#ifndef STAN_MATH_REV_MAT_FUN_SOFTMAX_HPP
#define STAN_MATH_REV_MAT_FUN_SOFTMAX_HPP

#include <stan/math/prim/arr/err/check_nonzero_size.hpp>
#include <stan/math/prim/mat/fun/Eigen.hpp>
#include <stan/math/prim/mat/fun/softmax.hpp>
#include <stan/math/rev/core.hpp>
#include <vector>

namespace stan {
namespace math {

namespace {
class softmax_elt_vari : public vari {
 private:
  vari** alpha_;
  const double* softmax_alpha_;
  const int size_;  // array sizes
  const int idx_;   // in in softmax output

 public:
  softmax_elt_vari(double val, vari** alpha, const double* softmax_alpha,
                   int size, int idx)
      : vari(val),
        alpha_(alpha),
        softmax_alpha_(softmax_alpha),
        size_(size),
        idx_(idx) {}
  void chain() {
    for (int m = 0; m < size_; ++m) {
      if (m == idx_) {
        alpha_[m]->adj_
            += adj_ * softmax_alpha_[idx_] * (1 - softmax_alpha_[m]);
      } else {
        alpha_[m]->adj_ -= adj_ * softmax_alpha_[idx_] * softmax_alpha_[m];
      }
    }
  }
};
}  // namespace

/**
 * Return the softmax of the specified Eigen vector.  Softmax is
 * guaranteed to return a simplex.
 *
 * The gradient calculations are unfolded.
 *
 * @param alpha Unconstrained input vector.
 * @return Softmax of the input.
 * @throw std::domain_error If the input vector is size 0.
 */
inline Eigen::Matrix<var, Eigen::Dynamic, 1> softmax(
    const Eigen::Matrix<var, Eigen::Dynamic, 1>& alpha) {
  using Eigen::Dynamic;
  using Eigen::Matrix;

  check_nonzero_size("softmax", "alpha", alpha);

  vari** alpha_vi_array = reinterpret_cast<vari**>(
      ChainableStack::instance().memalloc_.alloc(sizeof(vari*) * alpha.size()));
  for (int i = 0; i < alpha.size(); ++i)
    alpha_vi_array[i] = alpha(i).vi_;

  Matrix<double, Dynamic, 1> alpha_d(alpha.size());
  for (int i = 0; i < alpha_d.size(); ++i)
    alpha_d(i) = alpha(i).val();

  Matrix<double, Dynamic, 1> softmax_alpha_d = softmax(alpha_d);

  double* softmax_alpha_d_array
      = reinterpret_cast<double*>(ChainableStack::instance().memalloc_.alloc(
          sizeof(double) * alpha_d.size()));
  for (int i = 0; i < alpha_d.size(); ++i)
    softmax_alpha_d_array[i] = softmax_alpha_d(i);

  Matrix<var, Dynamic, 1> softmax_alpha(alpha.size());
  for (int k = 0; k < softmax_alpha.size(); ++k)
    softmax_alpha(k)
        = var(new softmax_elt_vari(softmax_alpha_d[k], alpha_vi_array,
                                   softmax_alpha_d_array, alpha.size(), k));
  return softmax_alpha;
}

}  // namespace math
}  // namespace stan
#endif
