I might be an order of magnitude off here, but I believe there's only around 1 billion unique numbers per card once you take away check sum digits and look at how they are issued.
Assuming that's correct, it really wouldn't take up much memory or computing power to create a lookup table for every credit card number with hash x.
Assuming that's correct, it really wouldn't take up much memory or computing power to create a lookup table for every credit card number with hash x.