#!/usr/bin/perl
use strict;
use warnings;

print OpenSubtitlesHash('breakdance.avi');

sub OpenSubtitlesHash {
	my $filename = shift or die("Need video filename");

	open my $handle, "<", $filename or die $!;
	my $fsize = -s $filename;

	my $hash = [$fsize & 0xFFFF, ($fsize >> 16) & 0xFFFF, 0, 0];

	$hash = AddUINT64($hash, ReadUINT64($handle)) for (1..8192);

    my $offset = $fsize - 65536;
    seek($handle, $offset > 0 ? $offset : 0, 0) or die $!;

    $hash = AddUINT64($hash, ReadUINT64($handle)) for (1..8192);

    close $handle or die $!;
    return UINT64FormatHex($hash);
}

sub ReadUINT64 {
	read($_[0], my $u, 8);
   	return [unpack("vvvv", $u)];
}

sub AddUINT64 {
    my $o = [0,0,0,0];
    my $carry = 0;
    for my $i (0..3) {
        if (($_[0]->[$i] + $_[1]->[$i] + $carry) > 0xffff ) {
			$o->[$i] += ($_[0]->[$i] + $_[1]->[$i] + $carry) & 0xffff;
			$carry = 1;
		} else {
			$o->[$i] += ($_[0]->[$i] + $_[1]->[$i] + $carry);
			$carry = 0;
		}
	}
    return $o;
}

sub UINT64FormatHex {
    return sprintf("%04x%04x%04x%04x", $_[0]->[3], $_[0]->[2], $_[0]->[1], $_[0]->[0]);
}