#ifndef STAN_MATH_REV_SCAL_FUN_EXP2_HPP
#define STAN_MATH_REV_SCAL_FUN_EXP2_HPP

#include <stan/math/rev/core.hpp>
#include <stan/math/prim/scal/fun/constants.hpp>
#include <cmath>
#include <valarray>

namespace stan {
  namespace math {

    namespace {
      class exp2_vari : public op_v_vari {
      public:
        explicit exp2_vari(vari* avi) :
          op_v_vari(std::pow(2.0, avi->val_), avi) {
        }
        void chain() {
          avi_->adj_ += adj_ * val_ * LOG_2;
        }
      };
    }

    /**
     * Exponentiation base 2 function for variables (C99).
     *
     * The derivative is
     *
     * \f$\frac{d}{dx} 2^x = (\log 2) 2^x\f$.
     *
       \f[
       \mbox{exp2}(x) =
       \begin{cases}
         2^x & \mbox{if } -\infty\leq x \leq \infty \\[6pt]
         \textrm{NaN} & \mbox{if } x = \textrm{NaN}
       \end{cases}
       \f]

       \f[
       \frac{\partial\, \mbox{exp2}(x)}{\partial x} =
       \begin{cases}
         2^x\ln2 & \mbox{if } -\infty\leq x\leq \infty \\[6pt]
         \textrm{NaN} & \mbox{if } x = \textrm{NaN}
       \end{cases}
       \f]
     *
     * @param a The variable.
     * @return Two to the power of the specified variable.
     */
    inline var exp2(const var& a) {
      return var(new exp2_vari(a.vi_));
    }

  }
}
#endif
