log_falling_factorial.hpp

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#ifndef STAN_MATH_PRIM_FUN_LOG_FALLING_FACTORIAL_HPP
#define STAN_MATH_PRIM_FUN_LOG_FALLING_FACTORIAL_HPP
 
#include <stan/math/prim/meta.hpp>
#include <stan/math/prim/err.hpp>
#include <stan/math/prim/fun/is_any_nan.hpp>
#include <stan/math/prim/fun/lgamma.hpp>
#include <stan/math/prim/fun/constants.hpp>
#include <stan/math/prim/functor/apply_scalar_binary.hpp>
 
namespace stan {
namespace math {
 
template <typename T1, typename T2, require_all_arithmetic_t<T1, T2>* = nullptr>
inline return_type_t<T1, T2> log_falling_factorial(const T1 x, const T2 n) {
  if (is_any_nan(x, n)) {
    return NOT_A_NUMBER;
  }
  static constexpr const char* function = "log_falling_factorial";
  check_positive(function, "first argument", x);
  return lgamma(x + 1) - lgamma(x - n + 1);
}
 
template <typename T1, typename T2, require_any_container_t<T1, T2>* = nullptr>
inline auto log_falling_factorial(const T1& a, const T2& b) {
  return apply_scalar_binary(a, b, [&](const auto& c, const auto& d) {
    return log_falling_factorial(c, d);
  });
}
 
}  // namespace math
}  // namespace stan
#endif