Luhn (modulo 10) check digit calculator

Hi all. Perhaps a lazy request, but does anybody have some cut-n-paste code to calculate a Luhn check-digit? It's a check-digit often added to the end of things like credit card or account numbers to make detecting typo's a bit easier. I found lots of code to validate a number once the check-digit is applied, but nothing to calculate the digit in the first place. Thanks in advance ! Angus