#!/usr/bin/perl
use Algorithm::Diff;
my ($len, $off, $file1, $file2) = @ARGV;
$len or die "Usage: $0 <length> <offset> <file1> <file2>";

my ($buf1, $buf2);
open ONE, $file1 or die "$file1: $!"; read(ONE, $buf1, $len, $off);
open TWO, $file2 or die "$file2: $!"; read(TWO, $buf2, $len, $off);

@buf1 = split //, $buf1;
@buf2 = split //, $buf2;

my $diff = Algorithm::Diff->new(\@buf1, \@buf2);
@differences;
while ($diff->Next) {
    if ($diff->Same and $diff->Items(1) > 3 ) { 
        clear_difference_buffer();
        print_same($diff);
    } else {
        if (!@differences) { push @differences, [$diff->Get(qw( Min1 Max1 Min2 Max2 )) ] }
        push @differences, [ $diff->Items(1), $diff->Items(2) ];
    }
}

sub print_position {
    my ($min1, $max1, $min2, $max2) = @_;
    if ($min1 == $min2 and $max1 == $max2) {
        printf "%d-%d : ", $min1+$off, $max1+$off;
    } else { 
        printf "%d-%d / %d-%d: ", $min1+$off, $max1+$off, $min1+$off, $max2+$off;
    }
}

sub printable {
    my $string = shift;
    my $in_len = length($string);
    $string =~ s/([^[:print:]]|\s)/sprintf("\\x{%x}", ord($1))/eg;
    $in_len > 1 ? "[$string]" : $string;
}

sub print_same { 
    my $d = shift;
    print_position($d->Get(qw( Min1 Max1 Min2 Max2 )));
    my $stuff = join "", $d->Items(1);
    print length($stuff), " identical characters";
    print ": ", printable($stuff) if length($stuff) < 40;
    print "\n";
}
sub clear_difference_buffer {
    return unless @differences;
    my @offsets = @{ shift @differences };
    my ($left, $right);
    for (@differences) {
        $left .= $_->[0];
        $right .= $_->[1];
    }
    $offsets[1] += length($left); $offsets[3] += length($right);
    print_position(@offsets);
    if (length($left) != length($right)) { 
        print length($left), " / " , length($right);
    } else { print length($left) }
    print " different characters";
    my ($p_left, $p_right) = (printable($left), printable($right));
    if (length($p_left.$p_right) < 40) {
        print ": $p_left versus $p_right";
    }
    print "\n";
    @differences = ();
}