...
To handle alphanumeric digits (numbers and letters), we actually use the ASCII value (the computer's internal code) for each character and subtract 48 to derive the "digit" used in the Luhn algorithm. We subtract 48 because the characters "0" through "9" are assigned values 48 to 57 in the ASCII table. Subtracting 48 lets the characters "0" to "9" assume the values 0 to 9 we'd expect. The letters "A" through "Z" are values 65 to 90 in the ASCII table (and become values 17 to 42 in our algorithm after subtracting 48). To keep life simple, we convert identifiers to uppercase and remove any spaces before applying the algorithm.
The Luhn CheckDigit Validator uses this variation to allow for letters, whereas the Luhn Mod-10 Check-Digit Validator uses the standard Luhn Algorithm using only numbers 0-9.
Mod 25 and Mod 30
The idgen module supports additional algorithms, including Mod25 and Mod30 algorithms. These algorithms not only allow letters and numbers to be used throughout the identifier, but also allow the check "digit" to be a letter. Typically, letters than can easily be confused with numbers (B, I, O, Q, S, and Z) are omitted. In fact, the Mod25 algorithm omits both numbers and letters that look similar and can be confused with each other (0, 1, 2, 5, 8, B, I, O, Q, S, and Z); the Mod25 Mod30 algorithm omits only the potentially confusing letters. The LuhnModNIdentifierValidator.java class contains the code that computes a check digit using "baseCharacters" as the set of possible characters for the identifier or check digit.
...