shiftLeftSat

    pub fn shiftLeftSat(r: *Mutable, a: Const, shift: usize, signedness: Signedness, bit_count: usize) void {
        // Special case: When the argument is negative, but the result is supposed to be unsigned,
        // return 0 in all cases.
        if (!a.positive and signedness == .unsigned) {
            r.set(0);
            return;
        }

        // Check whether the shift is going to overflow. This is the case
        // when (in 2s complement) any bit above `bit_count - shift` is set in the unshifted value.
        // Note, the sign bit is not counted here.

        // Handle shifts larger than the target type. This also deals with
        // 0-bit integers.
        if (bit_count <= shift) {
            // In this case, there is only no overflow if `a` is zero.
            if (a.eqlZero()) {
                r.set(0);
            } else {
                r.setTwosCompIntLimit(if (a.positive) .max else .min, signedness, bit_count);
            }
            return;
        }

        const checkbit = bit_count - shift - @intFromBool(signedness == .signed);
        // If `checkbit` and more significant bits are zero, no overflow will take place.

        if (checkbit >= a.limbs.len * limb_bits) {
            // `checkbit` is outside the range of a, so definitely no overflow will take place. We
            // can defer to a normal shift.
            // Note that if `a` is normalized (which we assume), this checks for set bits in the upper limbs.

            // Note, in this case r should already have enough limbs required to perform the normal shift.
            // In this case the shift of the most significant limb may still overflow.
            r.shiftLeft(a, shift);
            return;
        } else if (checkbit < (a.limbs.len - 1) * limb_bits) {
            // `checkbit` is not in the most significant limb. If `a` is normalized the most significant
            // limb will not be zero, so in this case we need to saturate. Note that `a.limbs.len` must be
            // at least one according to normalization rules.

            r.setTwosCompIntLimit(if (a.positive) .max else .min, signedness, bit_count);
            return;
        }

        // Generate a mask with the bits to check in the most significant limb. We'll need to check
        // all bits with equal or more significance than checkbit.
        // const msb = @truncate(Log2Limb, checkbit);
        // const checkmask = (@as(Limb, 1) << msb) -% 1;

        if (a.limbs[a.limbs.len - 1] >> @as(Log2Limb, @truncate(checkbit)) != 0) {
            // Need to saturate.
            r.setTwosCompIntLimit(if (a.positive) .max else .min, signedness, bit_count);
            return;
        }

        // This shift should not be able to overflow, so invoke llshl and normalize manually
        // to avoid the extra required limb.
        const new_len = llshl(r.limbs, a.limbs, shift);
        r.normalize(new_len);
        r.positive = a.positive;
    }