log1m_exp.hpp

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#ifndef STAN__AGRAD__REV__FUNCTIONS__LOG1M_EXP_HPP
#define STAN__AGRAD__REV__FUNCTIONS__LOG1M_EXP_HPP

#include <stan/agrad/rev/var.hpp>
#include <stan/agrad/rev/internal/v_vari.hpp>
#include <stan/math/functions/log1m_exp.hpp>
#include <stan/agrad/rev/calculate_chain.hpp>
#include <boost/math/special_functions/expm1.hpp>

namespace stan {
  namespace agrad {

    namespace {
      class log1m_exp_v_vari : public op_v_vari {
      public:
        log1m_exp_v_vari(vari* avi) :
          op_v_vari(stan::math::log1m_exp(avi->val_),
                    avi) {
        }
        void chain() {
          //derivative of log(1-exp(x)) = -exp(x)/(1-exp(x)) = -1/(exp(-x)-1) = -1/expm1(-x)
          avi_->adj_ -= adj_ / boost::math::expm1(-(avi_->val_));
        }
      };      
    }

    inline var log1m_exp(const stan::agrad::var& a) {
      return var(new log1m_exp_v_vari(a.vi_));
    }

  }
}
#endif