#ifndef STAN_MATH_REV_CORE_OPERATOR_ADDITION_HPP
#define STAN_MATH_REV_CORE_OPERATOR_ADDITION_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/prim/scal/fun/is_nan.hpp>
#include <limits>

namespace stan {
namespace math {

namespace {
class add_vv_vari : public op_vv_vari {
 public:
  add_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_->adj_ += adj_;
    }
  }
};

class add_vd_vari : public op_vd_vari {
 public:
  add_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_;
  }
};
}  // namespace

/**
 * Addition operator for variables (C++).
 *
 * The partial derivatives are defined by
 *
 * \f$\frac{\partial}{\partial x} (x+y) = 1\f$, and
 *
 * \f$\frac{\partial}{\partial y} (x+y) = 1\f$.
 *
 *
   \f[
   \mbox{operator+}(x, y) =
   \begin{cases}
     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}
     1 & \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}
     1 & \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 adding two variables.
 */
inline var operator+(const var& a, const var& b) {
  return var(new add_vv_vari(a.vi_, b.vi_));
}

/**
 * Addition operator for variable and scalar (C++).
 *
 * The derivative with respect to the variable is
 *
 * \f$\frac{d}{dx} (x + c) = 1\f$.
 *
 * @param a First variable operand.
 * @param b Second scalar operand.
 * @return Result of adding variable and scalar.
 */
inline var operator+(const var& a, double b) {
  if (b == 0.0)
    return a;
  return var(new add_vd_vari(a.vi_, b));
}

/**
 * Addition operator for scalar and variable (C++).
 *
 * The derivative with respect to the variable is
 *
 * \f$\frac{d}{dy} (c + y) = 1\f$.
 *
 * @param a First scalar operand.
 * @param b Second variable operand.
 * @return Result of adding variable and scalar.
 */
inline var operator+(double a, const var& b) {
  if (a == 0.0)
    return b;
  return var(new add_vd_vari(b.vi_, a));  // by symmetry
}

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