--- 3m-810.pl	2008/10/01 18:46:03	14
+++ 3m-810.pl	2008/10/02 21:20:10	15
@@ -39,6 +39,8 @@
 
 L<perl(1)>
 
+L<http://stackoverflow.com/questions/149617/how-could-i-guess-a-checksum-algorithm>
+
 =head1 AUTHOR
 
 Dobrica Pavlinusic <dpavlin@rot13.org> L<http://www.rot13.org/~dpavlin/>
@@ -219,12 +221,25 @@
 	}
 }
 
+use Digest::CRC;
+
+sub crcccitt {
+	my $bytes = shift;
+	my $crc = Digest::CRC->new(
+		# midified CCITT to xor with 0xffff instead of 0x0000
+		width => 16, init => 0xffff, xorout => 0xffff, refout => 0, poly => 0x1021, refin => 0,
+	) or die $!;
+	$crc->add( $bytes );
+	pack('n', $crc->digest);
+}
+
 # my $checksum = checksum( $bytes );
 # my $checksum = checksum( $bytes, $original_checksum );
 sub checksum {
 	my ( $bytes, $checksum ) = @_;
 
-	my $xor = $checksum; # FIXME
+	my $xor = crcccitt( substr($bytes,1) );	# skip D6
+	warn "## checksum ",dump( $bytes, $xor, $checksum ) if $debug;
 
 	if ( defined $checksum && $xor ne $checksum ) {
 		print "checksum doesn't match: ", as_hex($xor), " != ", as_hex($checksum), " data: ", as_hex($bytes), "\n";