SafeSum.h

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// -*- LSST-C++ -*-
/*
 * Developed for the LSST Data Management System.
 * This product includes software developed by the LSST Project
 * (https://www.lsst.org).
 * See the COPYRIGHT file at the top-level directory of this distribution
 * for details of code ownership.
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
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#ifndef LSST_AFW_MATH_POLYNOMIALS_SafeSum_h_INCLUDED
#define LSST_AFW_MATH_POLYNOMIALS_SafeSum_h_INCLUDED

#include <cmath>

namespace lsst { namespace geom { namespace polynomials {

enum class SumMode {
    FAST,
    SAFE
};

template <typename T>
class SafeSum {
public:

    explicit SafeSum(T initial=static_cast<T>(0)) noexcept :
        _sum(initial),
        _correction(static_cast<T>(0))
    {}

    SafeSum & operator=(T value) noexcept {
        _sum = value;
        _correction = static_cast<T>(0);
        return *this;
    }

    SafeSum & operator+=(T value) noexcept {
        T t = _sum + value;
        // update _correction to account for lost low-order bits of the
        // greater of _sum and value
        if (std::abs(_sum) >= std::abs(value)) {
            _correction += (_sum - t) + value;
        } else {
            _correction += (value - t) + _sum;
        }
        _sum = t;
        return *this;
    }

    SafeSum & operator-=(T value) noexcept {
        return operator+=(-value);
    }

    explicit operator T() const noexcept {
        return _sum + _correction;
    }

private:
    T _sum;
    T _correction;
};

}}} // namespace lsst::geom::polynomials

#endif // !LSST_AFW_MATH_POLYNOMIALS_SafeSum_h_INCLUDED