Repository: bitcoin/bitcoin
File: src/util/overflow.h (C/C++)

Code before:
            return std::numeric_limits<T>::min();
        }
    } else {
        if (std::numeric_limits<T>::max() - i < j) {
            return std::numeric_limits<T>::max();
        }
    }
    return i + j;
}

/**
 * @brief Left bit shift with overflow checking.
 * @param input The input value to be left shifted.
 * @param shift The number of bits to left shift.
 * @return The result of the left shift, or std::nullopt in case of
 *         overflow or too high shift value.
 */
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 >= std::numeric_limits<T>::digits) return std::nullopt;
    // If input << shift is too big to fit in T, return nullopt.
    const auto max_allowed_input{std::numeric_limits<T>::max() >> shift};
    if (input > 0 && input > max_allowed_input) return std::nullopt;
    if constexpr (std::is_signed_v<T>) {
        if (input < 0 && input < -max_allowed_input - 1) return std::nullopt;
    }
    return input << shift;
}

/**
 * @brief Left bit shift with safe minimum and maximum values.
 * @param input The input value to be left shifted.
 * @param shift The number of bits to left shift.
 * @return The result of the left shift, with the return value saturated
 *         to the maximum value of T if the input is positive and the
 *         minimum value of T if the input is negative, on left shift
 *         overflow.
 */
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

Review comment (refactor):
In commit "util: Add integer left shift helpers" (https://github.com/bitcoin/bitcoin/commit/a85c07a891f76cd9e606aa83c0671506b1173f30)

Could consider dropping this line. Code is correct without it and it seems like a distraction. Or, if it is important to check type of T for some reason, presumably that reason would also apply in the `SaturatingLeftShift` function below and this same check should be done there before checking for `input < 0`.

Code after:
            return std::numeric_limits<T>::min();
        }
    } else {
        if (std::numeric_limits<T>::max() - i < j) {
            return std::numeric_limits<T>::max();
        }
    }
    return i + j;
}

/**
 * @brief Left bit shift with overflow checking.
 * @param input The input value to be left shifted.
 * @param shift The number of bits to left shift.
 * @return (input * 2^shift) or nullopt if it would not fit in the return type.
 */
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;
}

/**
 * @brief Left bit shift with safe minimum and maximum values.
 * @param input The input value to be left shifted.
 * @param shift The number of bits to left shift.
 * @return (input * 2^shift) clamped to fit between the lowest and highest
 *         representable values of the type T.
 */
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