expm1.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 <type_traits>
 
#include "num_collect/constants/exp.h"
#include "num_collect/constants/impl/expm1_maclaurin.h"
#include "num_collect/constants/one.h"   // IWYU pragma: keep
#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 expm1(T x) -> T {
    if (x < -one<T> || one<T> < x) {
        return exp(x) - one<T>;
    }
 
    if (x >= zero<T>) {
        return impl::expm1_maclaurin(x);
    }
 
    T expm1_neg = impl::expm1_maclaurin(-x);
    return -expm1_neg / (expm1_neg + one<T>);
}
 
}  // namespace num_collect::constants