inv_logit.hpp

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#ifndef STAN_MATH_PRIM_FUN_INV_LOGIT_HPP
#define STAN_MATH_PRIM_FUN_INV_LOGIT_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/inv.hpp>
#include <stan/math/prim/functor/apply_scalar_unary.hpp>
#include <cmath>
 
namespace stan {
namespace math {
 
template <typename T, require_arithmetic_t<T>* = nullptr>
inline double inv_logit(T&& a) {
  if (a < 0) {
    double exp_a = std::exp(a);
    if (a < LOG_EPSILON) {
      return exp_a;
    }
    return exp_a / (1.0 + exp_a);
  }
  return inv(1.0 + std::exp(-a));
}
 
struct inv_logit_fun {
  template <typename T>
  static inline auto fun(T&& x) {
    return inv_logit(std::forward<T>(x));
  }
};
 
template <typename Container, require_ad_container_t<Container>* = nullptr,
          require_all_not_nonscalar_prim_or_rev_kernel_expression_t<
              Container>* = nullptr,
          require_not_rev_matrix_t<Container>* = nullptr>
inline auto inv_logit(Container&& x) {
  return apply_scalar_unary<inv_logit_fun, Container>::apply(
      std::forward<Container>(x));
}
 
template <typename Container,
          require_container_bt<std::is_arithmetic, Container>* = nullptr,
          require_all_not_nonscalar_prim_or_rev_kernel_expression_t<
              Container>* = nullptr>
inline auto inv_logit(Container&& x) {
  return apply_vector_unary<Container>::apply(
      std::forward<Container>(x),
      [](auto&& v) { return v.array().logistic(); });
}
}  // namespace math
}  // namespace stan
 
#endif