source: git/lib/launchpad-merge-po

#!/usr/bin/perl -w
use strict;

open O, '<', $ARGV[0] or die $!;
if ($ARGV[1] eq '-') {
    *N = *STDIN;
} else {
    open N, '<', $ARGV[1] or die $!;
}

my $fuzzy = 0;
my $diff = 0;
my ($o, $n);
while (1) {
    $o //= <O>;
    $n //= <N>;
    defined $o and $o =~ s/[ \t]+$//;
    defined $n and $n =~ s/[ \t]+$//;
    if (!defined $n) {
        last unless defined $o;
        print '-' if $diff;
        print $o;
        $o = undef;
        next;
    }
    if (!defined $o) {
        print '+' if $diff;
        print $n;
        $n = undef;
        next;
    }

    if ($o eq "\n" || $n eq "\n") {
        $fuzzy = 0;
    }

    if ($o eq $n) {
        print ' ' if $diff;
        print $n;
        $n = $o = undef;
        next;
    }

    if (substr($o, 0, 3) eq '#~ ' && substr($n, 0, 3) ne '#~ ') {
        print ' ' if $diff;
        print "#~ $n";
        $n = $o = undef;
        next;
    }

    if ($o =~ /^("PO(?:-Revision|T-Creation)-Date: \d+-\d+-\d+ \d+:\d+):\d\d (.*\n)$/ &&
        $n eq "$1:00 $2") {
        print ' ' if $diff;
        print $o;
        $n = $o = undef;
        next;
    }

    if ($o =~ /^"PO-Revision-Date:/ && $n =~ /^"PO-Revision-Date:/) {
        print ' ' if $diff;
        print $o;
        $n = $o = undef;
        next;
    }

    if ($o =~ /(^"Last-Translator: .*<).*?(>.*\n)/ && $n eq "$1Unknown$2") {
        print ' ' if $diff;
        print $o;
        $n = $o = undef;
        next;
    }

    if ($o eq "\n") {
        print '+' if $diff;
        print $n;
        $n = undef;
        next;
    }

    if ($n eq "\n") {
        print '-$o' if $diff;
        $o = undef;
        next;
    }

    if ($o eq "#, fuzzy\n") {
        print ' ' if $diff;
        print $o;
        $o = undef;
        $fuzzy = 1;
        next;
    }

    my $tmp = $o;
    if ($tmp =~ s/, fuzzy// && $n eq $tmp) {
        print ' ' if $diff;
        print $o;
        $n = $o = undef;
        $fuzzy = 1;
        next;
    }

    if ($fuzzy && $o =~ /^(?:#~ )?msgstr / && $n eq "msgstr \"\"\n") {
        print ' ' if $diff;
        print $o;
        $n = $o = undef;
        $fuzzy = 0;
        next;
    }

    print "-$o" if $diff;
    print '+' if $diff;
    print $n;
    $n = $o = undef;
}