llaccum

fn llaccum(comptime op: AccOp, r: []Limb, a: []const Limb) void {
    assert(!slicesOverlap(r, a) or @intFromPtr(r.ptr) <= @intFromPtr(a.ptr));
    if (op == .sub) {
        _ = llsubcarry(r, r, a);
        return;
    }

    assert(r.len != 0 and a.len != 0);
    assert(r.len >= a.len);

    var i: usize = 0;
    var carry: Limb = 0;

    while (i < a.len) : (i += 1) {
        const ov1 = @addWithOverflow(r[i], a[i]);
        r[i] = ov1[0];
        const ov2 = @addWithOverflow(r[i], carry);
        r[i] = ov2[0];
        carry = @as(Limb, ov1[1]) + ov2[1];
    }

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