kahan_adder.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 "num_collect/util/concepts/kahan_addable.h"
 
namespace num_collect::util {
 
template <concepts::kahan_addable T>
class kahan_adder {
public:
    kahan_adder() : kahan_adder(static_cast<T>(0)) {}
 
    explicit kahan_adder(const T& zero)
        : sum_(zero), rem_(zero), prev_sum_(zero) {}
 
    auto add(const T& value) -> kahan_adder& {
        prev_sum_ = sum_;
        rem_ += value;
        sum_ += rem_;
        rem_ -= sum_ - prev_sum_;
        return *this;
    }
 
    auto sub(const T& value) -> kahan_adder& {
        prev_sum_ = sum_;
        rem_ += -value;
        sum_ += rem_;
        rem_ -= sum_ - prev_sum_;
        return *this;
    }
 
    auto operator+=(const T& value) -> kahan_adder& {
        add(value);
        return *this;
    }
 
    auto operator-=(const T& value) -> kahan_adder& {
        sub(value);
        return *this;
    }
 
    [[nodiscard]] auto sum() const noexcept -> const T& { return sum_; }
 
    operator T() const {  // NOLINT: Allow implicit conversion.
        return sum_;
    }
 
private:
    T sum_;
 
    T rem_;
 
    T prev_sum_;
};
 
}  // namespace num_collect::util