exp.h

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/*
 * Copyright 2021 MusicScience37 (Kenta Kabashima)
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
#pragma once
 
#include <limits>
#include <type_traits>
 
#include "num_collect/constants/impl/exp_maclaurin.h"
#include "num_collect/constants/impl/pow_pos_int.h"
#include "num_collect/constants/napier.h"  // IWYU pragma: keep
#include "num_collect/constants/one.h"     // IWYU pragma: keep
#include "num_collect/constants/trunc.h"
#include "num_collect/constants/zero.h"  // IWYU pragma: keep
 
namespace num_collect::constants {
 
template <typename T, typename = std::enable_if_t<std::is_floating_point_v<T>>>
constexpr auto exp(T x) -> T {
    if (x < zero<T>) {
        return one<T> / exp(-x);
    }
    if (x < std::numeric_limits<T>::min()) {
        return one<T>;
    }
 
    constexpr auto thresh_inf =
        static_cast<T>(std::numeric_limits<T>::max_exponent);
    if (x >= thresh_inf) {
        return std::numeric_limits<T>::infinity();
    }
 
    int int_part = static_cast<int>(trunc(x));
    T rem_part = x - static_cast<T>(int_part);
    T exp_int_part = impl::pow_pos_int(napier<T>, int_part);
    T exp_rem_part = impl::exp_maclaurin(rem_part);
    return exp_int_part * exp_rem_part;
}
 
}  // namespace num_collect::constants