#!/usr/bin/perl -w
#
# git-me-backwards -- convert a git patch to a regular patch
# Copyright (C) 2006 Lennert Buytenhek
#
#

sub parse_hunk {
	my $hunk = shift;
	my $removed_or_identical;
	my $added_or_identical;

	die "corrupt hunk" unless ($hunk =~ /\@\@ -\d+,(\d+) \+\d+,(\d+) \@\@/);
	$removed_or_identical = $1;
	$added_or_identical = $2;

	while ($removed_or_identical > 0 or $added_or_identical > 0) {
		my $line;

		$line = <STDIN>;
		die "corrupt hunk" if not defined $line;

		$hunk .= $line;

		if ($line =~ /^-/) {
			$removed_or_identical--;
		} elsif ($line =~ /^\+/) {
			$added_or_identical--;
		} else {
			$removed_or_identical--;
			$added_or_identical--;
		}
	}

	die "corrupt hunk" if ($removed_or_identical or $added_or_identical);

	return $hunk;
}

my $currentline;

sub parse_patch {
	my $from;
	my $to;
	my $patch;

	$currentline =~ /^--- (.*)$/ or return;
	$from = $1;

	$currentline = <STDIN>;
	die "corrupt patch" if not defined $currentline;
	$currentline =~ /^\+\+\+ (.*)$/ or die "corrupt patch";
	$to = $1;

	$currentline = <STDIN>;
	while (defined $currentline) {
		last if ($currentline !~ /^\@\@ /);
		$patch .= parse_hunk($currentline);
		$currentline = <STDIN>;
	}

	return ($from, $to, $patch);
}

my $in_diff_header;
my $from;
my $to;
my $copying;
my $renaming;

$currentline = <STDIN>;
$in_diff_header = 0;
while (defined $currentline) {
	if ($currentline =~ /^diff --git a\/(\S+) b\/(\S+)/) {
		$currentline = <STDIN>;
		$in_diff_header = 1;
		$from = $1;
		$to = $2;
		$copying = 0;
		$renaming = 0;
		next;
	}

	if (not $in_diff_header) {
		print $currentline;
		$currentline = <STDIN>;
		next;
	}

	if ($currentline =~ /^copy /) {
		$currentline = <STDIN>;
		$copying = 1;
		next;
	}

#	if ($currentline =~ /^deleted /) {
#		$currentline = <STDIN>;
#		next;
#	}

#	if ($currentline =~ /^new /) {
#		$currentline = <STDIN>;
#		next;
#	}

	if ($currentline =~ /^rename /) {
		$currentline = <STDIN>;
		$renaming = 1;
		next;
	}

	if ($currentline =~ /^index /) {
		my $hdrfrom;
		my $hdrto;
		my $patch;

		$currentline = <STDIN>;
		$in_diff_header = 0;
		($hdrfrom, $hdrto, $patch) = parse_patch();

		if (not $copying and not $renaming) {
			if (defined $hdrfrom) {
				print "diff -u a/$from b/$to\n";
				print "--- a/$from\n";
				print "+++ b/$to\n";
				print $patch;
			}
			next;
		}

		if ($renaming) {
			# delete original
			print "diff -u a/$from b/$from\n";
			print "--- a/$from\n";
			print "+++ b/$from\n";
			system "diff -up $from /dev/null | tail +3";
		}

		if (not defined $from) {
			# create new
			print "diff -u a/$to b/$to\n";
			print "--- a/$to\n";
			print "+++ b/$to\n";
			system "diff -up /dev/null $from | tail +3\n";

			next;
		}

		system "cp $from /tmp/mergetemp";

		open PATCH, "| patch --quiet /tmp/mergetemp";
		print PATCH "diff -u $hdrfrom $hdrto\n";
		print PATCH "--- $hdrfrom\n";
		print PATCH "+++ $hdrto\n";
		print PATCH $patch;
		close PATCH;

		print "diff -u a/$to b/$to\n";
		print "--- a/$to\n";
		print "+++ b/$to\n";
		system "diff -up /dev/null /tmp/mergetemp | tail +3\n";

		unlink "/tmp/mergetemp";

		next;
	}

	$currentline = <STDIN>;
}