greatest_common_divisor.h

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/*
 * Copyright 2023 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 "num_collect/base/concepts/integral.h"
#include "num_collect/base/exception.h"
#include "num_collect/base/precondition.h"
#include "num_collect/logging/logging_macros.h"
 
namespace num_collect::util {
 
template <base::concepts::integral T>
[[nodiscard]] constexpr auto greatest_common_divisor(T a, T b) -> T {
    NUM_COLLECT_PRECONDITION(a > static_cast<T>(0) && b > static_cast<T>(0),
        "greatest_common_divisor requires positive integers.");
    while (true) {
        const T x = a % b;
        if (x == static_cast<T>(0)) {
            return b;
        }
        a = b;
        b = x;
    }
}
 
}  // namespace num_collect::util