softmax.hpp

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#ifndef STAN_MATH_PRIM_FUN_SOFTMAX_HPP
#define STAN_MATH_PRIM_FUN_SOFTMAX_HPP
 
#include <stan/math/prim/meta.hpp>
#include <stan/math/prim/err.hpp>
#include <stan/math/prim/fun/Eigen.hpp>
#include <stan/math/prim/fun/to_ref.hpp>
#include <stan/math/prim/functor/apply_vector_unary.hpp>
 
namespace stan {
namespace math {
 
template <typename Container, require_st_arithmetic<Container>* = nullptr,
          require_container_t<Container>* = nullptr,
          require_not_t<bool_constant<
              is_eigen<std::decay_t<Container>>::value
              && !is_eigen_vector<std::decay_t<Container>>::value>>* = nullptr>
inline auto softmax(Container&& x) {
  return make_holder(
      [](auto&& a) {
        return apply_vector_unary<ref_type_t<Container>>::apply(
            std::forward<decltype(a)>(a),
            [](auto&& v) -> plain_type_t<decltype(v)> {
              if (v.size() == 0) {
                return v;
              }
              const auto theta = (v.array() - v.maxCoeff()).exp();
              return (theta / theta.sum()).matrix();
            });
      },
      to_ref(std::forward<Container>(x)));
}
 
}  // namespace math
}  // namespace stan
#endif