log1p_exp.hpp

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#ifndef STAN__MATH__FUNCTIONS__LOG1P_EXP_HPP
#define STAN__MATH__FUNCTIONS__LOG1P_EXP_HPP

#include <boost/math/tools/promotion.hpp>
#include <stan/math/functions/log1p.hpp>

namespace stan {
  namespace math {

    template <typename T>
    inline typename boost::math::tools::promote_args<T>::type
    log1p_exp(const T a) {
      using std::exp;
      // like log_sum_exp below with b=0.0; prevents underflow
      if (a > 0.0)
        return a + log1p(exp(-a)); 
      return log1p(exp(a));
    }

  }
}

#endif