overflow.h

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// Copyright (c) 2021-2022 The Bitcoin Core developers
// Distributed under the MIT software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
#ifndef BITCOIN_UTIL_OVERFLOW_H
#define BITCOIN_UTIL_OVERFLOW_H
 
#include <climits>
#include <limits>
#include <optional>
#include <type_traits>
 
template <class T>
[[nodiscard]] bool AdditionOverflow(const T i, const T j) noexcept {
    static_assert(std::is_integral<T>::value, "Integral required.");
    if (std::numeric_limits<T>::is_signed) {
        return (i > 0 && j > std::numeric_limits<T>::max() - i) ||
               (i < 0 && j < std::numeric_limits<T>::min() - i);
    }
    return std::numeric_limits<T>::max() - i < j;
}
 
template <class T>
[[nodiscard]] std::optional<T> CheckedAdd(const T i, const T j) noexcept {
    if (AdditionOverflow(i, j)) {
        return std::nullopt;
    }
    return i + j;
}
 
template <std::unsigned_integral T, std::unsigned_integral U>
[[nodiscard]] constexpr bool TrySub(T &i, const U j) noexcept {
    if (i < T{j}) {
        return false;
    }
    i -= T{j};
    return true;
}
 
template <std::integral T>
constexpr std::optional<T> CheckedLeftShift(T input, unsigned shift) noexcept {
    if (shift == 0 || input == 0) {
        return input;
    }
    // Avoid undefined c++ behaviour if shift is >= number of bits in T.
    if (shift >= sizeof(T) * CHAR_BIT) {
        return std::nullopt;
    }
    // If input << shift is too big to fit in T, return nullopt.
    if (input > (std::numeric_limits<T>::max() >> shift)) {
        return std::nullopt;
    }
    if (input < (std::numeric_limits<T>::min() >> shift)) {
        return std::nullopt;
    }
    return input << shift;
}
 
template <std::integral T>
constexpr T SaturatingLeftShift(T input, unsigned shift) noexcept {
    if (auto result{CheckedLeftShift(input, shift)}) {
        return *result;
    }
    // If input << shift is too big to fit in T, return biggest positive or
    // negative number that fits.
    return input < 0 ? std::numeric_limits<T>::min()
                     : std::numeric_limits<T>::max();
}
 
#endif // BITCOIN_UTIL_OVERFLOW_H