#ifndef STAN_MATH_REV_SCAL_FUN_TRUNC_HPP
#define STAN_MATH_REV_SCAL_FUN_TRUNC_HPP

#include <stan/math/rev/core.hpp>
#include <stan/math/prim/scal/fun/is_nan.hpp>
#include <stan/math/prim/scal/fun/trunc.hpp>
#include <limits>

namespace stan {
namespace math {

namespace {
class trunc_vari : public op_v_vari {
 public:
  explicit trunc_vari(vari* avi) : op_v_vari(trunc(avi->val_), avi) {}
  void chain() {
    if (unlikely(is_nan(avi_->val_)))
      avi_->adj_ = std::numeric_limits<double>::quiet_NaN();
  }
};
}  // namespace

/**
 * Returns the truncatation of the specified variable (C99).
 *
 * See ::trunc() for the double-based version.
 *
 * The derivative is zero everywhere but at integer values, so for
 * convenience the derivative is defined to be everywhere zero,
 *
 * \f$\frac{d}{dx} \mbox{trunc}(x) = 0\f$.
 *
 *
   \f[
   \mbox{trunc}(x) =
   \begin{cases}
     \lfloor x \rfloor & \mbox{if } -\infty\leq x\leq \infty \\[6pt]
     \textrm{NaN} & \mbox{if } x = \textrm{NaN}
   \end{cases}
   \f]

   \f[
   \frac{\partial\, \mbox{trunc}(x)}{\partial x} =
   \begin{cases}
     0 & \mbox{if } -\infty\leq x\leq \infty \\[6pt]
     \textrm{NaN} & \mbox{if } x = \textrm{NaN}
   \end{cases}
   \f]
 *
 * @param a Specified variable.
 * @return Truncation of the variable.
 */
inline var trunc(const var& a) { return var(new trunc_vari(a.vi_)); }

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