#ifndef STAN_MATH_PRIM_SCAL_FUN_DIGAMMA_HPP
#define STAN_MATH_PRIM_SCAL_FUN_DIGAMMA_HPP

#include <stan/math/prim/scal/fun/boost_policy.hpp>
#include <boost/math/special_functions/digamma.hpp>

namespace stan {
  namespace math {

    /**
     * Return the derivative of the log gamma function
     * at the specified value.
     *
       \f[
       \mbox{digamma}(x) =
       \begin{cases}
         \textrm{error} & \mbox{if } x\in \{\dots, -3, -2, -1, 0\}\\
         \Psi(x) & \mbox{if } x\not\in \{\dots, -3, -2, -1, 0\}\\[6pt]
         \textrm{NaN} & \mbox{if } x = \textrm{NaN}
       \end{cases}
       \f]

       \f[
       \frac{\partial\, \mbox{digamma}(x)}{\partial x} =
       \begin{cases}
         \textrm{error} & \mbox{if } x\in \{\dots, -3, -2, -1, 0\}\\
         \frac{\partial\, \Psi(x)}{\partial x} & \mbox{if } x\not\in \{\dots, -3, -2, -1, 0\}\\[6pt]
         \textrm{NaN} & \mbox{if } x = \textrm{NaN}
       \end{cases}
       \f]

       \f[
       \Psi(x)=\frac{\Gamma'(x)}{\Gamma(x)}
       \f]

       \f[
       \frac{\partial \, \Psi(x)}{\partial x} = \frac{\Gamma''(x)\Gamma(x)-(\Gamma'(x))^2}{\Gamma^2(x)}
       \f]

     *
     * The design follows the standard C++ library in returning NaN
     * rather than throwing exceptions.
     *
     * @param[in] x argument
     * @return derivative of log gamma function at argument
     */
    inline double digamma(double x) {
      return boost::math::digamma(x, boost_policy_t());
    }

  }
}
#endif
