These subroutines tell you whether a credit card number is self-consistent -- whether the last digit of the number is a valid checksum for the preceding digits. The validate() subroutine returns 1 if the card number provided passes the checksum test, and 0 otherwise. The cardtype() subroutine returns a string containing the type of card: "MasterCard", "VISA", and so on. My list is not complete; I welcome additions. The generate_last_digit() subroutine computes and returns the last digit of the card given the preceding digits. With a 16-digit card, you provide the first 15 digits; the subroutine returns the sixteenth.