
    attachmentId,
    rev,
    blob,
    type,
    callback,
  ) {
    if (typeof type === 'function') {
      callback = type;
      type = blob;
      blob = rev;
      rev = null;
    }
    const id = encodeDocId(docId) + '/' + encodeAttachmentId(attachmentId);
    let url = genDBUrl(host, id);
    if (rev) {
      url += '?rev=' + rev;
    }

    if (typeof blob === 'string') {
      // input is assumed to be a base64 string
      let binary;
      try {
        binary = thisAtob(blob);
      } catch (err) {
        return callback(createError(BAD_ARG,
                        'Attachment is not a valid base64 string'));
      }
      blob = binary ? binStringToBluffer(binary, type) : '';
    }

    try {
      // Add the attachment
      const result = await fetchJSON(url, {
        headers: new nodeFetch.Headers({'Content-Type': type}),
        method: 'PUT',
        body: blob
      });
      callback(null, result.data);
    } catch (error) {
      callback(error);
    }
  });

  // Update/create multiple documents given by req in the database
  // given by host.
  api._bulkDocs = async function (req, opts, callback) {
    // If new_edits=false then it prevents the database from creating
    // new revision numbers for the documents. Instead it just uses
    // the old ones. This is used in database replication.
    req.new_edits = opts.new_edits;

    try {
      await setup();
      await Promise.all(req.docs.map(preprocessAttachments$1));

      // Update/create the documents
      const result = await fetchJSON(genDBUrl(host, '_bulk_docs'), {
        method: 'POST',
        body: JSON.stringify(req)
      });
      callback(null, result.data);
    } catch (error) {
      callback(error);
    }
  };

  // Update/create document
  api._put = async function (doc, opts, callback) {
    try {
      await setup();
      await preprocessAttachments$1(doc);

      const result = await fetchJSON(genDBUrl(host, encodeDocId(doc._id)), {
        method: 'PUT',
        body: JSON.stringify(doc)
      });
      callback(null, result.data);
    } catch (error) {
      error.docId = doc && doc._id;
      callback(error);
    }
  };


  // Get a listing of the documents in the database given
  // by host and ordered by increasing id.
  api.allDocs = adapterFun$$1('allDocs', async function (opts, callback) {
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
    opts = clone(opts);

    // List of parameters to add to the GET request
    let params = {};
    let body;
    let method = 'GET';

    if (opts.conflicts) {
      params.conflicts = true;
    }

    /* istanbul ignore if */
    if (opts.update_seq) {
      params.update_seq = true;
    }

    if (opts.descending) {
      params.descending = true;
    }

    if (opts.include_docs) {
      params.include_docs = true;
    }

    // added in CouchDB 1.6.0
    if (opts.attachments) {
      params.attachments = true;
    }

    if (opts.key) {
      params.key = JSON.stringify(opts.key);
    }

    if (opts.start_key) {
      opts.startkey = opts.start_key;
    }

    if (opts.startkey) {
      params.startkey = JSON.stringify(opts.startkey);
    }

    if (opts.end_key) {
      opts.endkey = opts.end_key;
    }

    if (opts.endkey) {
      params.endkey = JSON.stringify(opts.endkey);
    }

    if (typeof opts.inclusive_end !== 'undefined') {
      params.inclusive_end = !!opts.inclusive_end;
    }

    if (typeof opts.limit !== 'undefined') {
      params.limit = opts.limit;
    }

    if (typeof opts.skip !== 'undefined') {
      params.skip = opts.skip;
    }

    let paramStr = paramsToStr(params);

    if (typeof opts.keys !== 'undefined') {
      method = 'POST';
      body = {keys: opts.keys};
    }

    try {
      const result = await fetchJSON(genDBUrl(host, '_all_docs' + paramStr), {
        method: method,
        body: JSON.stringify(body)
      });
      if (opts.include_docs && opts.attachments && opts.binary) {
        result.data.rows.forEach(readAttachmentsAsBlobOrBuffer);
      }
      callback(null, result.data);
    } catch (error) {
      callback(error);
    }
  });

  // Get a list of changes made to documents in the database given by host.
  // TODO According to the README, there should be two other methods here,
  // api.changes.addListener and api.changes.removeListener.
  api._changes = function (opts) {

    // We internally page the results of a changes request, this means
    // if there is a large set of changes to be returned we can start
    // processing them quicker instead of waiting on the entire
    // set of changes to return and attempting to process them at once
    let batchSize = 'batch_size' in opts ? opts.batch_size : CHANGES_BATCH_SIZE;

    opts = clone(opts);

    if (opts.continuous && !('heartbeat' in opts)) {
      opts.heartbeat = DEFAULT_HEARTBEAT;
    }

    let requestTimeout = ('timeout' in opts) ? opts.timeout : 30 * 1000;

    // ensure CHANGES_TIMEOUT_BUFFER applies
    if ('timeout' in opts && opts.timeout &&
      (requestTimeout - opts.timeout) < CHANGES_TIMEOUT_BUFFER) {
        requestTimeout = opts.timeout + CHANGES_TIMEOUT_BUFFER;
    }

    /* istanbul ignore if */
    if ('heartbeat' in opts && opts.heartbeat &&
       (requestTimeout - opts.heartbeat) < CHANGES_TIMEOUT_BUFFER) {
        requestTimeout = opts.heartbeat + CHANGES_TIMEOUT_BUFFER;
    }

    let params = {};
    if ('timeout' in opts && opts.timeout) {
      params.timeout = opts.timeout;
    }

    let limit = (typeof opts.limit !== 'undefined') ? opts.limit : false;
    let leftToFetch = limit;

    if (opts.style) {
      params.style = opts.style;
    }

    if (opts.include_docs || opts.filter && typeof opts.filter === 'function') {
      params.include_docs = true;
    }

    if (opts.attachments) {
      params.attachments = true;
    }

    if (opts.continuous) {
      params.feed = 'longpoll';
    }

    if (opts.seq_interval) {
      params.seq_interval = opts.seq_interval;
    }

    if (opts.conflicts) {
      params.conflicts = true;
    }

    if (opts.descending) {
      params.descending = true;
    }

    /* istanbul ignore if */
    if (opts.update_seq) {
      params.update_seq = true;
    }

    if ('heartbeat' in opts) {
      // If the heartbeat value is false, it disables the default heartbeat
      if (opts.heartbeat) {
        params.heartbeat = opts.heartbeat;
      }
    }

    if (opts.filter && typeof opts.filter === 'string') {
      params.filter = opts.filter;
    }

    if (opts.view && typeof opts.view === 'string') {
      params.filter = '_view';
      params.view = opts.view;
    }

    // If opts.query_params exists, pass it through to the changes request.
    // These parameters may be used by the filter on the source database.
    if (opts.query_params && typeof opts.query_params === 'object') {
      for (let param_name in opts.query_params) {
        /* istanbul ignore else */
        if (Object.prototype.hasOwnProperty.call(opts.query_params, param_name)) {
          params[param_name] = opts.query_params[param_name];
        }
      }
    }

    let method = 'GET';
    let body;

    if (opts.doc_ids) {
      // set this automagically for the user; it's annoying that couchdb
      // requires both a "filter" and a "doc_ids" param.
      params.filter = '_doc_ids';
      method = 'POST';
      body = {doc_ids: opts.doc_ids };
    }
    /* istanbul ignore next */
    else if (opts.selector) {
      // set this automagically for the user, similar to above
      params.filter = '_selector';
      method = 'POST';
      body = {selector: opts.selector };
    }

    let controller = new AbortController();
    let lastFetchedSeq;

    // Get all the changes starting wtih the one immediately after the
    // sequence number given by since.
    const fetchData = async function (since, callback) {
      if (opts.aborted) {
        return;
      }
      params.since = since;
      // "since" can be any kind of json object in Cloudant/CouchDB 2.x
      /* istanbul ignore next */
      if (typeof params.since === "object") {
        params.since = JSON.stringify(params.since);
      }

      if (opts.descending) {
        if (limit) {
          params.limit = leftToFetch;
        }
      } else {
        params.limit = (!limit || leftToFetch > batchSize) ?
          batchSize : leftToFetch;
      }

      // Set the options for the ajax call
      let url = genDBUrl(host, '_changes' + paramsToStr(params));
      let fetchOpts = {
        signal: controller.signal,
        method: method,
        body: JSON.stringify(body)
      };
      lastFetchedSeq = since;

      /* istanbul ignore if */
      if (opts.aborted) {
        return;
      }

      // Get the changes
      try {
        await setup();
        const result = await fetchJSON(url, fetchOpts);
        callback(null, result.data);
      } catch (error) {
        callback(error);
      }
    };

    // If opts.since exists, get all the changes from the sequence
    // number given by opts.since. Otherwise, get all the changes
    // from the sequence number 0.
    let results = {results: []};

    const fetched = function (err, res$$1) {
      if (opts.aborted) {
        return;
      }
      let raw_results_length = 0;
      // If the result of the ajax call (res) contains changes (res.results)
      if (res$$1 && res$$1.results) {
        raw_results_length = res$$1.results.length;
        results.last_seq = res$$1.last_seq;
        let pending = null;
        let lastSeq = null;
        // Attach 'pending' property if server supports it (CouchDB 2.0+)
        /* istanbul ignore if */
        if (typeof res$$1.pending === 'number') {
          pending = res$$1.pending;
        }
        if (typeof results.last_seq === 'string' || typeof results.last_seq === 'number') {
          lastSeq = results.last_seq;
        }
        // For each change
        let req = {};
        req.query = opts.query_params;
        res$$1.results = res$$1.results.filter(function (c) {
          leftToFetch--;
          let ret = filterChange(opts)(c);
          if (ret) {
            if (opts.include_docs && opts.attachments && opts.binary) {
              readAttachmentsAsBlobOrBuffer(c);
            }
            if (opts.return_docs) {
              results.results.push(c);
            }
            opts.onChange(c, pending, lastSeq);
          }
          return ret;
        });
      } else if (err) {
        // In case of an error, stop listening for changes and call
        // opts.complete
        opts.aborted = true;
        opts.complete(err);
        return;
      }

      // The changes feed may have timed out with no results
      // if so reuse last update sequence
      if (res$$1 && res$$1.last_seq) {
        lastFetchedSeq = res$$1.last_seq;
      }

      let finished = (limit && leftToFetch <= 0) ||
        (res$$1 && raw_results_length < batchSize) ||
        (opts.descending);

      if ((opts.continuous && !(limit && leftToFetch <= 0)) || !finished) {
        // Queue a call to fetch again with the newest sequence number
        nextTick(function () { fetchData(lastFetchedSeq, fetched); });
      } else {
        // We're done, call the callback
        opts.complete(null, results);
      }
    };

    fetchData(opts.since || 0, fetched);

    // Return a method to cancel this method from processing any more
    return {
      cancel: function () {
        opts.aborted = true;
        controller.abort();
      }
    };
  };

  // Given a set of document/revision IDs (given by req), tets the subset of
  // those that do NOT correspond to revisions stored in the database.
  // See http://wiki.apache.org/couchdb/HttpPostRevsDiff
  api.revsDiff = adapterFun$$1('revsDiff', async function (req, opts, callback) {
    // If no options were given, set the callback to be the second parameter
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }

    try {
      // Get the missing document/revision IDs
      const result = await fetchJSON(genDBUrl(host, '_revs_diff'), {
        method: 'POST',
        body: JSON.stringify(req)
      });
      callback(null, result.data);
    } catch (error) {
      callback(error);
    }
  });

  api._close = function (callback) {
    callback();
  };

  api._destroy = async function (options, callback) {
    try {
      const json = await fetchJSON(genDBUrl(host, ''), {method: 'DELETE'});
      callback(null, json);
    } catch (error) {
      if (error.status === 404) {
        callback(null, {ok: true});
      } else {
        callback(error);
      }
    }
  };
}

// HttpPouch is a valid adapter.
HttpPouch.valid = function () {
  return true;
};

function HttpPouch$1 (PouchDB) {
  PouchDB.adapter('http', HttpPouch, false);
  PouchDB.adapter('https', HttpPouch, false);
}

class QueryParseError extends Error {
  constructor(message) {
    super();
    this.status = 400;
    this.name = 'query_parse_error';
    this.message = message;
    this.error = true;
    try {
      Error.captureStackTrace(this, QueryParseError);
    } catch (e) {}
  }
}

class NotFoundError$1 extends Error {
  constructor(message) {
    super();
    this.status = 404;
    this.name = 'not_found';
    this.message = message;
    this.error = true;
    try {
      Error.captureStackTrace(this, NotFoundError$1);
    } catch (e) {}
  }
}

class BuiltInError extends Error {
  constructor(message) {
    super();
    this.status = 500;
    this.name = 'invalid_value';
    this.message = message;
    this.error = true;
    try {
      Error.captureStackTrace(this, BuiltInError);
    } catch (e) {}
  }
}

function promisedCallback(promise, callback) {
  if (callback) {
    promise.then(function (res$$1) {
      nextTick(function () {
        callback(null, res$$1);
      });
    }, function (reason) {
      nextTick(function () {
        callback(reason);
      });
    });
  }
  return promise;
}

function callbackify(fun) {
  return function (...args) {
    var cb = args.pop();
    var promise = fun.apply(this, args);
    if (typeof cb === 'function') {
      promisedCallback(promise, cb);
    }
    return promise;
  };
}

// Promise finally util similar to Q.finally
function fin(promise, finalPromiseFactory) {
  return promise.then(function (res$$1) {
    return finalPromiseFactory().then(function () {
      return res$$1;
    });
  }, function (reason) {
    return finalPromiseFactory().then(function () {
      throw reason;
    });
  });
}

function sequentialize(queue, promiseFactory) {
  return function () {
    var args = arguments;
    var that = this;
    return queue.add(function () {
      return promiseFactory.apply(that, args);
    });
  };
}

// uniq an array of strings, order not guaranteed
// similar to underscore/lodash _.uniq
function uniq(arr) {
  var theSet = new ExportedSet(arr);
  var result = new Array(theSet.size);
  var index = -1;
  theSet.forEach(function (value) {
    result[++index] = value;
  });
  return result;
}

function mapToKeysArray(map) {
  var result = new Array(map.size);
  var index = -1;
  map.forEach(function (value, key) {
    result[++index] = key;
  });
  return result;
}

function createBuiltInError(name) {
  var message = 'builtin ' + name +
    ' function requires map values to be numbers' +
    ' or number arrays';
  return new BuiltInError(message);
}

function sum(values) {
  var result = 0;
  for (var i = 0, len = values.length; i < len; i++) {
    var num = values[i];
    if (typeof num !== 'number') {
      if (Array.isArray(num)) {
        // lists of numbers are also allowed, sum them separately
        result = typeof result === 'number' ? [result] : result;
        for (var j = 0, jLen = num.length; j < jLen; j++) {
          var jNum = num[j];
          if (typeof jNum !== 'number') {
            throw createBuiltInError('_sum');
          } else if (typeof result[j] === 'undefined') {
            result.push(jNum);
          } else {
            result[j] += jNum;
          }
        }
      } else { // not array/number
        throw createBuiltInError('_sum');
      }
    } else if (typeof result === 'number') {
      result += num;
    } else { // add number to array
      result[0] += num;
    }
  }
  return result;
}

// Inside of 'vm' for Node, we need a way to translate a pseudo-error
// back into a real error once it's out of the VM.
function createBuiltInErrorInVm(name) {
  return {
    builtInError: true,
    name: name
  };
}

function convertToTrueError(err) {
  return createBuiltInError(err.name);
}

function isBuiltInError(obj) {
  return obj && obj.builtInError;
}

// All of this vm hullaballoo is to be able to run arbitrary code in a sandbox
// for security reasons.
function evalFunctionInVm(func, emit) {
  return function (arg1, arg2, arg3) {
    var code = '(function() {"use strict";' +
      'var createBuiltInError = ' + createBuiltInErrorInVm.toString() + ';' +
      'var sum = ' + sum.toString() + ';' +
      'var log = function () {};' +
      'var isArray = Array.isArray;' +
      'var toJSON = JSON.parse;' +
      'var __emitteds__ = [];' +
      'var emit = function (key, value) {__emitteds__.push([key, value]);};' +
      'var __result__ = (' +
      func.replace(/;\s*$/, '') + ')' + '(' +
      JSON.stringify(arg1) + ',' +
      JSON.stringify(arg2) + ',' +
      JSON.stringify(arg3) + ');' +
      'return {result: __result__, emitteds: __emitteds__};' +
      '})()';

    var output = vm.runInNewContext(code);

    output.emitteds.forEach(function (emitted) {
      emit(emitted[0], emitted[1]);
    });
    if (isBuiltInError(output.result)) {
      output.result = convertToTrueError(output.result);
    }
    return output.result;
  };
}

var log = guardedConsole.bind(null, 'log');
var toJSON = JSON.parse;

// The "stringify, then execute in a VM" strategy totally breaks Istanbul due
// to missing __coverage global objects. As a solution, export different
// code during coverage testing and during regular execution.
// Note that this doesn't get shipped to consumers because Rollup replaces it
// with rollup-plugin-replace, so false is replaced with `false`
var evalFunc;
/* istanbul ignore else */
{
  evalFunc = evalFunctionInVm;
}

var evalFunction = evalFunc;

/*
 * Simple task queue to sequentialize actions. Assumes
 * callbacks will eventually fire (once).
 */


class TaskQueue$1 {
  constructor() {
    this.promise = new Promise(function (fulfill) {fulfill(); });
  }

  add(promiseFactory) {
    this.promise = this.promise.catch(function () {
      // just recover
    }).then(function () {
      return promiseFactory();
    });
    return this.promise;
  }

  finish() {
    return this.promise;
  }
}

function stringify(input) {
  if (!input) {
    return 'undefined'; // backwards compat for empty reduce
  }
  // for backwards compat with mapreduce, functions/strings are stringified
  // as-is. everything else is JSON-stringified.
  switch (typeof input) {
    case 'function':
      // e.g. a mapreduce map
      return input.toString();
    case 'string':
      // e.g. a mapreduce built-in _reduce function
      return input.toString();
    default:
      // e.g. a JSON object in the case of mango queries
      return JSON.stringify(input);
  }
}

/* create a string signature for a view so we can cache it and uniq it */
function createViewSignature(mapFun, reduceFun) {
  // the "undefined" part is for backwards compatibility
  return stringify(mapFun) + stringify(reduceFun) + 'undefined';
}

async function createView(sourceDB, viewName, mapFun, reduceFun, temporary, localDocName) {
  const viewSignature = createViewSignature(mapFun, reduceFun);

  let cachedViews;
  if (!temporary) {
    // cache this to ensure we don't try to update the same view twice
    cachedViews = sourceDB._cachedViews = sourceDB._cachedViews || {};
    if (cachedViews[viewSignature]) {
      return cachedViews[viewSignature];
    }
  }

  const promiseForView = sourceDB.info().then(async function (info) {
    const depDbName = info.db_name + '-mrview-' +
    (temporary ? 'temp' : stringMd5(viewSignature));

    // save the view name in the source db so it can be cleaned up if necessary
    // (e.g. when the _design doc is deleted, remove all associated view data)
    function diffFunction(doc) {
      doc.views = doc.views || {};
      let fullViewName = viewName;
      if (fullViewName.indexOf('/') === -1) {
        fullViewName = viewName + '/' + viewName;
      }
      const depDbs = doc.views[fullViewName] = doc.views[fullViewName] || {};
      /* istanbul ignore if */
      if (depDbs[depDbName]) {
        return; // no update necessary
      }
      depDbs[depDbName] = true;
      return doc;
    }
    await upsert(sourceDB, '_local/' + localDocName, diffFunction);
    const res$$1 = await sourceDB.registerDependentDatabase(depDbName);
    const db = res$$1.db;
    db.auto_compaction = true;
    const view = {
      name: depDbName,
      db: db,
      sourceDB: sourceDB,
      adapter: sourceDB.adapter,
      mapFun: mapFun,
      reduceFun: reduceFun
    };

    let lastSeqDoc;
    try {
      lastSeqDoc = await view.db.get('_local/lastSeq');
    } catch (err) {
        /* istanbul ignore if */
      if (err.status !== 404) {
        throw err;
      }
    }

    view.seq = lastSeqDoc ? lastSeqDoc.seq : 0;
    if (cachedViews) {
      view.db.once('destroyed', function () {
        delete cachedViews[viewSignature];
      });
    }
    return view;
  });

  if (cachedViews) {
    cachedViews[viewSignature] = promiseForView;
  }
  return promiseForView;
}

var persistentQueues = {};
var tempViewQueue = new TaskQueue$1();
var CHANGES_BATCH_SIZE$1 = 50;

function parseViewName(name) {
  // can be either 'ddocname/viewname' or just 'viewname'
  // (where the ddoc name is the same)
  return name.indexOf('/') === -1 ? [name, name] : name.split('/');
}

function isGenOne(changes) {
  // only return true if the current change is 1-
  // and there are no other leafs
  return changes.length === 1 && /^1-/.test(changes[0].rev);
}

function emitError(db, e, data) {
  try {
    db.emit('error', e);
  } catch (err) {
    guardedConsole('error',
      'The user\'s map/reduce function threw an uncaught error.\n' +
      'You can debug this error by doing:\n' +
      'myDatabase.on(\'error\', function (err) { debugger; });\n' +
      'Please double-check your map/reduce function.');
    guardedConsole('error', e, data);
  }
}

/**
 * Returns an "abstract" mapreduce object of the form:
 *
 *   {
 *     query: queryFun,
 *     viewCleanup: viewCleanupFun
 *   }
 *
 * Arguments are:
 *
 * localDoc: string
 *   This is for the local doc that gets saved in order to track the
 *   "dependent" DBs and clean them up for viewCleanup. It should be
 *   unique, so that indexer plugins don't collide with each other.
 * mapper: function (mapFunDef, emit)
 *   Returns a map function based on the mapFunDef, which in the case of
 *   normal map/reduce is just the de-stringified function, but may be
 *   something else, such as an object in the case of pouchdb-find.
 * reducer: function (reduceFunDef)
 *   Ditto, but for reducing. Modules don't have to support reducing
 *   (e.g. pouchdb-find).
 * ddocValidator: function (ddoc, viewName)
 *   Throws an error if the ddoc or viewName is not valid.
 *   This could be a way to communicate to the user that the configuration for the
 *   indexer is invalid.
 */
function createAbstractMapReduce(localDocName, mapper, reducer, ddocValidator) {

  function tryMap(db, fun, doc) {
    // emit an event if there was an error thrown by a map function.
    // putting try/catches in a single function also avoids deoptimizations.
    try {
      fun(doc);
    } catch (e) {
      emitError(db, e, {fun: fun, doc: doc});
    }
  }

  function tryReduce(db, fun, keys, values, rereduce) {
    // same as above, but returning the result or an error. there are two separate
    // functions to avoid extra memory allocations since the tryCode() case is used
    // for custom map functions (common) vs this function, which is only used for
    // custom reduce functions (rare)
    try {
      return {output : fun(keys, values, rereduce)};
    } catch (e) {
      emitError(db, e, {fun: fun, keys: keys, values: values, rereduce: rereduce});
      return {error: e};
    }
  }

  function sortByKeyThenValue(x, y) {
    const keyCompare = collate(x.key, y.key);
    return keyCompare !== 0 ? keyCompare : collate(x.value, y.value);
  }

  function sliceResults(results, limit, skip) {
    skip = skip || 0;
    if (typeof limit === 'number') {
      return results.slice(skip, limit + skip);
    } else if (skip > 0) {
      return results.slice(skip);
    }
    return results;
  }

  function rowToDocId(row) {
    const val = row.value;
    // Users can explicitly specify a joined doc _id, or it
    // defaults to the doc _id that emitted the key/value.
    const docId = (val && typeof val === 'object' && val._id) || row.id;
    return docId;
  }

  function readAttachmentsAsBlobOrBuffer(res$$1) {
    res$$1.rows.forEach(function (row) {
      const atts = row.doc && row.doc._attachments;
      if (!atts) {
        return;
      }
      Object.keys(atts).forEach(function (filename) {
        const att = atts[filename];
        atts[filename].data = b64ToBluffer(att.data, att.content_type);
      });
    });
  }

  function postprocessAttachments(opts) {
    return function (res$$1) {
      if (opts.include_docs && opts.attachments && opts.binary) {
        readAttachmentsAsBlobOrBuffer(res$$1);
      }
      return res$$1;
    };
  }

  function addHttpParam(paramName, opts, params, asJson) {
    // add an http param from opts to params, optionally json-encoded
    let val = opts[paramName];
    if (typeof val !== 'undefined') {
      if (asJson) {
        val = encodeURIComponent(JSON.stringify(val));
      }
      params.push(paramName + '=' + val);
    }
  }

  function coerceInteger(integerCandidate) {
    if (typeof integerCandidate !== 'undefined') {
      const asNumber = Number(integerCandidate);
      // prevents e.g. '1foo' or '1.1' being coerced to 1
      if (!isNaN(asNumber) && asNumber === parseInt(integerCandidate, 10)) {
        return asNumber;
      } else {
        return integerCandidate;
      }
    }
  }

  function coerceOptions(opts) {
    opts.group_level = coerceInteger(opts.group_level);
    opts.limit = coerceInteger(opts.limit);
    opts.skip = coerceInteger(opts.skip);
    return opts;
  }

  function checkPositiveInteger(number) {
    if (number) {
      if (typeof number !== 'number') {
        return  new QueryParseError(`Invalid value for integer: "${number}"`);
      }
      if (number < 0) {
        return new QueryParseError(`Invalid value for positive integer: "${number}"`);
      }
    }
  }

  function checkQueryParseError(options, fun) {
    const startkeyName = options.descending ? 'endkey' : 'startkey';
    const endkeyName = options.descending ? 'startkey' : 'endkey';

    if (typeof options[startkeyName] !== 'undefined' &&
      typeof options[endkeyName] !== 'undefined' &&
      collate(options[startkeyName], options[endkeyName]) > 0) {
      throw new QueryParseError('No rows can match your key range, ' +
        'reverse your start_key and end_key or set {descending : true}');
    } else if (fun.reduce && options.reduce !== false) {
      if (options.include_docs) {
        throw new QueryParseError('{include_docs:true} is invalid for reduce');
      } else if (options.keys && options.keys.length > 1 &&
        !options.group && !options.group_level) {
        throw new QueryParseError('Multi-key fetches for reduce views must use ' +
          '{group: true}');
      }
    }
    ['group_level', 'limit', 'skip'].forEach(function (optionName) {
      const error = checkPositiveInteger(options[optionName]);
      if (error) {
        throw error;
      }
    });
  }

  async function httpQuery(db, fun, opts) {
    // List of parameters to add to the PUT request
    let params = [];
    let body;
    let method = 'GET';
    let ok;

    // If opts.reduce exists and is defined, then add it to the list
    // of parameters.
    // If reduce=false then the results are that of only the map function
    // not the final result of map and reduce.
    addHttpParam('reduce', opts, params);
    addHttpParam('include_docs', opts, params);
    addHttpParam('attachments', opts, params);
    addHttpParam('limit', opts, params);
    addHttpParam('descending', opts, params);
    addHttpParam('group', opts, params);
    addHttpParam('group_level', opts, params);
    addHttpParam('skip', opts, params);
    addHttpParam('stale', opts, params);
    addHttpParam('conflicts', opts, params);
    addHttpParam('startkey', opts, params, true);
    addHttpParam('start_key', opts, params, true);
    addHttpParam('endkey', opts, params, true);
    addHttpParam('end_key', opts, params, true);
    addHttpParam('inclusive_end', opts, params);
    addHttpParam('key', opts, params, true);
    addHttpParam('update_seq', opts, params);

    // Format the list of parameters into a valid URI query string
    params = params.join('&');
    params = params === '' ? '' : '?' + params;

    // If keys are supplied, issue a POST to circumvent GET query string limits
    // see http://wiki.apache.org/couchdb/HTTP_view_API#Querying_Options
    if (typeof opts.keys !== 'undefined') {
      const MAX_URL_LENGTH = 2000;
      // according to http://stackoverflow.com/a/417184/680742,
      // the de facto URL length limit is 2000 characters

      const keysAsString = `keys=${encodeURIComponent(JSON.stringify(opts.keys))}`;
      if (keysAsString.length + params.length + 1 <= MAX_URL_LENGTH) {
        // If the keys are short enough, do a GET. we do this to work around
        // Safari not understanding 304s on POSTs (see pouchdb/pouchdb#1239)
        params += (params[0] === '?' ? '&' : '?') + keysAsString;
      } else {
        method = 'POST';
        if (typeof fun === 'string') {
          body = {keys: opts.keys};
        } else { // fun is {map : mapfun}, so append to this
          fun.keys = opts.keys;
        }
      }
    }

    // We are referencing a query defined in the design doc
    if (typeof fun === 'string') {
      const parts = parseViewName(fun);

      const response = await db.fetch('_design/' + parts[0] + '/_view/' + parts[1] + params, {
        headers: new nodeFetch.Headers({'Content-Type': 'application/json'}),
        method: method,
        body: JSON.stringify(body)
      });
      ok = response.ok;
      // status = response.status;
      const result = await response.json();

      if (!ok) {
        result.status = response.status;
        throw generateErrorFromResponse(result);
      }

      // fail the entire request if the result contains an error
      result.rows.forEach(function (row) {
        /* istanbul ignore if */
        if (row.value && row.value.error && row.value.error === "builtin_reduce_error") {
          throw new Error(row.reason);
        }
      });

      return new Promise(function (resolve) {
        resolve(result);
      }).then(postprocessAttachments(opts));
    }

    // We are using a temporary view, terrible for performance, good for testing
    body = body || {};
    Object.keys(fun).forEach(function (key) {
      if (Array.isArray(fun[key])) {
        body[key] = fun[key];
      } else {
        body[key] = fun[key].toString();
      }
    });

    const response = await db.fetch('_temp_view' + params, {
      headers: new nodeFetch.Headers({'Content-Type': 'application/json'}),
      method: 'POST',
      body: JSON.stringify(body)
    });

    ok = response.ok;
    // status = response.status;
    const result = await response.json();
    if (!ok) {
      result.status = response.status;
      throw generateErrorFromResponse(result);
    }

    return new Promise(function (resolve) {
      resolve(result);
    }).then(postprocessAttachments(opts));
  }

  // custom adapters can define their own api._query
  // and override the default behavior
  /* istanbul ignore next */
  function customQuery(db, fun, opts) {
    return new Promise(function (resolve, reject) {
      db._query(fun, opts, function (err, res$$1) {
        if (err) {
          return reject(err);
        }
        resolve(res$$1);
      });
    });
  }

  // custom adapters can define their own api._viewCleanup
  // and override the default behavior
  /* istanbul ignore next */
  function customViewCleanup(db) {
    return new Promise(function (resolve, reject) {
      db._viewCleanup(function (err, res$$1) {
        if (err) {
          return reject(err);
        }
        resolve(res$$1);
      });
    });
  }

  function defaultsTo(value) {
    return function (reason) {
      /* istanbul ignore else */
      if (reason.status === 404) {
        return value;
      } else {
        throw reason;
      }
    };
  }

  // returns a promise for a list of docs to update, based on the input docId.
  // the order doesn't matter, because post-3.2.0, bulkDocs
  // is an atomic operation in all three adapters.
  async function getDocsToPersist(docId, view, docIdsToChangesAndEmits) {
    const metaDocId = '_local/doc_' + docId;
    const defaultMetaDoc = {_id: metaDocId, keys: []};
    const docData = docIdsToChangesAndEmits.get(docId);
    const indexableKeysToKeyValues = docData[0];
    const changes = docData[1];

    function getMetaDoc() {
      if (isGenOne(changes)) {
        // generation 1, so we can safely assume initial state
        // for performance reasons (avoids unnecessary GETs)
        return Promise.resolve(defaultMetaDoc);
      }
      return view.db.get(metaDocId).catch(defaultsTo(defaultMetaDoc));
    }

    function getKeyValueDocs(metaDoc) {
      if (!metaDoc.keys.length) {
        // no keys, no need for a lookup
        return Promise.resolve({rows: []});
      }
      return view.db.allDocs({
        keys: metaDoc.keys,
        include_docs: true
      });
    }

    function processKeyValueDocs(metaDoc, kvDocsRes) {
      const kvDocs = [];
      const oldKeys = new ExportedSet();

      for (let i = 0, len = kvDocsRes.rows.length; i < len; i++) {
        const row = kvDocsRes.rows[i];
        const doc = row.doc;
        if (!doc) { // deleted
          continue;
        }
        kvDocs.push(doc);
        oldKeys.add(doc._id);
        doc._deleted = !indexableKeysToKeyValues.has(doc._id);
        if (!doc._deleted) {
          const keyValue = indexableKeysToKeyValues.get(doc._id);
          if ('value' in keyValue) {
            doc.value = keyValue.value;
          }
        }
      }
      const newKeys = mapToKeysArray(indexableKeysToKeyValues);
      newKeys.forEach(function (key) {
        if (!oldKeys.has(key)) {
          // new doc
          const kvDoc = {
            _id: key
          };
          const keyValue = indexableKeysToKeyValues.get(key);
          if ('value' in keyValue) {
            kvDoc.value = keyValue.value;
          }
          kvDocs.push(kvDoc);
        }
      });
      metaDoc.keys = uniq(newKeys.concat(metaDoc.keys));
      kvDocs.push(metaDoc);

      return kvDocs;
    }

    const metaDoc = await getMetaDoc();
    const keyValueDocs = await getKeyValueDocs(metaDoc);
    return processKeyValueDocs(metaDoc, keyValueDocs);
  }

  function updatePurgeSeq(view) {
    // with this approach, we just assume to have processed all missing purges and write the latest
    // purgeSeq into the _local/purgeSeq doc.
    return view.sourceDB.get('_local/purges').then(function (res$$1) {
      const purgeSeq = res$$1.purgeSeq;
      return view.db.get('_local/purgeSeq').then(function (res$$1) {
        return res$$1._rev;
      }).catch(function (err) {
        if (err.status !== 404) {
          throw err;
        }
        return undefined;
      }).then(function (rev) {
        return view.db.put({
          _id: '_local/purgeSeq',
          _rev: rev,
          purgeSeq,
        });
      });
    }).catch(function (err) {
      if (err.status !== 404) {
        throw err;
      }
    });
  }

  // updates all emitted key/value docs and metaDocs in the mrview database
  // for the given batch of documents from the source database
  function saveKeyValues(view, docIdsToChangesAndEmits, seq) {
    var seqDocId = '_local/lastSeq';
    return view.db.get(seqDocId)
      .catch(defaultsTo({_id: seqDocId, seq: 0}))
      .then(function (lastSeqDoc) {
        var docIds = mapToKeysArray(docIdsToChangesAndEmits);
        return Promise.all(docIds.map(function (docId) {
          return getDocsToPersist(docId, view, docIdsToChangesAndEmits);
        })).then(function (listOfDocsToPersist) {
          var docsToPersist = flatten(listOfDocsToPersist);
          lastSeqDoc.seq = seq;
          docsToPersist.push(lastSeqDoc);
          // write all docs in a single operation, update the seq once
          return view.db.bulkDocs({docs : docsToPersist});
        })
          // TODO: this should be placed somewhere else, probably? we're querying both docs twice
          //   (first time when getting the actual purges).
          .then(() => updatePurgeSeq(view));
      });
  }

  function getQueue(view) {
    const viewName = typeof view === 'string' ? view : view.name;
    let queue = persistentQueues[viewName];
    if (!queue) {
      queue = persistentQueues[viewName] = new TaskQueue$1();
    }
    return queue;
  }

  async function updateView(view, opts) {
    return sequentialize(getQueue(view), function () {
      return updateViewInQueue(view, opts);
    })();
  }

  async function updateViewInQueue(view, opts) {
    // bind the emit function once
    let mapResults;
    let doc;
    let taskId;

    function emit(key, value) {
      const output = {id: doc._id, key: normalizeKey(key)};
      // Don't explicitly store the value unless it's defined and non-null.
      // This saves on storage space, because often people don't use it.
      if (typeof value !== 'undefined' && value !== null) {
        output.value = normalizeKey(value);
      }
      mapResults.push(output);
    }

    const mapFun = mapper(view.mapFun, emit);

    let currentSeq = view.seq || 0;

    function createTask() {
      return view.sourceDB.info().then(function (info) {
        taskId = view.sourceDB.activeTasks.add({
          name: 'view_indexing',
          total_items: info.update_seq - currentSeq,
        });
      });
    }

    function processChange(docIdsToChangesAndEmits, seq) {
      return function () {
        return saveKeyValues(view, docIdsToChangesAndEmits, seq);
      };
    }

    let indexed_docs = 0;
    const progress = {
      view: view.name,
      indexed_docs: indexed_docs
    };
    view.sourceDB.emit('indexing', progress);

    const queue = new TaskQueue$1();

    async function processNextBatch() {
      const response = await view.sourceDB.changes({
        return_docs: true,
        conflicts: true,
        include_docs: true,
        style: 'all_docs',
        since: currentSeq,
        limit: opts.changes_batch_size
      });
      const purges = await getRecentPurges();
      return processBatch(response, purges);
    }

    function getRecentPurges() {
      return view.db.get('_local/purgeSeq').then(function (res$$1) {
        return res$$1.purgeSeq;
      }).catch(function (err) {
        if (err && err.status !== 404) {
          throw err;
        }
        return -1;
      }).then(function (purgeSeq) {
        return view.sourceDB.get('_local/purges').then(function (res$$1) {
          const recentPurges = res$$1.purges.filter(function (purge, index) {
            return index > purgeSeq;
          }).map((purge) => purge.docId);

          const uniquePurges = recentPurges.filter(function (docId, index) {
            return recentPurges.indexOf(docId) === index;
          });

          return Promise.all(uniquePurges.map(function (docId) {
            return view.sourceDB.get(docId).then(function (doc) {
              return { docId, doc };
            }).catch(function (err) {
              if (err.status !== 404) {
                throw err;
              }
              return { docId };
            });
          }));
        }).catch(function (err) {
          if (err && err.status !== 404) {
            throw err;
          }
          return [];
        });
      });
    }

    function processBatch(response, purges) {
      var results = response.results;
      if (!results.length && !purges.length) {
        return;
      }

      for (let purge of purges) {
        const index = results.findIndex(function (change) {
          return change.id === purge.docId;
        });
        if (index < 0) {
          // mimic a db.remove() on the changes feed
          const entry = {
            _id: purge.docId,
            doc: {
              _id: purge.docId,
              _deleted: 1,
            },
            changes: [],
          };

          if (purge.doc) {
            // update with new winning rev after purge
            entry.doc = purge.doc;
            entry.changes.push({ rev: purge.doc._rev });
          }

          results.push(entry);
        }
      }

      var docIdsToChangesAndEmits = createDocIdsToChangesAndEmits(results);

      queue.add(processChange(docIdsToChangesAndEmits, currentSeq));

      indexed_docs = indexed_docs + results.length;
      const progress = {
        view: view.name,
        last_seq: response.last_seq,
        results_count: results.length,
        indexed_docs: indexed_docs
      };
      view.sourceDB.emit('indexing', progress);
      view.sourceDB.activeTasks.update(taskId, {completed_items: indexed_docs});

      if (results.length < opts.changes_batch_size) {
        return;
      }
      return processNextBatch();
    }

    function createDocIdsToChangesAndEmits(results) {
      const docIdsToChangesAndEmits = new ExportedMap();
      for (let i = 0, len = results.length; i < len; i++) {
        const change = results[i];
        if (change.doc._id[0] !== '_') {
          mapResults = [];
          doc = change.doc;

          if (!doc._deleted) {
            tryMap(view.sourceDB, mapFun, doc);
          }
          mapResults.sort(sortByKeyThenValue);

          const indexableKeysToKeyValues = createIndexableKeysToKeyValues(mapResults);
          docIdsToChangesAndEmits.set(change.doc._id, [
            indexableKeysToKeyValues,
            change.changes
          ]);
        }
        currentSeq = change.seq;
      }
      return docIdsToChangesAndEmits;
    }

    function createIndexableKeysToKeyValues(mapResults) {
      const indexableKeysToKeyValues = new ExportedMap();
      let lastKey;
      for (let i = 0, len = mapResults.length; i < len; i++) {
        const emittedKeyValue = mapResults[i];
        const complexKey = [emittedKeyValue.key, emittedKeyValue.id];
        if (i > 0 && collate(emittedKeyValue.key, lastKey) === 0) {
          complexKey.push(i); // dup key+id, so make it unique
        }
        indexableKeysToKeyValues.set(toIndexableString(complexKey), emittedKeyValue);
        lastKey = emittedKeyValue.key;
      }
      return indexableKeysToKeyValues;
    }

    try {
      await createTask();
      await processNextBatch();
      await queue.finish();
      view.seq = currentSeq;
      view.sourceDB.activeTasks.remove(taskId);
    } catch (error) {
      view.sourceDB.activeTasks.remove(taskId, error);      
    }
  }

  function reduceView(view, results, options) {
    if (options.group_level === 0) {
      delete options.group_level;
    }

    const shouldGroup = options.group || options.group_level;

    const reduceFun = reducer(view.reduceFun);

    const groups = [];
    const lvl = isNaN(options.group_level) ? Number.POSITIVE_INFINITY :
      options.group_level;
    results.forEach(function (e) {
      const last = groups[groups.length - 1];
      let groupKey = shouldGroup ? e.key : null;

      // only set group_level for array keys
      if (shouldGroup && Array.isArray(groupKey)) {
        groupKey = groupKey.slice(0, lvl);
      }

      if (last && collate(last.groupKey, groupKey) === 0) {
        last.keys.push([e.key, e.id]);
        last.values.push(e.value);
        return;
      }
      groups.push({
        keys: [[e.key, e.id]],
        values: [e.value],
        groupKey: groupKey
      });
    });
    results = [];
    for (let i = 0, len = groups.length; i < len; i++) {
      const e = groups[i];
      const reduceTry = tryReduce(view.sourceDB, reduceFun, e.keys, e.values, false);
      if (reduceTry.error && reduceTry.error instanceof BuiltInError) {
        // CouchDB returns an error if a built-in errors out
        throw reduceTry.error;
      }
      results.push({
        // CouchDB just sets the value to null if a non-built-in errors out
        value: reduceTry.error ? null : reduceTry.output,
        key: e.groupKey
      });
    }
    // no total_rows/offset when reducing
    return {rows: sliceResults(results, options.limit, options.skip)};
  }

  function queryView(view, opts) {
    return sequentialize(getQueue(view), function () {
      return queryViewInQueue(view, opts);
    })();
  }

  async function queryViewInQueue(view, opts) {
    let totalRows;
    const shouldReduce = view.reduceFun && opts.reduce !== false;
    const skip = opts.skip || 0;
    if (typeof opts.keys !== 'undefined' && !opts.keys.length) {
      // equivalent query
      opts.limit = 0;
      delete opts.keys;
    }

    async function fetchFromView(viewOpts) {
      viewOpts.include_docs = true;
      const res$$1 = await view.db.allDocs(viewOpts);
      totalRows = res$$1.total_rows;

      return res$$1.rows.map(function (result) {
        // implicit migration - in older versions of PouchDB,
        // we explicitly stored the doc as {id: ..., key: ..., value: ...}
        // this is tested in a migration test
        /* istanbul ignore next */
        if ('value' in result.doc && typeof result.doc.value === 'object' &&
          result.doc.value !== null) {
          const keys = Object.keys(result.doc.value).sort();
          // this detection method is not perfect, but it's unlikely the user
          // emitted a value which was an object with these 3 exact keys
          const expectedKeys = ['id', 'key', 'value'];
          if (!(keys < expectedKeys || keys > expectedKeys)) {
            return result.doc.value;
          }
        }

        const parsedKeyAndDocId = parseIndexableString(result.doc._id);
        return {
          key: parsedKeyAndDocId[0],
          id: parsedKeyAndDocId[1],
          value: ('value' in result.doc ? result.doc.value : null)
        };
      });
    }

    async function onMapResultsReady(rows) {
      let finalResults;
      if (shouldReduce) {
        finalResults = reduceView(view, rows, opts);
      } else if (typeof opts.keys === 'undefined') {
        finalResults = {
          total_rows: totalRows,
          offset: skip,
          rows: rows
        };
      } else {
        // support limit, skip for keys query
        finalResults = {
          total_rows: totalRows,
          offset: skip,
          rows: sliceResults(rows,opts.limit,opts.skip)
        };
      }
      /* istanbul ignore if */
      if (opts.update_seq) {
        finalResults.update_seq = view.seq;
      }
      if (opts.include_docs) {
        const docIds = uniq(rows.map(rowToDocId));

        const allDocsRes = await view.sourceDB.allDocs({
          keys: docIds,
          include_docs: true,
          conflicts: opts.conflicts,
          attachments: opts.attachments,
          binary: opts.binary
        });
        var docIdsToDocs = new ExportedMap();
        allDocsRes.rows.forEach(function (row) {
          docIdsToDocs.set(row.id, row.doc);
        });
        rows.forEach(function (row) {
          var docId = rowToDocId(row);
          var doc = docIdsToDocs.get(docId);
          if (doc) {
            row.doc = doc;
          }
        });
        return finalResults;
      } else {
        return finalResults;
      }
    }

    if (typeof opts.keys !== 'undefined') {
      const keys = opts.keys;
      const fetchPromises = keys.map(function (key) {
        const viewOpts = {
          startkey : toIndexableString([key]),
          endkey   : toIndexableString([key, {}])
        };
        /* istanbul ignore if */
        if (opts.update_seq) {
          viewOpts.update_seq = true;
        }
        return fetchFromView(viewOpts);
      });
      const result = await Promise.all(fetchPromises);
      const flattenedResult = flatten(result);
      return onMapResultsReady(flattenedResult);
    } else { // normal query, no 'keys'
      const viewOpts = {
        descending : opts.descending
      };
      /* istanbul ignore if */
      if (opts.update_seq) {
        viewOpts.update_seq = true;
      }
      let startkey;
      let endkey;
      if ('start_key' in opts) {
        startkey = opts.start_key;
      }
      if ('startkey' in opts) {
        startkey = opts.startkey;
      }
      if ('end_key' in opts) {
        endkey = opts.end_key;
      }
      if ('endkey' in opts) {
        endkey = opts.endkey;
      }
      if (typeof startkey !== 'undefined') {
        viewOpts.startkey = opts.descending ?
          toIndexableString([startkey, {}]) :
          toIndexableString([startkey]);
      }
      if (typeof endkey !== 'undefined') {
        let inclusiveEnd = opts.inclusive_end !== false;
        if (opts.descending) {
          inclusiveEnd = !inclusiveEnd;
        }

        viewOpts.endkey = toIndexableString(
          inclusiveEnd ? [endkey, {}] : [endkey]);
      }
      if (typeof opts.key !== 'undefined') {
        const keyStart = toIndexableString([opts.key]);
        const keyEnd = toIndexableString([opts.key, {}]);
        if (viewOpts.descending) {
          viewOpts.endkey = keyStart;
          viewOpts.startkey = keyEnd;
        } else {
          viewOpts.startkey = keyStart;
          viewOpts.endkey = keyEnd;
        }
      }
      if (!shouldReduce) {
        if (typeof opts.limit === 'number') {
          viewOpts.limit = opts.limit;
        }
        viewOpts.skip = skip;
      }

      const result = await fetchFromView(viewOpts);
      return onMapResultsReady(result);
    }
  }

  async function httpViewCleanup(db) {
    const response = await db.fetch('_view_cleanup', {
      headers: new nodeFetch.Headers({'Content-Type': 'application/json'}),
      method: 'POST'
    });
    return response.json();
  }

  async function localViewCleanup(db) {
    try {
      const metaDoc = await db.get('_local/' + localDocName);
      const docsToViews = new ExportedMap();

      Object.keys(metaDoc.views).forEach(function (fullViewName) {
        const parts = parseViewName(fullViewName);
        const designDocName = '_design/' + parts[0];
        const viewName = parts[1];
        let views = docsToViews.get(designDocName);
        if (!views) {
          views = new ExportedSet();
          docsToViews.set(designDocName, views);
        }
        views.add(viewName);
      });
      const opts = {
        keys : mapToKeysArray(docsToViews),
        include_docs : true
      };

      const res$$1 = await db.allDocs(opts);
      const viewsToStatus = {};
      res$$1.rows.forEach(function (row) {
        const ddocName = row.key.substring(8); // cuts off '_design/'
        docsToViews.get(row.key).forEach(function (viewName) {
          let fullViewName = ddocName + '/' + viewName;
          /* istanbul ignore if */
          if (!metaDoc.views[fullViewName]) {
            // new format, without slashes, to support PouchDB 2.2.0
            // migration test in pouchdb's browser.migration.js verifies this
            fullViewName = viewName;
          }
          const viewDBNames = Object.keys(metaDoc.views[fullViewName]);
          // design doc deleted, or view function nonexistent
          const statusIsGood = row.doc && row.doc.views &&
            row.doc.views[viewName];
          viewDBNames.forEach(function (viewDBName) {
            viewsToStatus[viewDBName] =
              viewsToStatus[viewDBName] || statusIsGood;
          });
        });
      });

      const dbsToDelete = Object.keys(viewsToStatus)
        .filter(function (viewDBName) { return !viewsToStatus[viewDBName]; });

      const destroyPromises = dbsToDelete.map(function (viewDBName) {
        return sequentialize(getQueue(viewDBName), function () {
          return new db.constructor(viewDBName, db.__opts).destroy();
        })();
      });

      return Promise.all(destroyPromises).then(function () {
        return {ok: true};
      });
    } catch (err) {
      if (err.status === 404) {
        return {ok: true};
      } else {
        throw err;
      }
    }
  }

  async function queryPromised(db, fun, opts) {
    /* istanbul ignore next */
    if (typeof db._query === 'function') {
      return customQuery(db, fun, opts);
    }
    if (isRemote(db)) {
      return httpQuery(db, fun, opts);
    }

    const updateViewOpts = {
      changes_batch_size: db.__opts.view_update_changes_batch_size || CHANGES_BATCH_SIZE$1
    };

    if (typeof fun !== 'string') {
      // temp_view
      checkQueryParseError(opts, fun);

      tempViewQueue.add(async function () {
        const view = await createView(
          /* sourceDB */ db,
          /* viewName */ 'temp_view/temp_view',
          /* mapFun */ fun.map,
          /* reduceFun */ fun.reduce,
          /* temporary */ true,
          /* localDocName */ localDocName);

        return fin(updateView(view, updateViewOpts).then(
          function () { return queryView(view, opts); }),
          function () { return view.db.destroy(); }
        );
      });
      return tempViewQueue.finish();
    } else {
      // persistent view
      const fullViewName = fun;
      const parts = parseViewName(fullViewName);
      const designDocName = parts[0];
      const viewName = parts[1];

      const doc = await db.get('_design/' + designDocName);
      fun = doc.views && doc.views[viewName];

      if (!fun) {
        // basic validator; it's assumed that every subclass would want this
        throw new NotFoundError$1(`ddoc ${doc._id} has no view named ${viewName}`);
      }

      ddocValidator(doc, viewName);
      checkQueryParseError(opts, fun);

      const view = await createView(
        /* sourceDB */ db,
        /* viewName */ fullViewName,
        /* mapFun */ fun.map,
        /* reduceFun */ fun.reduce,
        /* temporary */ false,
        /* localDocName */ localDocName);

      if (opts.stale === 'ok' || opts.stale === 'update_after') {
        if (opts.stale === 'update_after') {
          nextTick(function () {
            updateView(view, updateViewOpts);
          });
        }
        return queryView(view, opts);
      } else { // stale not ok
        await updateView(view, updateViewOpts);
        return queryView(view, opts);
      }
    }
  }

  function abstractQuery(fun, opts, callback) {
    const db = this;
    if (typeof opts === 'function') {
      callback = opts;
      opts = {};
    }
    opts = opts ? coerceOptions(opts) : {};

    if (typeof fun === 'function') {
      fun = {map : fun};
    }

    const promise = Promise.resolve().then(function () {
      return queryPromised(db, fun, opts);
    });
    promisedCallback(promise, callback);
    return promise;
  }

  const abstractViewCleanup = callbackify(function () {
    const db = this;
    /* istanbul ignore next */
    if (typeof db._viewCleanup === 'function') {
      return customViewCleanup(db);
    }
    if (isRemote(db)) {
      return httpViewCleanup(db);
    }
    return localViewCleanup(db);
  });

  return {
    query: abstractQuery,
    viewCleanup: abstractViewCleanup
  };
}

var builtInReduce = {
  _sum: function (keys, values) {
    return sum(values);
  },

  _count: function (keys, values) {
    return values.length;
  },

  _stats: function (keys, values) {
    // no need to implement rereduce=true, because Pouch
    // will never call it
    function sumsqr(values) {
      var _sumsqr = 0;
      for (var i = 0, len = values.length; i < len; i++) {
        var num = values[i];
        _sumsqr += (num * num);
      }
      return _sumsqr;
    }
    return {
      sum     : sum(values),
      min     : Math.min.apply(null, values),
      max     : Math.max.apply(null, values),
      count   : values.length,
      sumsqr : sumsqr(values)
    };
  }
};

function getBuiltIn(reduceFunString) {
  if (/^_sum/.test(reduceFunString)) {
    return builtInReduce._sum;
  } else if (/^_count/.test(reduceFunString)) {
    return builtInReduce._count;
  } else if (/^_stats/.test(reduceFunString)) {
    return builtInReduce._stats;
  } else if (/^_/.test(reduceFunString)) {
    throw new Error(reduceFunString + ' is not a supported reduce function.');
  }
}

function mapper(mapFun, emit) {
  // for temp_views one can use emit(doc, emit), see #38
  if (typeof mapFun === "function" && mapFun.length === 2) {
    var origMap = mapFun;
    return function (doc) {
      return origMap(doc, emit);
    };
  } else {
    return evalFunction(mapFun.toString(), emit);
  }
}

function reducer(reduceFun) {
  var reduceFunString = reduceFun.toString();
  var builtIn = getBuiltIn(reduceFunString);
  if (builtIn) {
    return builtIn;
  } else {
    return evalFunction(reduceFunString);
  }
}

function ddocValidator(ddoc, viewName) {
  var fun = ddoc.views && ddoc.views[viewName];
  if (typeof fun.map !== 'string') {
    throw new NotFoundError$1('ddoc ' + ddoc._id + ' has no string view named ' +
      viewName + ', instead found object of type: ' + typeof fun.map);
  }
}

var localDocName = 'mrviews';
var abstract = createAbstractMapReduce(localDocName, mapper, reducer, ddocValidator);

function query(fun, opts, callback) {
  return abstract.query.call(this, fun, opts, callback);
}

function viewCleanup(callback) {
  return abstract.viewCleanup.call(this, callback);
}

var mapreduce = {
  query: query,
  viewCleanup: viewCleanup
};

function fileHasChanged(localDoc, remoteDoc, filename) {
  return !localDoc._attachments ||
         !localDoc._attachments[filename] ||
         localDoc._attachments[filename].digest !== remoteDoc._attachments[filename].digest;
}

function getDocAttachments(db, doc) {
  var filenames = Object.keys(doc._attachments);
  return Promise.all(filenames.map(function (filename) {
    return db.getAttachment(doc._id, filename, {rev: doc._rev});
  }));
}

function getDocAttachmentsFromTargetOrSource(target, src, doc) {
  var doCheckForLocalAttachments = isRemote(src) && !isRemote(target);
  var filenames = Object.keys(doc._attachments);

  if (!doCheckForLocalAttachments) {
    return getDocAttachments(src, doc);
  }

  return target.get(doc._id).then(function (localDoc) {
    return Promise.all(filenames.map(function (filename) {
      if (fileHasChanged(localDoc, doc, filename)) {
        return src.getAttachment(doc._id, filename);
      }

      return target.getAttachment(localDoc._id, filename);
    }));
  }).catch(function (error) {
    /* istanbul ignore if */
    if (error.status !== 404) {
      throw error;
    }

    return getDocAttachments(src, doc);
  });
}

function createBulkGetOpts(diffs) {
  var requests = [];
  Object.keys(diffs).forEach(function (id) {
    var missingRevs = diffs[id].missing;
    missingRevs.forEach(function (missingRev) {
      requests.push({
        id: id,
        rev: missingRev
      });
    });
  });

  return {
    docs: requests,
    revs: true,
    latest: true
  };
}

//
// Fetch all the documents from the src as described in the "diffs",
// which is a mapping of docs IDs to revisions. If the state ever
// changes to "cancelled", then the returned promise will be rejected.
// Else it will be resolved with a list of fetched documents.
//
function getDocs(src, target, diffs, state) {
  diffs = clone(diffs); // we do not need to modify this

  var resultDocs = [],
      ok = true;

  function getAllDocs() {

    var bulkGetOpts = createBulkGetOpts(diffs);

    if (!bulkGetOpts.docs.length) { // optimization: skip empty requests
      return;
    }

    return src.bulkGet(bulkGetOpts).then(function (bulkGetResponse) {
      /* istanbul ignore if */
      if (state.cancelled) {
        throw new Error('cancelled');
      }
      return Promise.all(bulkGetResponse.results.map(function (bulkGetInfo) {
        return Promise.all(bulkGetInfo.docs.map(function (doc) {
          var remoteDoc = doc.ok;

          if (doc.error) {
            // when AUTO_COMPACTION is set, docs can be returned which look
            // like this: {"missing":"1-7c3ac256b693c462af8442f992b83696"}
            ok = false;
          }

          if (!remoteDoc || !remoteDoc._attachments) {
            return remoteDoc;
          }

          return getDocAttachmentsFromTargetOrSource(target, src, remoteDoc)
                   .then(function (attachments) {
                           var filenames = Object.keys(remoteDoc._attachments);
                           attachments
                             .forEach(function (attachment, i) {
                                        var att = remoteDoc._attachments[filenames[i]];
                                        delete att.stub;
                                        delete att.length;
                                        att.data = attachment;
                                      });

                                      return remoteDoc;
                                    });
        }));
      }))

      .then(function (results) {
        resultDocs = resultDocs.concat(flatten(results).filter(Boolean));
      });
    });
  }

  function returnResult() {
    return { ok:ok, docs:resultDocs };
  }

  return Promise.resolve()
    .then(getAllDocs)
    .then(returnResult);
}

var CHECKPOINT_VERSION = 1;
var REPLICATOR = "pouchdb";
// This is an arbitrary number to limit the
// amount of replication history we save in the checkpoint.
// If we save too much, the checkpoing docs will become very big,
// if we save fewer, we'll run a greater risk of having to
// read all the changes from 0 when checkpoint PUTs fail
// CouchDB 2.0 has a more involved history pruning,
// but let's go for the simple version for now.
var CHECKPOINT_HISTORY_SIZE = 5;
var LOWEST_SEQ = 0;

function updateCheckpoint(db, id, checkpoint, session, returnValue) {
  return db.get(id).catch(function (err) {
    if (err.status === 404) {
      if (db.adapter === 'http' || db.adapter === 'https') ;
      return {
        session_id: session,
        _id: id,
        history: [],
        replicator: REPLICATOR,
        version: CHECKPOINT_VERSION
      };
    }
    throw err;
  }).then(function (doc) {
    if (returnValue.cancelled) {
      return;
    }

    // if the checkpoint has not changed, do not update
    if (doc.last_seq === checkpoint) {
      return;
    }

    // Filter out current entry for this replication
    doc.history = (doc.history || []).filter(function (item) {
      return item.session_id !== session;
    });

    // Add the latest checkpoint to history
    doc.history.unshift({
      last_seq: checkpoint,
      session_id: session
    });

    // Just take the last pieces in history, to
    // avoid really big checkpoint docs.
    // see comment on history size above
    doc.history = doc.history.slice(0, CHECKPOINT_HISTORY_SIZE);

    doc.version = CHECKPOINT_VERSION;
    doc.replicator = REPLICATOR;

    doc.session_id = session;
    doc.last_seq = checkpoint;

    return db.put(doc).catch(function (err) {
      if (err.status === 409) {
        // retry; someone is trying to write a checkpoint simultaneously
        return updateCheckpoint(db, id, checkpoint, session, returnValue);
      }
      throw err;
    });
  });
}

class CheckpointerInternal {
  constructor(src, target, id, returnValue, opts) {
    this.src = src;
    this.target = target;
    this.id = id;
    this.returnValue = returnValue;
    this.opts = opts || {};
  }

  writeCheckpoint(checkpoint, session) {
    var self = this;
    return this.updateTarget(checkpoint, session).then(function () {
      return self.updateSource(checkpoint, session);
    });
  }

  updateTarget(checkpoint, session) {
    if (this.opts.writeTargetCheckpoint) {
      return updateCheckpoint(this.target, this.id, checkpoint,
        session, this.returnValue);
    } else {
      return Promise.resolve(true);
    }
  }

  updateSource(checkpoint, session) {
    if (this.opts.writeSourceCheckpoint) {
      var self = this;
      return updateCheckpoint(this.src, this.id, checkpoint,
        session, this.returnValue)
        .catch(function (err) {
          if (isForbiddenError(err)) {
            self.opts.writeSourceCheckpoint = false;
            return true;
          }
          throw err;
        });
    } else {
      return Promise.resolve(true);
    }
  }

  getCheckpoint() {
    var self = this;
  
    if (self.opts && self.opts.writeSourceCheckpoint && !self.opts.writeTargetCheckpoint) {
      return self.src.get(self.id).then(function (sourceDoc) {
        return sourceDoc.last_seq || LOWEST_SEQ;
      }).catch(function (err) {
        /* istanbul ignore if */
        if (err.status !== 404) {
          throw err;
        }
        return LOWEST_SEQ;
      });
    }
  
    return self.target.get(self.id).then(function (targetDoc) {
      if (self.opts && self.opts.writeTargetCheckpoint && !self.opts.writeSourceCheckpoint) {
        return targetDoc.last_seq || LOWEST_SEQ;
      }
  
      return self.src.get(self.id).then(function (sourceDoc) {
        // Since we can't migrate an old version doc to a new one
        // (no session id), we just go with the lowest seq in this case
        /* istanbul ignore if */
        if (targetDoc.version !== sourceDoc.version) {
          return LOWEST_SEQ;
        }
  
        var version;
        if (targetDoc.version) {
          version = targetDoc.version.toString();
        } else {
          version = "undefined";
        }
  
        if (version in comparisons) {
          return comparisons[version](targetDoc, sourceDoc);
        }
        /* istanbul ignore next */
        return LOWEST_SEQ;
      }, function (err) {
        if (err.status === 404 && targetDoc.last_seq) {
          return self.src.put({
            _id: self.id,
            last_seq: LOWEST_SEQ
          }).then(function () {
            return LOWEST_SEQ;
          }, function (err) {
            if (isForbiddenError(err)) {
              self.opts.writeSourceCheckpoint = false;
              return targetDoc.last_seq;
            }
            /* istanbul ignore next */
            return LOWEST_SEQ;
          });
        }
        throw err;
      });
    }).catch(function (err) {
      if (err.status !== 404) {
        throw err;
      }
      return LOWEST_SEQ;
    });
  }
}

var comparisons = {
  "undefined": function (targetDoc, sourceDoc) {
    // This is the previous comparison function
    if (collate(targetDoc.last_seq, sourceDoc.last_seq) === 0) {
      return sourceDoc.last_seq;
    }
    /* istanbul ignore next */
    return 0;
  },
  "1": function (targetDoc, sourceDoc) {
    // This is the comparison function ported from CouchDB
    return compareReplicationLogs(sourceDoc, targetDoc).last_seq;
  }
};

// This checkpoint comparison is ported from CouchDBs source
// they come from here:
// https://github.com/apache/couchdb-couch-replicator/blob/master/src/couch_replicator.erl#L863-L906

function compareReplicationLogs(srcDoc, tgtDoc) {
  if (srcDoc.session_id === tgtDoc.session_id) {
    return {
      last_seq: srcDoc.last_seq,
      history: srcDoc.history
    };
  }

  return compareReplicationHistory(srcDoc.history, tgtDoc.history);
}

function compareReplicationHistory(sourceHistory, targetHistory) {
  // the erlang loop via function arguments is not so easy to repeat in JS
  // therefore, doing this as recursion
  var S = sourceHistory[0];
  var sourceRest = sourceHistory.slice(1);
  var T = targetHistory[0];
  var targetRest = targetHistory.slice(1);

  if (!S || targetHistory.length === 0) {
    return {
      last_seq: LOWEST_SEQ,
      history: []
    };
  }

  var sourceId = S.session_id;
  /* istanbul ignore if */
  if (hasSessionId(sourceId, targetHistory)) {
    return {
      last_seq: S.last_seq,
      history: sourceHistory
    };
  }

  var targetId = T.session_id;
  if (hasSessionId(targetId, sourceRest)) {
    return {
      last_seq: T.last_seq,
      history: targetRest
    };
  }

  return compareReplicationHistory(sourceRest, targetRest);
}

function hasSessionId(sessionId, history) {
  var props = history[0];
  var rest = history.slice(1);

  if (!sessionId || history.length === 0) {
    return false;
  }

  if (sessionId === props.session_id) {
    return true;
  }

  return hasSessionId(sessionId, rest);
}

function isForbiddenError(err) {
  return typeof err.status === 'number' && Math.floor(err.status / 100) === 4;
}

function Checkpointer(src, target, id, returnValue, opts) {
  if (!(this instanceof CheckpointerInternal)) {
    return new CheckpointerInternal(src, target, id, returnValue, opts);
  }
  return Checkpointer;
}

var STARTING_BACK_OFF = 0;

function backOff(opts, returnValue, error, callback) {
  if (opts.retry === false) {
    returnValue.emit('error', error);
    returnValue.removeAllListeners();
    return;
  }
  /* istanbul ignore if */
  if (typeof opts.back_off_function !== 'function') {
    opts.back_off_function = defaultBackOff;
  }
  returnValue.emit('requestError', error);
  if (returnValue.state === 'active' || returnValue.state === 'pending') {
    returnValue.emit('paused', error);
    returnValue.state = 'stopped';
    var backOffSet = function backoffTimeSet() {
      opts.current_back_off = STARTING_BACK_OFF;
    };
    var removeBackOffSetter = function removeBackOffTimeSet() {
      returnValue.removeListener('active', backOffSet);
    };
    returnValue.once('paused', removeBackOffSetter);
    returnValue.once('active', backOffSet);
  }

  opts.current_back_off = opts.current_back_off || STARTING_BACK_OFF;
  opts.current_back_off = opts.back_off_function(opts.current_back_off);
  setTimeout(callback, opts.current_back_off);
}

function sortObjectPropertiesByKey(queryParams) {
  return Object.keys(queryParams).sort(collate).reduce(function (result, key) {
    result[key] = queryParams[key];
    return result;
  }, {});
}

// Generate a unique id particular to this replication.
// Not guaranteed to align perfectly with CouchDB's rep ids.
function generateReplicationId(src, target, opts) {
  var docIds = opts.doc_ids ? opts.doc_ids.sort(collate) : '';
  var filterFun = opts.filter ? opts.filter.toString() : '';
  var queryParams = '';
  var filterViewName =  '';
  var selector = '';

  // possibility for checkpoints to be lost here as behaviour of
  // JSON.stringify is not stable (see #6226)
  /* istanbul ignore if */
  if (opts.selector) {
    selector = JSON.stringify(opts.selector);
  }

  if (opts.filter && opts.query_params) {
    queryParams = JSON.stringify(sortObjectPropertiesByKey(opts.query_params));
  }

  if (opts.filter && opts.filter === '_view') {
    filterViewName = opts.view.toString();
  }

  return Promise.all([src.id(), target.id()]).then(function (res) {
    var queryData = res[0] + res[1] + filterFun + filterViewName +
      queryParams + docIds + selector;
    return new Promise(function (resolve) {
      binaryMd5(queryData, resolve);
    });
  }).then(function (md5sum) {
    // can't use straight-up md5 alphabet, because
    // the char '/' is interpreted as being for attachments,
    // and + is also not url-safe
    md5sum = md5sum.replace(/\//g, '.').replace(/\+/g, '_');
    return '_local/' + md5sum;
  });
}

function replicate(src, target, opts, returnValue, result) {
  var batches = [];               // list of batches to be processed
  var currentBatch;               // the batch currently being processed
  var pendingBatch = {
    seq: 0,
    changes: [],
    docs: []
  }; // next batch, not yet ready to be processed
  var writingCheckpoint = false;  // true while checkpoint is being written
  var changesCompleted = false;   // true when all changes received
  var replicationCompleted = false; // true when replication has completed
  // initial_last_seq is the state of the source db before
  // replication started, and it is _not_ updated during
  // replication or used anywhere else, as opposed to last_seq
  var initial_last_seq = 0;
  var last_seq = 0;
  var continuous = opts.continuous || opts.live || false;
  var batch_size = opts.batch_size || 100;
  var batches_limit = opts.batches_limit || 10;
  var style = opts.style || 'all_docs';
  var changesPending = false;     // true while src.changes is running
  var doc_ids = opts.doc_ids;
  var selector = opts.selector;
  var repId;
  var checkpointer;
  var changedDocs = [];
  // Like couchdb, every replication gets a unique session id
  var session = uuid$1();
  var taskId;

  result = result || {
    ok: true,
    start_time: new Date().toISOString(),
    docs_read: 0,
    docs_written: 0,
    doc_write_failures: 0,
    errors: []
  };

  var changesOpts = {};
  returnValue.ready(src, target);

  function initCheckpointer() {
    if (checkpointer) {
      return Promise.resolve();
    }
    return generateReplicationId(src, target, opts).then(function (res$$1) {
      repId = res$$1;

      var checkpointOpts = {};
      if (opts.checkpoint === false) {
        checkpointOpts = { writeSourceCheckpoint: false, writeTargetCheckpoint: false };
      } else if (opts.checkpoint === 'source') {
        checkpointOpts = { writeSourceCheckpoint: true, writeTargetCheckpoint: false };
      } else if (opts.checkpoint === 'target') {
        checkpointOpts = { writeSourceCheckpoint: false, writeTargetCheckpoint: true };
      } else {
        checkpointOpts = { writeSourceCheckpoint: true, writeTargetCheckpoint: true };
      }

      checkpointer = new Checkpointer(src, target, repId, returnValue, checkpointOpts);
    });
  }

  function writeDocs() {
    changedDocs = [];

    if (currentBatch.docs.length === 0) {
      return;
    }
    var docs = currentBatch.docs;
    var bulkOpts = {timeout: opts.timeout};
    return target.bulkDocs({docs: docs, new_edits: false}, bulkOpts).then(function (res$$1) {
      /* istanbul ignore if */
      if (returnValue.cancelled) {
        completeReplication();
        throw new Error('cancelled');
      }

      // `res` doesn't include full documents (which live in `docs`), so we create a map of
      // (id -> error), and check for errors while iterating over `docs`
      var errorsById = Object.create(null);
      res$$1.forEach(function (res$$1) {
        if (res$$1.error) {
          errorsById[res$$1.id] = res$$1;
        }
      });

      var errorsNo = Object.keys(errorsById).length;
      result.doc_write_failures += errorsNo;
      result.docs_written += docs.length - errorsNo;

      docs.forEach(function (doc) {
        var error = errorsById[doc._id];
        if (error) {
          result.errors.push(error);
          // Normalize error name. i.e. 'Unauthorized' -> 'unauthorized' (eg Sync Gateway)
          var errorName = (error.name || '').toLowerCase();
          if (errorName === 'unauthorized' || errorName === 'forbidden') {
            returnValue.emit('denied', clone(error));
          } else {
            throw error;
          }
        } else {
          changedDocs.push(doc);
        }
      });

    }, function (err) {
      result.doc_write_failures += docs.length;
      throw err;
    });
  }

  function finishBatch() {
    if (currentBatch.error) {
      throw new Error('There was a problem getting docs.');
    }
    result.last_seq = last_seq = currentBatch.seq;
    var outResult = clone(result);
    if (changedDocs.length) {
      outResult.docs = changedDocs;
      // Attach 'pending' property if server supports it (CouchDB 2.0+)
      /* istanbul ignore if */
      if (typeof currentBatch.pending === 'number') {
        outResult.pending = currentBatch.pending;
        delete currentBatch.pending;
      }
      returnValue.emit('change', outResult);
    }
    writingCheckpoint = true;

    src.info().then(function (info) {
      var task = src.activeTasks.get(taskId);
      if (!currentBatch || !task) {
        return;
      }

      var completed = task.completed_items || 0;
      var total_items = parseInt(info.update_seq, 10) - parseInt(initial_last_seq, 10);
      src.activeTasks.update(taskId, {
        completed_items: completed + currentBatch.changes.length,
        total_items
      });
    });

    return checkpointer.writeCheckpoint(currentBatch.seq,
        session).then(function () {
      returnValue.emit('checkpoint', { 'checkpoint': currentBatch.seq });
      writingCheckpoint = false;
      /* istanbul ignore if */
      if (returnValue.cancelled) {
        completeReplication();
        throw new Error('cancelled');
      }
      currentBatch = undefined;
      getChanges();
    }).catch(function (err) {
      onCheckpointError(err);
      throw err;
    });
  }

  function getDiffs() {
    var diff = {};
    currentBatch.changes.forEach(function (change) {
      returnValue.emit('checkpoint', { 'revs_diff': change });
      // Couchbase Sync Gateway emits these, but we can ignore them
      /* istanbul ignore if */
      if (change.id === "_user/") {
        return;
      }
      diff[change.id] = change.changes.map(function (x) {
        return x.rev;
      });
    });
    return target.revsDiff(diff).then(function (diffs) {
      /* istanbul ignore if */
      if (returnValue.cancelled) {
        completeReplication();
        throw new Error('cancelled');
      }
      // currentBatch.diffs elements are deleted as the documents are written
      currentBatch.diffs = diffs;
    });
  }

  function getBatchDocs() {
    return getDocs(src, target, currentBatch.diffs, returnValue).then(function (got) {
      currentBatch.error = !got.ok;
      got.docs.forEach(function (doc) {
        delete currentBatch.diffs[doc._id];
        result.docs_read++;
        currentBatch.docs.push(doc);
      });
    });
  }

  function startNextBatch() {
    if (returnValue.cancelled || currentBatch) {
      return;
    }
    if (batches.length === 0) {
      processPendingBatch(true);
      return;
    }
    currentBatch = batches.shift();
    returnValue.emit('checkpoint', { 'start_next_batch': currentBatch.seq });
    getDiffs()
      .then(getBatchDocs)
      .then(writeDocs)
      .then(finishBatch)
      .then(startNextBatch)
      .catch(function (err) {
        abortReplication('batch processing terminated with error', err);
      });
  }


  function processPendingBatch(immediate) {
    if (pendingBatch.changes.length === 0) {
      if (batches.length === 0 && !currentBatch) {
        if ((continuous && changesOpts.live) || changesCompleted) {
          returnValue.state = 'pending';
          returnValue.emit('paused');
        }
        if (changesCompleted) {
          completeReplication();
        }
      }
      return;
    }
    if (
      immediate ||
      changesCompleted ||
      pendingBatch.changes.length >= batch_size
    ) {
      batches.push(pendingBatch);
      pendingBatch = {
        seq: 0,
        changes: [],
        docs: []
      };
      if (returnValue.state === 'pending' || returnValue.state === 'stopped') {
        returnValue.state = 'active';
        returnValue.emit('active');
      }
      startNextBatch();
    }
  }


  function abortReplication(reason, err) {
    if (replicationCompleted) {
      return;
    }
    if (!err.message) {
      err.message = reason;
    }
    result.ok = false;
    result.status = 'aborting';
    batches = [];
    pendingBatch = {
      seq: 0,
      changes: [],
      docs: []
    };
    completeReplication(err);
  }


  function completeReplication(fatalError) {
    if (replicationCompleted) {
      return;
    }
    /* istanbul ignore if */
    if (returnValue.cancelled) {
      result.status = 'cancelled';
      if (writingCheckpoint) {
        return;
      }
    }
    result.status = result.status || 'complete';
    result.end_time = new Date().toISOString();
    result.last_seq = last_seq;
    replicationCompleted = true;

    src.activeTasks.remove(taskId, fatalError);

    if (fatalError) {
      // need to extend the error because Firefox considers ".result" read-only
      fatalError = createError(fatalError);
      fatalError.result = result;

      // Normalize error name. i.e. 'Unauthorized' -> 'unauthorized' (eg Sync Gateway)
      var errorName = (fatalError.name || '').toLowerCase();
      if (errorName === 'unauthorized' || errorName === 'forbidden') {
        returnValue.emit('error', fatalError);
        returnValue.removeAllListeners();
      } else {
        backOff(opts, returnValue, fatalError, function () {
          replicate(src, target, opts, returnValue);
        });
      }
    } else {
      returnValue.emit('complete', result);
      returnValue.removeAllListeners();
    }
  }

  function onChange(change, pending, lastSeq) {
    /* istanbul ignore if */
    if (returnValue.cancelled) {
      return completeReplication();
    }
    // Attach 'pending' property if server supports it (CouchDB 2.0+)
    /* istanbul ignore if */
    if (typeof pending === 'number') {
      pendingBatch.pending = pending;
    }

    var filter = filterChange(opts)(change);
    if (!filter) {
      // update processed items count by 1
      var task = src.activeTasks.get(taskId);
      if (task) {
        // we can assume that task exists here? shouldn't be deleted by here.
        var completed = task.completed_items || 0;
        src.activeTasks.update(taskId, {completed_items: ++completed});
      }
      return;
    }
    pendingBatch.seq = change.seq || lastSeq;
    pendingBatch.changes.push(change);
    returnValue.emit('checkpoint', { 'pending_batch': pendingBatch.seq });
    nextTick(function () {
      processPendingBatch(batches.length === 0 && changesOpts.live);
    });
  }


  function onChangesComplete(changes) {
    changesPending = false;
    /* istanbul ignore if */
    if (returnValue.cancelled) {
      return completeReplication();
    }

    // if no results were returned then we're done,
    // else fetch more
    if (changes.results.length > 0) {
      changesOpts.since = changes.results[changes.results.length - 1].seq;
      getChanges();
      processPendingBatch(true);
    } else {

      var complete = function () {
        if (continuous) {
          changesOpts.live = true;
          getChanges();
        } else {
          changesCompleted = true;
        }
        processPendingBatch(true);
      };

      // update the checkpoint so we start from the right seq next time
      if (!currentBatch && changes.results.length === 0) {
        writingCheckpoint = true;
        checkpointer.writeCheckpoint(changes.last_seq,
            session).then(function () {
          writingCheckpoint = false;
          result.last_seq = last_seq = changes.last_seq;
          if (returnValue.cancelled) {
            completeReplication();
            throw new Error('cancelled');
          } else {
            complete();
          }
        })
        .catch(onCheckpointError);
      } else {
        complete();
      }
    }
  }


  function onChangesError(err) {
    changesPending = false;
    /* istanbul ignore if */
    if (returnValue.cancelled) {
      return completeReplication();
    }
    abortReplication('changes rejected', err);
  }


  function getChanges() {
    if (!(
      !changesPending &&
      !changesCompleted &&
      batches.length < batches_limit
      )) {
      return;
    }
    changesPending = true;
    function abortChanges() {
      changes.cancel();
    }
    function removeListener() {
      returnValue.removeListener('cancel', abortChanges);
    }

    if (returnValue._changes) { // remove old changes() and listeners
      returnValue.removeListener('cancel', returnValue._abortChanges);
      returnValue._changes.cancel();
    }
    returnValue.once('cancel', abortChanges);

    var changes = src.changes(changesOpts)
      .on('change', onChange);
    changes.then(removeListener, removeListener);
    changes.then(onChangesComplete)
      .catch(onChangesError);

    if (opts.retry) {
      // save for later so we can cancel if necessary
      returnValue._changes = changes;
      returnValue._abortChanges = abortChanges;
    }
  }

  function createTask(checkpoint) {
    return src.info().then(function (info) {
      var total_items = typeof opts.since === 'undefined' ?
        parseInt(info.update_seq, 10) - parseInt(checkpoint, 10) :
        parseInt(info.update_seq, 10);

      taskId = src.activeTasks.add({
        name: `${continuous ? 'continuous ' : ''}replication from ${info.db_name}` ,
        total_items,
      });

      return checkpoint;
    });
  }

  function startChanges() {
    initCheckpointer().then(function () {
      /* istanbul ignore if */
      if (returnValue.cancelled) {
        completeReplication();
        return;
      }
      return checkpointer.getCheckpoint().then(createTask).then(function (checkpoint) {
        last_seq = checkpoint;
        initial_last_seq = checkpoint;
        changesOpts = {
          since: last_seq,
          limit: batch_size,
          batch_size: batch_size,
          style: style,
          doc_ids: doc_ids,
          selector: selector,
          return_docs: true // required so we know when we're done
        };
        if (opts.filter) {
          if (typeof opts.filter !== 'string') {
            // required for the client-side filter in onChange
            changesOpts.include_docs = true;
          } else { // ddoc filter
            changesOpts.filter = opts.filter;
          }
        }
        if ('heartbeat' in opts) {
          changesOpts.heartbeat = opts.heartbeat;
        }
        if ('timeout' in opts) {
          changesOpts.timeout = opts.timeout;
        }
        if (opts.query_params) {
          changesOpts.query_params = opts.query_params;
        }
        if (opts.view) {
          changesOpts.view = opts.view;
        }
        getChanges();
      });
    }).catch(function (err) {
      abortReplication('getCheckpoint rejected with ', err);
    });
  }

  /* istanbul ignore next */
  function onCheckpointError(err) {
    writingCheckpoint = false;
    abortReplication('writeCheckpoint completed with error', err);
  }

  /* istanbul ignore if */
  if (returnValue.cancelled) { // cancelled immediately
    completeReplication();
    return;
  }

  if (!returnValue._addedListeners) {
    returnValue.once('cancel', completeReplication);

    if (typeof opts.complete === 'function') {
      returnValue.once('error', opts.complete);
      returnValue.once('complete', function (result) {
        opts.complete(null, result);
      });
    }
    returnValue._addedListeners = true;
  }

  if (typeof opts.since === 'undefined') {
    startChanges();
  } else {
    initCheckpointer().then(function () {
      writingCheckpoint = true;
      return checkpointer.writeCheckpoint(opts.since, session);
    }).then(function () {
      writingCheckpoint = false;
      /* istanbul ignore if */
      if (returnValue.cancelled) {
        completeReplication();
        return;
      }
      last_seq = opts.since;
      startChanges();
    }).catch(onCheckpointError);
  }
}

// We create a basic promise so the caller can cancel the replication possibly
// before we have actually started listening to changes etc
class Replication extends EE {
  constructor() {
    super();
    this.cancelled = false;
    this.state = 'pending';
    const promise = new Promise((fulfill, reject) => {
      this.once('complete', fulfill);
      this.once('error', reject);
    });
    this.then = function (resolve, reject) {
      return promise.then(resolve, reject);
    };
    this.catch = function (reject) {
      return promise.catch(reject);
    };
    // As we allow error handling via "error" event as well,
    // put a stub in here so that rejecting never throws UnhandledError.
    this.catch(function () {});
  }

  cancel() {
    this.cancelled = true;
    this.state = 'cancelled';
    this.emit('cancel');
  }

  ready(src, target) {
    if (this._readyCalled) {
      return;
    }
    this._readyCalled = true;
  
    const onDestroy = () => {
      this.cancel();
    };
    src.once('destroyed', onDestroy);
    target.once('destroyed', onDestroy);
    function cleanup() {
      src.removeListener('destroyed', onDestroy);
      target.removeListener('destroyed', onDestroy);
    }
    this.once('complete', cleanup);
    this.once('error', cleanup);
  }
}

function toPouch(db, opts) {
  var PouchConstructor = opts.PouchConstructor;
  if (typeof db === 'string') {
    return new PouchConstructor(db, opts);
  } else {
    return db;
  }
}

function replicateWrapper(src, target, opts, callback) {

  if (typeof opts === 'function') {
    callback = opts;
    opts = {};
  }
  if (typeof opts === 'undefined') {
    opts = {};
  }

  if (opts.doc_ids && !Array.isArray(opts.doc_ids)) {
    throw createError(BAD_REQUEST,
                       "`doc_ids` filter parameter is not a list.");
  }

  opts.complete = callback;
  opts = clone(opts);
  opts.continuous = opts.continuous || opts.live;
  opts.retry = ('retry' in opts) ? opts.retry : false;
  /*jshint validthis:true */
  opts.PouchConstructor = opts.PouchConstructor || this;
  var replicateRet = new Replication(opts);
  var srcPouch = toPouch(src, opts);
  var targetPouch = toPouch(target, opts);
  replicate(srcPouch, targetPouch, opts, replicateRet);
  return replicateRet;
}

function sync(src, target, opts, callback) {
  if (typeof opts === 'function') {
    callback = opts;
    opts = {};
  }
  if (typeof opts === 'undefined') {
    opts = {};
  }
  opts = clone(opts);
  /*jshint validthis:true */
  opts.PouchConstructor = opts.PouchConstructor || this;
  src = toPouch(src, opts);
  target = toPouch(target, opts);
  return new Sync(src, target, opts, callback);
}

class Sync extends EE {
  constructor(src, target, opts, callback) {
    super();
    this.canceled = false;

    const optsPush = opts.push ? $inject_Object_assign({}, opts, opts.push) : opts;
    const optsPull = opts.pull ? $inject_Object_assign({}, opts, opts.pull) : opts;

    this.push = replicateWrapper(src, target, optsPush);
    this.pull = replicateWrapper(target, src, optsPull);

    this.pushPaused = true;
    this.pullPaused = true;

    const pullChange = (change) => {
      this.emit('change', {
        direction: 'pull',
        change: change
      });
    };
    const pushChange = (change) => {
      this.emit('change', {
        direction: 'push',
        change: change
      });
    };
    const pushDenied = (doc) => {
      this.emit('denied', {
        direction: 'push',
        doc: doc
      });
    };
    const pullDenied = (doc) => {
      this.emit('denied', {
        direction: 'pull',
        doc: doc
      });
    };
    const pushPaused = () => {
      this.pushPaused = true;
      /* istanbul ignore if */
      if (this.pullPaused) {
        this.emit('paused');
      }
    };
    const pullPaused = () => {
      this.pullPaused = true;
      /* istanbul ignore if */
      if (this.pushPaused) {
        this.emit('paused');
      }
    };
    const pushActive = () => {
      this.pushPaused = false;
      /* istanbul ignore if */
      if (this.pullPaused) {
        this.emit('active', {
          direction: 'push'
        });
      }
    };
    const pullActive = () => {
      this.pullPaused = false;
      /* istanbul ignore if */
      if (this.pushPaused) {
        this.emit('active', {
          direction: 'pull'
        });
      }
    };

    let removed = {};

    const removeAll = (type) => { // type is 'push' or 'pull'
      return (event, func) => {
        const isChange = event === 'change' &&
          (func === pullChange || func === pushChange);
        const isDenied = event === 'denied' &&
          (func === pullDenied || func === pushDenied);
        const isPaused = event === 'paused' &&
          (func === pullPaused || func === pushPaused);
        const isActive = event === 'active' &&
          (func === pullActive || func === pushActive);

        if (isChange || isDenied || isPaused || isActive) {
          if (!(event in removed)) {
            removed[event] = {};
          }
          removed[event][type] = true;
          if (Object.keys(removed[event]).length === 2) {
            // both push and pull have asked to be removed
            this.removeAllListeners(event);
          }
        }
      };
    };

    if (opts.live) {
      this.push.on('complete', this.pull.cancel.bind(this.pull));
      this.pull.on('complete', this.push.cancel.bind(this.push));
    }

    function addOneListener(ee, event, listener) {
      if (ee.listeners(event).indexOf(listener) == -1) {
        ee.on(event, listener);
      }
    }

    this.on('newListener', function (event) {
      if (event === 'change') {
        addOneListener(this.pull, 'change', pullChange);
        addOneListener(this.push, 'change', pushChange);
      } else if (event === 'denied') {
        addOneListener(this.pull, 'denied', pullDenied);
        addOneListener(this.push, 'denied', pushDenied);
      } else if (event === 'active') {
        addOneListener(this.pull, 'active', pullActive);
        addOneListener(this.push, 'active', pushActive);
      } else if (event === 'paused') {
        addOneListener(this.pull, 'paused', pullPaused);
        addOneListener(this.push, 'paused', pushPaused);
      }
    });

    this.on('removeListener', function (event) {
      if (event === 'change') {
        this.pull.removeListener('change', pullChange);
        this.push.removeListener('change', pushChange);
      } else if (event === 'denied') {
        this.pull.removeListener('denied', pullDenied);
        this.push.removeListener('denied', pushDenied);
      } else if (event === 'active') {
        this.pull.removeListener('active', pullActive);
        this.push.removeListener('active', pushActive);
      } else if (event === 'paused') {
        this.pull.removeListener('paused', pullPaused);
        this.push.removeListener('paused', pushPaused);
      }
    });

    this.pull.on('removeListener', removeAll('pull'));
    this.push.on('removeListener', removeAll('push'));

    const promise = Promise.all([
      this.push,
      this.pull
    ]).then((resp) => {
      const out = {
        push: resp[0],
        pull: resp[1]
      };
      this.emit('complete', out);
      if (callback) {
        callback(null, out);
      }
      this.removeAllListeners();
      return out;
    }, (err) => {
      this.cancel();
      if (callback) {
        // if there's a callback, then the callback can receive
        // the error event
        callback(err);
      } else {
        // if there's no callback, then we're safe to emit an error
        // event, which would otherwise throw an unhandled error
        // due to 'error' being a special event in EventEmitters
        this.emit('error', err);
      }
      this.removeAllListeners();
      if (callback) {
        // no sense throwing if we're already emitting an 'error' event
        throw err;
      }
    });

    this.then = function (success, err) {
      return promise.then(success, err);
    };

    this.catch = function (err) {
      return promise.catch(err);
    };
  }

  cancel() {
    if (!this.canceled) {
      this.canceled = true;
      this.push.cancel();
      this.pull.cancel();
    }
  }
}

function replication(PouchDB) {
  PouchDB.replicate = replicateWrapper;
  PouchDB.sync = sync;

  Object.defineProperty(PouchDB.prototype, 'replicate', {
    get: function () {
      var self = this;
      if (typeof this.replicateMethods === 'undefined') {
        this.replicateMethods = {
          from: function (other, opts, callback) {
            return self.constructor.replicate(other, self, opts, callback);
          },
          to: function (other, opts, callback) {
            return self.constructor.replicate(self, other, opts, callback);
          }
        };
      }
      return this.replicateMethods;
    }
  });

  PouchDB.prototype.sync = function (dbName, opts, callback) {
    return this.constructor.sync(this, dbName, opts, callback);
  };
}

PouchDB.plugin(LevelPouch$1)
  .plugin(HttpPouch$1)
  .plugin(mapreduce)
  .plugin(replication);

// Pull from src because pouchdb-node/pouchdb-browser themselves

module.exports = PouchDB;
