LUHN is a C library 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 GNU LGPL 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 library 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 library which can determine the check digit for an International Standard Book Number or report whether a given ISBN is valid.
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You can go up one level to the C source codes.