File Coverage

blib/lib/Mail/Exchange/CRC.pm

Criterion	Covered	Total	%
statement	44	44	100.0
branch	7	8	87.5
condition			n/a
subroutine	10	10	100.0
pod	4	4	100.0
total	65	66	98.4

package Mail::Exchange::CRC;

=head1 NAME
Mail::Exchange::CRC - implement the CRC algorithm used in RTF compression
and the named property to index PPS streams

=head1 SYNOPSIS

    use Mail::Exchange::CRC;

    my $crc=Mail::Exchange::CRC::new();
    while () {
	    $crc->append($_);
    }
    print $crc->value;

    print Mail::Exchange::CRC::crc($string);

=head1 DESCRIPTION

Mail::Exchange::CRC can be used in function mode or in object oriented mode.
In function mode, you pass a string and get back the crc immediately,
while in object mode, you initialize an object via C, then append data
to the object as needed, and fetch the resulting value at the end.

The crc algorithm is documented in [MS-OXRTFCP], and happens to be the CRC-32
algorithm that is used in a lot of different places as well, for example
in the the IEEE 802.3 Ethernet CRC specification.

=cut

use strict;
use warnings;
use 5.008;

use Exporter;
use vars qw(@ISA @EXPORT_OK $VERSION);

@ISA=qw(Exporter);
@EXPORT_OK=qw(crc);
$VERSION = "0.02";

our @crctable;
my $initialized;

# taken from Image::Dot which uses the same values

sub _make_crc_table {
	my ($c, $n, $k);
	for ($n = 0; $n < 256; $n++) {
		$c = $n;
		for ($k = 0; $k < 8; $k++) {
			if ($c & 1) {
				$c = 0xEDB88320 ^ ($c >> 1);
			} else {
				$c = $c >> 1;
			}
		}
		$crctable[$n] = $c;
	}
}

=head2 new()

$crc=Mail::Exchange::CRC::new([string]) - initialize a new CRC value

Initialize a new CRC calculator, and calculate the CRC of C if
provided.

=cut

sub new {
	my $class=shift;
	my $string=shift;

	unless ($initialized) {
		_make_crc_table();
		die "internal error" unless $crctable[255] == 0x2D02EF8D;
		$initialized=1;
	}

	my $self={};
	bless($self, $class);

	$self->{currval}=0;
	if ($string) {
		$self->append($string);
	}
	return $self;
}

=head2 append()

$crc->append(string)

Appends another string to a CRC, calculating the CRC of the two strings
concatenated to each other

The following are supposed to be equal:

	$crc1=Mail::Exchange::CRC::new("hello world");


	$crc2=Mail::Exchange::CRC::new("hello");
	$crc2->append(" world");
=cut

sub append {
	my $self=shift;
	my $string=shift;

	foreach my $byte (unpack("C*", $string)) {
		$self->{currval}=$crctable[($self->{currval} ^ $byte) & 0xff]
				^ ($self->{currval} >> 8);
	}
	return $self->{currval};
}

=head2 value()

$crcval=$crc->value()

Returns the calculated value of the CRC.

=cut

sub value {
	my $self=shift;
	return $self->{currval};
}

=head2 crc()

$crc=Mail::Exchange::CRC::crc($string)

Calculates the CRC of a string in an easy-to-use, non-object-oriented way.

=cut

sub crc {
	my $string=shift;
	return Mail::Exchange::CRC->new($string)->value;
}

1;