#!/usr/bin/perl
# This is a generic update/post-receive hook script for GIT repositories.
# Written by Martin Mares <mj@ucw.cz> and placed into public domain.

use strict;
use warnings;

use Getopt::Long;
use IO::File;
use File::Temp;
use POSIX;

my $mail_to;
my $subject_prefix = "GIT";
my $max_diff_size;

GetOptions(
	'mail-to=s' => \$mail_to,
	'subject-prefix=s' => \$subject_prefix,
	'max-diff-size=s' => \$max_diff_size,
) and (@ARGV == 3 || !@ARGV) or die <<AMEN ;
Usage as update hook: $0 [<options>] <refname> <sha1-old> <sha1-new>
Usage as post-receive hook: $0 [<options>]

Options:
--mail-to=<address>	Send mail to the given address
--max-diff-size=<bytes>	If the diff is too long, send just a summary
--subject-prefix=<px>	Prefix subjects with [<px>] (default: GIT)
AMEN

my $repo = POSIX::getcwd();
$repo =~ s{.*/}{};

my @rev_list_options = ('--pretty', '--no-abbrev', '--date=iso');
my @diff_options = ('-C');

sub update_ref($$$);

open ORIG_STDOUT, '>&', \*STDOUT;

if (@ARGV) {
	update_ref($ARGV[0], $ARGV[1], $ARGV[2]);
} else {
	while (<STDIN>) {
		chomp;
		my ($old, $new, $ref) = /^(\S+) (\S+) (.*)/ or die "Error parsing hook input ($_)\n";
		update_ref($ref, $old, $new);
	}
}

sub scan_branches($$) {
	my ($ref, $new) = @_;
	# Is there any branch pointing to $new (other than $ref)?
	for (`git branch -v --no-abbrev`) {
		chomp;
		my ($name, $sha) = /^..(\S+)\s+(\S+)/ or die;
		if ((!defined($ref) || $name ne $ref) && $sha eq $new) {
			return $name;
		}
	}
	return;
}

sub scan_commits($$) {
	my ($old, $new) = @_;
	my @commits = ();
	for (`git rev-list $old..$new --pretty=format:"# %H (%P) %s"`) {
		chomp;
		/^# / or next;
		my ($hash, $parents, $subject) = m{^# (\S+) \(([^)]*)\) (.*)} or die;
		push @commits, {
			hash => $hash,
			parents => [ split /\s+/, $parents ],
			subject => $subject,
		};
	}
	return @commits;
}

sub most_recent($) {
	my ($new) = @_;
	print STDOUT "Most recent commits:\n\n";
	system 'git', 'rev-list', @rev_list_options, '--max-count=20', $new;
}

sub output_size($) {
	my ($out) = @_;
	$out->seek(0, 2);
	return $out->tell;
}

sub update_branch($$$$$)
{
	my ($branch, $old, $new, $out, $headers) = @_;

	my $subj = '[' . $subject_prefix . ($branch eq 'master' ? '' : "/$branch") . ']';
	if ($old =~ /^0+$/) {
		# Creation of a branch
		$subj .= ' Created branch';
		my $copy_of = scan_branches($branch, $new);
		if (defined $copy_of) {
			$subj .= " as a copy of $copy_of";
			print $out "Created branch $branch as a copy of $copy_of ($new).\n";
		} else {
			print $out "Created branch $branch ($new).\n\n";
			most_recent($new);
		}
	} elsif ($new =~ /^0+$/) {
		# Deletion of a branch
		$subj .= ' Branch deleted';
		print $out "Deleted branch $branch ($old).\n";
	} else {
		my $lca = `git merge-base $old $new`; die if $?;
		chomp $lca;
		if ($lca eq $old) {
			# Fast forward ... scan all objects
			my @commits = scan_commits($old, $new);
			my @nonmerges = grep { @{$_->{parents}} == 1 } @commits;
			@commits or return;

			# Construct subject
			# Try to recognize simple merges and display them as such
			my $c0 = $commits[0];
			my $n0 = $nonmerges[0];
			my $c0p = $c0->{parents};
			if (@{$c0p} == 2 &&
			    ($c0p->[0] eq $old || $c0p->[1] eq $old) &&
			    $c0->{subject} =~ m{^\s*Merge branch '([^']*)' into (\S+)} &&
			    (($1 eq $branch) != ($2 eq $branch))) {
				# Pushed a merge of the current branch with another local branch
				$subj .= ' ' . $c0->{subject};
			} elsif ($n0) {
				# Otherwise take the subject of the first non-merge commit
				$subj .= ' ' . $n0->{subject};
			} else {
				# If there is none, take the first merge
				$subj .= ' ' . $c0->{subject};
			}

			print $out "Push to branch $branch ($old -> $new)\n\n";

			# If there are multiple commits, mention that
			if (@nonmerges > 1) {
				$subj .= ' [' . (scalar @commits) . ' commits]';
				print $out 'Pushed ', (scalar @commits), " commits. Overall diffstat:\n\n";
			}

			# Print an overall diffstat
			system 'git', 'diff', '--stat', $old, $new;
			print $out "\n";
			my $pos_after_header = output_size($out);

			# Show individual commits with diffs
			system 'git', 'log', @rev_list_options, @diff_options, '-p', "$old..$new";

			# If the file is too long, truncate it and print just a summary
			if (defined($max_diff_size) && output_size($out) > $max_diff_size) {
				$out->truncate($pos_after_header);
				output_size($out);
				print $out "Diff was too long, printing just a summary.\n\n";
				system 'git', 'log', @rev_list_options, "$old..$new";
			}
		} elsif ($lca eq $new) {
			# Rewind
			$subj .= ' Branch rewound';
			print $out "Rewound branch $branch ($old -> $new).\n\n";
			most_recent($new);
		} else {
			# Otherwise it is a rebase
			$subj .= ' Branch rebased';
			print $out "Rebased branch $branch ($old -> $new).\n\n";
			print $out "Commits from common ancestor:\n\n";
			system 'git', 'rev-list', @rev_list_options, $new, "^$old";
		}
	}

	$headers->{'Subject'} = $subj;
	$headers->{'X-Git-Branch'} = $branch;
	return 1;
}

sub update_tag($$$$$)
{
	my ($tag, $old, $new, $out, $headers) = @_;

	my $subj = '[' . $subject_prefix . ']';
	if ($new =~ /^0+$/) {
		$subj .= " Deleted tag $tag";
		print $out "Deleted tag $tag ($old).\n";
	} else {
		my $copy_of = scan_branches(undef, $new);
		my $cp = defined($copy_of) ? " to branch $copy_of" : "";
		if ($old =~ /^0+/) {
			$subj .= " Created tag $tag$cp";
			print $out "Created tag $tag$cp ($new).\n\n";
		} else {
			$subj .= " Changed tag $tag$cp";
			print $out "Changed tag $tag$cp ($old -> $new).\n\n";
		}
		most_recent($new);
	}

	$headers->{'Subject'} = $subj;
	$headers->{'X-Git-Tag'} = $tag;
	return 1;
}

sub update_ref($$$)
{
	my ($ref, $old, $new) = @_;
	$old ne $new or return;
	my ($type, $name) = ($ref =~ m{^refs/([^/]*)/(.*)}) or return;

	my $out = File::Temp->new() or die;
	my $outname = $out->filename;
	$out->autoflush(1);
	close STDOUT;
	open STDOUT, '>&', $out or die;

	my $headers = {
		'X-Git-Repo' => $repo,
		'X-Git-Old-SHA' => $old,
		'X-Git-New-SHA' => $new,
	};

	my $send;
	if ($type eq 'heads') { $send = update_branch($name, $old, $new, $out, $headers); }
	elsif ($type eq 'tags') { $send = update_tag($name, $old, $new, $out, $headers); }
	$out->close();
	$send or return;

	if (defined $mail_to) {
		close STDIN;
		open STDIN, '<', $outname;
		my @mutt = (
			'mutt',
			'-F/dev/null',
			'-x',
			'-e', 'set charset="utf-8"',
			'-e', 'set send_charset="us-ascii:iso-8859-2:utf-8"',
			'-s', $headers->{'Subject'},
		);
		delete $headers->{'Subject'};
		push @mutt, map { ('-e', "my_hdr $_: " . $headers->{$_}) } keys %$headers;
		system @mutt, $mail_to;
	} else {
		open STDOUT, '>&', \*ORIG_STDOUT;
		print map { "$_: " . $headers->{$_} . "\n" } sort keys %$headers;
		print "\n";
		system 'cat', $outname;
		print "\n";
	}
}