#!/usr/bin/perl

# Checksums data from standard input with the OSCAR file transfer checksum algorithm

use strict;
use warnings;

undef $/;
my $data = <STDIN>;

# Adopted from Gaim's implementation
sub checksum($) {
	my($part) = @_;
	my $check = sprintf("%lu", (0xFFFF0000 >> 16) & 0xFFFF);

	for(my $i = 0; $i < length($part); $i++) {
		my $oldcheck = $check;

		my $byte = ord(substr($part, $i, 1));
		my $val = ($i & 1) ? $byte : ($byte << 8);
		$check -= $val;
		$check = sprintf("%lu", $check);

		if($check > $oldcheck) {
			$check--;
			$check = sprintf("%lu", $check);
		}
	}

	$check = (($check & 0x0000FFFF) + ($check >> 16));
	$check = (($check & 0x0000FFFF) + ($check >> 16));
	$check = $check << 16;

	printf "%lu\n", $check;
}

checksum($data);