log1m_exp.hpp

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#ifndef STAN_MATH_PRIM_FUN_LOG1M_EXP_HPP
#define STAN_MATH_PRIM_FUN_LOG1M_EXP_HPP
 
#include <stan/math/prim/meta.hpp>
#include <stan/math/prim/fun/constants.hpp>
#include <stan/math/prim/fun/exp.hpp>
#include <stan/math/prim/fun/expm1.hpp>
#include <stan/math/prim/fun/log.hpp>
#include <stan/math/prim/fun/log1m.hpp>
#include <stan/math/prim/functor/apply_scalar_unary.hpp>
#include <cmath>
 
namespace stan {
namespace math {
 
inline double log1m_exp(double a) {
  using std::exp;
  using std::log;
  if (a > 0) {
    return NOT_A_NUMBER;
  } else if (a > -0.693147) {
    return log(-expm1(a));  // 0.693147 ~= log(2)
  } else {
    return log1m(exp(a));
  }
}
 
struct log1m_exp_fun {
  template <typename T>
  static inline auto fun(const T& x) {
    return log1m_exp(x);
  }
};
 
template <
    typename T, require_not_var_matrix_t<T>* = nullptr,
    require_all_not_nonscalar_prim_or_rev_kernel_expression_t<T>* = nullptr>
inline auto log1m_exp(const T& x) {
  return apply_scalar_unary<log1m_exp_fun, T>::apply(x);
}
 
}  // namespace math
}  // namespace stan
 
#endif