log1p_exp.hpp

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#ifndef STAN_MATH_PRIM_FUN_LOG1P_EXP_HPP
#define STAN_MATH_PRIM_FUN_LOG1P_EXP_HPP
 
#include <stan/math/prim/meta.hpp>
#include <stan/math/prim/fun/exp.hpp>
#include <stan/math/prim/fun/log.hpp>
#include <stan/math/prim/fun/log1p.hpp>
#include <stan/math/prim/functor/apply_scalar_unary.hpp>
#include <cmath>
 
namespace stan {
namespace math {
 
inline double log1p_exp(double 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));
}
 
struct log1p_exp_fun {
  template <typename T>
  static inline auto fun(const T& x) {
    return log1p_exp(x);
  }
};
 
template <typename T,
          require_not_nonscalar_prim_or_rev_kernel_expression_t<T>* = nullptr,
          require_not_var_matrix_t<T>* = nullptr>
inline auto log1p_exp(const T& x) {
  return apply_scalar_unary<log1p_exp_fun, T>::apply(x);
}
 
}  // namespace math
}  // namespace stan
 
#endif