luhn, a C code which demonstrates the calculation and use of the Luhn check digit for error detection.
The computer code and data files described and made available on this web page are distributed under the MIT license
luhn is available in a C version and a C++ version and a FORTRAN90 version and a MATLAB version and a Python version.
BANK, a C code which can compute the check digit associated with a US Bank Routing Number check digit, or it can report whether a 9-digit code is actually valid.
ISBN, a C code which can determine the check digit for an International Standard Book Number or report whether a given ISBN is valid.
UPC, a C code which can determine the check digit for a Uniform Product Code (UPC) or report whether a given UPC is valid.