NAME
    Algorithm::Damm - Calculate the Damm error correction check digit.

SYNOPSIS
      use Algorithm::Damm qw/check_digit is_valid/;

      $c = check_digit("43881234567");
      print "It works\n" if is_valid("43881234567$c");

DESCRIPTION
    This module implements the Damm algorithm for calculating a check digit.

    You can find information about the algorithm by searching the web for
    "Damm ECC". In particular, see the "SEE ALSO" section (below).

FUNCTIONS
    is_valid CHECKSUMMED_NUM
        This function returns 1 if the final character of CHECKSUMMED_NUM is
        the correct checksum for the rest of the number, 0 if not, and undef
        if CHECKSUMMED_NUM contains an invalid character or does not contain
        at least two digits (one for the number, and one for the checksum).

        This function is equivalent to

          substr $N,length($N)-1 eq check_digit(substr $N,0,length($N)-1)

        Additionally, due to the way this algorithm works, if you crank the
        checksum calculation through the last digit (checkdigit included),
        you will end up with a value of 0.

    check_digit NUM
        This function returns the checksum of the given number. It will
        return undef if it is not able to calculate the checksum.

HISTORY
    This module came about as I was reverse engineering a checksum method
    being used on another project of mine. History of the method used was
    lost to the sands of time, and I needed something to run some quick
    tests on various checksum algorithms.

    This module is a reimplementation of one of the algorithms I had tested.

SEE ALSO
    Algorithm::CheckDigits
        Algorithm::CheckDigits is a module that this probably should have
        been written as a part of. There is an open issue
        (<https://github.com/MidLifeXis/perl-algorithm-damm/issues/1>) to
        add an integration layer for this module to Algorithm::CheckDigits.

    Algorithm::LUHN
        Original code based on Algorithm::LUHN by Tim Ayers.

    Math::CheckDigits
        Slightly different approach to generating check digits.

    Wikipedia
        <http://en.wikipedia.org/wiki/Damm_algorithm> - Article explaining
        the Damm error correction algorithm.

REPOSITORY
    You can find the source at
    <https://www.github.org/MidLifeXis/perl-algorithm-damm>.

BUGS
    None known at this time, but feel free to submit them to RT or the issue
    tracker for this source at GitHub.

AUTHOR
    This module was written by Brian T. Wightman
    (http://search.cpan.org/search?author=MLX) based on the module
    Algorithm::LUHN by Tim Ayers.

COPYRIGHT
    Copyright (c) 2013 Brian T. Wightman. All rights reserved.

LICENSE
    This program is free software; you can redistribute it and/or modify it
    under the same terms as Perl itself.