llmulLimb

fn llmulLimb(comptime op: AccOp, acc: []Limb, y: []const Limb, xi: Limb) bool {
    assert(!slicesOverlap(acc, y) or @intFromPtr(acc.ptr) <= @intFromPtr(y.ptr));

    if (xi == 0) {
        return false;
    }

    const split = @min(y.len, acc.len);
    var a_lo = acc[0..split];
    var a_hi = acc[split..];

    switch (op) {
        .add => {
            var carry: Limb = 0;
            var j: usize = 0;
            while (j < a_lo.len) : (j += 1) {
                a_lo[j] = addMulLimbWithCarry(a_lo[j], y[j], xi, &carry);
            }

            j = 0;
            while ((carry != 0) and (j < a_hi.len)) : (j += 1) {
                const ov = @addWithOverflow(a_hi[j], carry);
                a_hi[j] = ov[0];
                carry = ov[1];
            }

            return carry != 0;
        },
        .sub => {
            var borrow: Limb = 0;
            var j: usize = 0;
            while (j < a_lo.len) : (j += 1) {
                a_lo[j] = subMulLimbWithBorrow(a_lo[j], y[j], xi, &borrow);
            }

            j = 0;
            while ((borrow != 0) and (j < a_hi.len)) : (j += 1) {
                const ov = @subWithOverflow(a_hi[j], borrow);
                a_hi[j] = ov[0];
                borrow = ov[1];
            }

            return borrow != 0;
        },
    }
}