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    print "$_ => $vc{$_}\n";
  }

  Algorithm::LUHN_XS::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 is an XS version of the original Perl Module Algorithm::LUHN, which
was written by Tim Ayers.  It should work exactly the same, only substantially
 faster. The supplied check_digit() routine is 100% compatible with the pure
Perl Algorithm::LUHN module, while the faster check_digit_fast() and really fast
check_digit_rff() are not. 

How much faster? Here's a benchmark, running on a 3.4GHz i7-2600:

C<Benchmark: timing 100 iterations>

C<Algorithm::LUHN: 69 secs (69.37 usr 0.00 sys)  1.44/s>

C<check_digit:      2 secs ( 1.98 usr 0.00 sys) 50.51/s>

C<check_digit_fast: 2 secs ( 1.68 usr 0.00 sys) 59.52/s>

C<check_digit_rff:  1 secs ( 1.29 usr 0.00 sys) 77.52/s>

So, it's 35x to 53x faster than the original pure Perl module, depending on
how much compatibility with the original module you need.  

The rest of the documentation is mostly a copy of the original docs, with some
additions for functions that are new.

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_XS::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

=item is_valid_fast CHECKSUMMED_NUM
=cut
=item is_valid_rff CHECKSUMMED_NUM

As with check_digit(), we have 3 versions of is_valid(), each one progressively
faster than the check_digit() that comes in the original pure Perl 
Algorithm::LUHN module.  Here's a benchmark of 1M total calls to is_valid():

C<Benchmark: timing 100 iterations>

C<Algorithm::LUHN: 100 secs (100.29 usr 0.01 sys)  1.00/s>

C<is_valid:          3 secs (  2.46 usr 0.11 sys) 38.91/s>

C<is_valid_fast:     2 secs (  2.38 usr 0.05 sys) 41.15/s> 

C<is_valid_rff:      2 secs (  1.97 usr 0.08 sys) 48.78/s>

Algorithm::LUHN_XS varies from 38x to 48x times faster than the original
pure perl Algorithm::LUHN module. The is_valid() routine is 100% compatible
with the original, returning either '1' for success or the empty string ''
for failure.   The is_valid_fast() routine returns 1 for success and 0 for 
failure.  Finally, the is_valid_rff() function also returns 1 for success 
and 0 for failure, but only works with numeric input.  If you supply any 
alpha characters, it will return 0.

=cut

# is_valid is an XS function

=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_XS::ERROR to 
contain the reason why.  This is much faster than the check_digit routine
in the pure perl Algorithm::LUHN module, but only about half as fast as
the check_digit_fast() function in this module, due to the need to return both
integers and undef, which isn't fast with XS.

=cut

=item check_digit_fast NUM

This function returns the checksum of the given number. If it cannot calculate
the check digit it will return -1 and set $Algorithm::LUHN_XS::ERROR to 
contain the reason why. It's about 20% faster than check_digit() because the XS
code in this case only has to return integers.

=cut