math/special_functions/logistic_sigmoid.hpp

0001 // Copyright Matt Borland 2025.
0002 // Use, modification and distribution are subject to the
0003 // Boost Software License, Version 1.0.
0004 // (See accompanying file LICENSE_1_0.txt
0005 // or copy at http://www.boost.org/LICENSE_1_0.txt)
0006
0007 #ifndef BOOST_MATH_SF_EXPIT_HPP
0008 #define BOOST_MATH_SF_EXPIT_HPP
0009
0010 #include <boost/math/policies/policy.hpp>
0011 #include <boost/math/tools/precision.hpp>
0012 #include <cmath>
0013
0014 namespace boost {
0015 namespace math {
0016
0017 template <typename RealType, typename Policy>
0018 RealType logistic_sigmoid(RealType x, const Policy&)
0019 {
0020     BOOST_MATH_STD_USING
0021
0022     using promoted_real_type = typename policies::evaluation<RealType, Policy>::type;
0023
0024     if(-x >= tools::log_max_value<RealType>())
0025     {
0026         return static_cast<RealType>(0);
0027     }
0028     if(-x <= -tools::log_max_value<RealType>())
0029     {
0030         return static_cast<RealType>(1);
0031     }
0032
0033     const auto res {static_cast<RealType>(1 / (1 + exp(static_cast<promoted_real_type>(-x))))};
0034     return res;
0035 }
0036
0037 template <typename RealType>
0038 RealType logistic_sigmoid(RealType x)
0039 {
0040     return logistic_sigmoid(x, policies::policy<>());
0041 }
0042
0043 } // namespace math
0044 } // namespace boost
0045
0046 #endif // BOOST_MATH_SF_EXPIT_HPP