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

@ARGV==2 or die;

sub readfile($)
{
  my($fname)=@_;

  local *F;
  open F,$fname or die $fname;
  my $F=do { local $/; <F>; };
  defined $F or die $fname;
  close F or die $fname;
  return $F;
}

sub writefile($$)
{
  my($fname,$content)=@_;

  local *F;
  open F,">$fname" or die $fname;
  print F $content or die $fname;
  close F or die $fname;
}

sub strip($$$)
{
  my($dst,$src,$locref)=@_;

  my $f=readfile $src;
  $f=~s{^LOC (.*)$}{push @$locref,$1;"LOC";}egm;
  writefile $dst,$f;
}

my $t="/tmp/locdiff.$$";
my($a,$b)=("$t.a","$t.b");
my(@a,@b);
strip $a,$ARGV[0],\@a;
strip $b,$ARGV[1],\@b;
my $f=readfile "diff -U-1 $a $b||true|";
die if 2!=unlink "$a","$b";
$f=~s{^([-+ ])LOC$}{
  my $l;
  if ($1 eq "-") {
    $l=shift @a;
  } elsif ($1 eq "+") {
    $l=shift @b;
  } else {
    die if $1 ne " ";
    $l=shift @a;
    my $r=shift @b;
    $l="$r ($l)" if $l ne $r;
  }
  "$1LOC $l";
}egm;
1 while $f=~s{^ LOC .*\n(?: .*?\n)*?(.LOC )}{$1}gm;
print $f;