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our @EXPORT_OK = qw/check_digit is_valid valid_chars/;
our $ERROR;

# The hash of valid characters.
my %map = map { $_ => $_ } 0..9;

=pod

=head1 NAME

Algorithm::LUHN - Calculate the Modulus 10 Double Add Double checksum

=head1 SYNOPSIS

  use Algorithm::LUHN qw/check_digit is_valid/;

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

  $c = check_digit("A2C4E6G8"); # this will cause an error

  print "Valid LUHN characters are:\n";
  my %vc = Algorithm::LUHN::valid_chars();
  for (sort keys %vc) {
    print "$_ => $vc{$_}\n";
  }

  Algorithm::LUHN::valid_chars(map {$_ => ord($_)-ord('A')+10} A..Z);
  $c = check_digit("A2C4E6G8");
  print "It worked again\n" if is_valid("A2C4E6G8$c");

=head1 DESCRIPTION

This module calculates the Modulus 10 Double Add Double checksum, also known as
the LUHN Formula. This algorithm is used to verify credit card numbers and
Standard & Poor's security identifiers such as CUSIP's and CSIN's.

You can find plenty of information about the algorithm by searching the web for
"modulus 10 double add double".

=head1 FUNCTION

=over 4

=cut

=item is_valid CHECKSUMMED_NUM

This function takes a credit-card number and returns true if
the number passes the LUHN check.

Ie it returns true if the final character of CHECKSUMMED_NUM is the
correct checksum for the rest of the number and false if not. Obviously the
final character does not factor into the checksum calculation. False will also
be returned if NUM contains in an invalid character as defined by
valid_chars(). If NUM is not valid, $Algorithm::LUHN::ERROR will contain the
reason.

This function is equivalent to

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

For example, C<4242 4242 4242 4242> is a valid Visa card number,
that is provided for test purposes. The final digit is '2',
which is the right check digit. If you change it to a '3', it's not
a valid card number. Ie:

    is_valid('4242424242424242');   # true
    is_valid('4242424242424243');   # false

=cut

sub is_valid {
  my $N = shift;
  my $c = check_digit(substr($N, 0,length($N)-1));
  if (defined $c) {
    if (substr($N,length($N)-1, 1) eq $c) {
      return 1;
    } else {
      $ERROR = "Check digit incorrect. Expected $c";
      return '';
    }
  } else {
    # $ERROR will have been set by check_digit
    return '';
  }
}

=item check_digit NUM

This function returns the checksum of the given number. If it cannot calculate
the check_digit it will return undef and set $Algorithm::LUHN::ERROR to contain
the reason why.

=cut
sub check_digit {
  my @buf = reverse split //, shift;

  my $totalVal = 0;
  my $flip = 1;
  foreach my $c (@buf) {
    unless (exists $map{$c}) {
      $ERROR = "Invalid character, '$c', in check_digit calculation";
      return;
    }
    my $posVal = $map{$c};

    $posVal *= 2 unless $flip = !$flip;

    while ($posVal) {
      $totalVal += $posVal % 10;
      $posVal = int($posVal / 10);
    }
  }

  return (10 - $totalVal % 10) % 10;
}

=item valid_chars LIST

By default this module only recognizes 0..9 as valid characters, but sometimes
you want to consider other characters as valid, e.g. Standard & Poor's
identifers may contain 0..9, A..Z, @, #, *. This function allows you to add
additional characters to the accepted list.

LIST is a mapping of C<character> =E<gt> C<value>.
For example, Standard & Poor's maps A..Z to 10..35
so the LIST to add these valid characters would be (A, 10, B, 11, C, 12, ...)

Please note that this I<adds> or I<re-maps> characters, so any characters
already considered valid but not in LIST will remain valid.

If you do not provide LIST,
this function returns the current valid character map.

=cut