upc, a C code which computes the check digit associated with a uniform product code (UPC), or reports whether a 12-digit UPC is actually valid.
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upc is available in a C version and a C++ version and a Fortran90 version and a MATLAB version and an Octave version and a Python version.
bank, a C code which computes the check digit associated with a US Bank Routing Number check digit, reports 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.
luhn, a C code which computes the Luhn check digit for a string, and can validate a string.