isbn, a C code which computes the check digit for an International Standard Book Number (ISBN), or reports whether a given ISBN is valid.
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isbn 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, or reports whether a 9-digit code is actually valid.
luhn, a C code which computes the Luhn check digit for a string, or validates a string.
upc, a C code which determines the check digit for a Uniform Product Code (UPC) or reports whether a given UPC is valid.