Function DivMod128

library/cpp/int128/int128.h:644–904

NOTE: division by zero is UB and can be changed in future

namespace NPrivateInt128 {
    // NOTE: division by zero is UB and can be changed in future
    constexpr void DivMod128(const ui128 lhs, const ui128 rhs, ui128* const quo, ui128* const rem) {
        if (!quo && !rem) {
            return;
        }

        constexpr size_t n_udword_bits = sizeof(ui64) * CHAR_BIT;
        constexpr size_t n_utword_bits = sizeof(ui128) * CHAR_BIT;

        ui128 q{};
        ui128 r{};

        unsigned sr{};

        /* special cases, X is unknown, K != 0 */
        if (GetHigh(lhs) == 0)
        {
            if (GetHigh(rhs) == 0)
            {
                /* 0 X
                 * ---
                 * 0 X
                 */
                if (rem) {
                    *rem = GetLow(lhs) % GetLow(rhs);
                }
                if (quo) {
                    *quo = GetLow(lhs) / GetLow(rhs);
                }
                return;
            }
            /* 0 X
             * ---
             * K X
             */
            if (rem) {
                *rem = GetLow(lhs);
            }
            if (quo) {
                *quo = 0;
            }
            return;
        }
        /* n.s.high != 0 */
        if (GetLow(rhs) == 0)
        {
            if (GetHigh(rhs) == 0)
            {
                /* K X
                 * ---
                 * 0 0
                 */
                if (rem) {
                    *rem = GetHigh(lhs) % GetLow(rhs);
                }
                if (quo) {
                    *quo = GetHigh(lhs) / GetLow(rhs);
                }
                return;