#ifndef STAN_MATH_REV_CORE_OPERATOR_DIVISION_HPP
#define STAN_MATH_REV_CORE_OPERATOR_DIVISION_HPP

#include <stan/math/rev/core/var.hpp>
#include <stan/math/rev/core/vv_vari.hpp>
#include <stan/math/rev/core/vd_vari.hpp>
#include <stan/math/rev/core/dv_vari.hpp>
#include <stan/math/prim/scal/fun/is_nan.hpp>
#include <limits>

namespace stan {
namespace math {

namespace {
// (a/b)' = a' * (1 / b) - b' * (a / [b * b])
class divide_vv_vari : public op_vv_vari {
 public:
  divide_vv_vari(vari* avi, vari* bvi)
      : op_vv_vari(avi->val_ / bvi->val_, avi, bvi) {}
  void chain() {
    if (unlikely(is_nan(avi_->val_) || is_nan(bvi_->val_))) {
      avi_->adj_ = std::numeric_limits<double>::quiet_NaN();
      bvi_->adj_ = std::numeric_limits<double>::quiet_NaN();
    } else {
      avi_->adj_ += adj_ / bvi_->val_;
      bvi_->adj_ -= adj_ * avi_->val_ / (bvi_->val_ * bvi_->val_);
    }
  }
};

class divide_vd_vari : public op_vd_vari {
 public:
  divide_vd_vari(vari* avi, double b) : op_vd_vari(avi->val_ / b, avi, b) {}
  void chain() {
    if (unlikely(is_nan(avi_->val_) || is_nan(bd_)))
      avi_->adj_ = std::numeric_limits<double>::quiet_NaN();
    else
      avi_->adj_ += adj_ / bd_;
  }
};

class divide_dv_vari : public op_dv_vari {
 public:
  divide_dv_vari(double a, vari* bvi) : op_dv_vari(a / bvi->val_, a, bvi) {}
  void chain() { bvi_->adj_ -= adj_ * ad_ / (bvi_->val_ * bvi_->val_); }
};
}  // namespace

/**
 * Division operator for two variables (C++).
 *
 * The partial derivatives for the variables are
 *
 * \f$\frac{\partial}{\partial x} (x/y) = 1/y\f$, and
 *
 * \f$\frac{\partial}{\partial y} (x/y) = -x / y^2\f$.
 *
   \f[
   \mbox{operator/}(x, y) =
   \begin{cases}
     \frac{x}{y} & \mbox{if } -\infty\leq x, y \leq \infty \\[6pt]
     \textrm{NaN} & \mbox{if } x = \textrm{NaN or } y = \textrm{NaN}
   \end{cases}
   \f]

   \f[
   \frac{\partial\, \mbox{operator/}(x, y)}{\partial x} =
   \begin{cases}
     \frac{1}{y} & \mbox{if } -\infty\leq x, y \leq \infty \\[6pt]
     \textrm{NaN} & \mbox{if } x = \textrm{NaN or } y = \textrm{NaN}
   \end{cases}
   \f]

   \f[
   \frac{\partial\, \mbox{operator/}(x, y)}{\partial y} =
   \begin{cases}
     -\frac{x}{y^2} & \mbox{if } -\infty\leq x, y \leq \infty \\[6pt]
     \textrm{NaN} & \mbox{if } x = \textrm{NaN or } y = \textrm{NaN}
   \end{cases}
   \f]
 *
 * @param a First variable operand.
 * @param b Second variable operand.
 * @return Variable result of dividing the first variable by the
 * second.
 */
inline var operator/(const var& a, const var& b) {
  return var(new divide_vv_vari(a.vi_, b.vi_));
}

/**
 * Division operator for dividing a variable by a scalar (C++).
 *
 * The derivative with respect to the variable is
 *
 * \f$\frac{\partial}{\partial x} (x/c) = 1/c\f$.
 *
 * @param a Variable operand.
 * @param b Scalar operand.
 * @return Variable result of dividing the variable by the scalar.
 */
inline var operator/(const var& a, double b) {
  if (b == 1.0)
    return a;
  return var(new divide_vd_vari(a.vi_, b));
}

/**
 * Division operator for dividing a scalar by a variable (C++).
 *
 * The derivative with respect to the variable is
 *
 * \f$\frac{d}{d y} (c/y) = -c / y^2\f$.
 *
 * @param a Scalar operand.
 * @param b Variable operand.
 * @return Variable result of dividing the scalar by the variable.
 */
inline var operator/(double a, const var& b) {
  return var(new divide_dv_vari(a, b.vi_));
}

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