falling_factorial.hpp

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#ifndef STAN_MATH_PRIM_FUN_FALLING_FACTORIAL_HPP
#define STAN_MATH_PRIM_FUN_FALLING_FACTORIAL_HPP
 
#include <stan/math/prim/meta.hpp>
#include <stan/math/prim/err.hpp>
#include <stan/math/prim/fun/boost_policy.hpp>
#include <stan/math/prim/functor/apply_scalar_binary.hpp>
#include <boost/math/special_functions/factorials.hpp>
 
namespace stan {
namespace math {
 
template <typename T, require_arithmetic_t<T>* = nullptr>
inline return_type_t<T> falling_factorial(const T& x, int n) {
  constexpr const char* function = "falling_factorial";
  check_not_nan(function, "first argument", x);
  check_nonnegative(function, "second argument", n);
  return boost::math::falling_factorial(x, n, boost_policy_t<>());
}
 
template <typename T1, typename T2, require_any_container_t<T1, T2>* = nullptr,
          require_all_not_var_matrix_t<T1, T2>* = nullptr>
inline auto falling_factorial(const T1& a, const T2& b) {
  return apply_scalar_binary(a, b, [&](const auto& c, const auto& d) {
    return falling_factorial(c, d);
  });
}
 
}  // namespace math
}  // namespace stan
 
#endif