#!/usr/bin/perl
#
# REuse REcorded REsolve.  This tool records a conflicted automerge
# result and its hand resolution, and helps to resolve future
# automerge that results in the same conflict.
#
# To enable this feature, create a directory 'rr-cache' under your
# .git/ directory.

use Digest;
use File::Path;
use File::Copy;

my $git_dir = $::ENV{GIT_DIR} || ".git";
my $rr_dir = "$git_dir/rr-cache";
my $merge_rr = "$git_dir/rr-cache/MERGE_RR";

my %merge_rr = ();

sub read_rr {
        if (!-f $merge_rr) {
                %merge_rr = ();
                return;
        }
        my $in;
        local $/ = "\0";
        open $in, "<$merge_rr" or die "$!: $merge_rr";
        while (<$in>) {
                chomp;
                my ($name, $path) = /^([0-9a-f]{40})\t(.*)$/s;
                $merge_rr{$path} = $name;
        }
        close $in;
}

sub write_rr {
        my $out;
        open $out, ">$merge_rr" or die "$!: $merge_rr";
        for my $path (sort keys %merge_rr) {
                my $name = $merge_rr{$path};
                print $out "$name\t$path\0";
        }
        close $out;
}

sub compute_conflict_name {
        my ($path) = @_;
        my @side = ();
        my $in;
        open $in, "<$path"  or die "$!: $path";

        my $sha1 = Digest->new("SHA-1");
        my $hunk = 0;
        while (<$in>) {
                if (/^<<<<<<< .*/) {
                        $hunk++;
                        @side = ([], undef);
                }
                elsif (/^=======$/) {
                        $side[1] = [];
                }
                elsif (/^>>>>>>> .*/) {
                        my ($one, $two);
                        $one = join('', @{$side[0]});
                        $two = join('', @{$side[1]});
                        if ($two le $one) {
                                ($one, $two) = ($two, $one);
                        }
                        $sha1->add($one);
                        $sha1->add("\0");
                        $sha1->add($two);
                        $sha1->add("\0");
                        @side = ();
                }
                elsif (@side == 0) {
                        next;
                }
                elsif (defined $side[1]) {
                        push @{$side[1]}, $_;
                }
                else {
                        push @{$side[0]}, $_;
                }
        }
        close $in;
        return ($sha1->hexdigest, $hunk);
}

sub record_preimage {
        my ($path, $name) = @_;
        my @side = ();
        my ($in, $out);
        open $in, "<$path"  or die "$!: $path";
        open $out, ">$name" or die "$!: $name";

        while (<$in>) {
                if (/^<<<<<<< .*/) {
                        @side = ([], undef);
                }
                elsif (/^=======$/) {
                        $side[1] = [];
                }
                elsif (/^>>>>>>> .*/) {
                        my ($one, $two);
                        $one = join('', @{$side[0]});
                        $two = join('', @{$side[1]});
                        if ($two le $one) {
                                ($one, $two) = ($two, $one);
                        }
                        print $out "<<<<<<<\n";
                        print $out $one;
                        print $out "=======\n";
                        print $out $two;
                        print $out ">>>>>>>\n";
                        @side = ();
                }
                elsif (@side == 0) {
                        print $out $_;
                }
                elsif (defined $side[1]) {
                        push @{$side[1]}, $_;
                }
                else {
                        push @{$side[0]}, $_;
                }
        }
        close $out;
        close $in;
}

sub find_conflict {
        my $in;
        local $/ = "\0";
        my $pid = open($in, '-|');
        die "$!" unless defined $pid;
        if (!$pid) {
                exec(qw(git ls-files -z -u)) or die "$!: ls-files";
        }
        my %path = ();
        my @path = ();
        while (<$in>) {
                chomp;
                my ($mode, $sha1, $stage, $path) =
                    /^([0-7]+) ([0-9a-f]{40}) ([123])\t(.*)$/s;
                $path{$path} |= (1 << $stage);
        }
        close $in;
        while (my ($path, $status) = each %path) {
                if ($status == 14) { push @path, $path; }
        }
        return @path;
}

sub merge {
        my ($name, $path) = @_;
        record_preimage($path, "$rr_dir/$name/thisimage");
        unless (system('git merge-file', map { "$rr_dir/$name/${_}image" }
                       qw(this pre post))) {
                my $in;
                open $in, "<$rr_dir/$name/thisimage" or
                    die "$!: $name/thisimage";
                my $out;
                open $out, ">$path" or die "$!: $path";
                while (<$in>) { print $out $_; }
                close $in;
                close $out;
                return 1;
        }
        return 0;
}

-d "$rr_dir" || exit(0);

read_rr();
my %conflict = map { $_ => 1 } find_conflict();

# MERGE_RR records paths with conflicts immediately after merge
# failed.  Some of the conflicted paths might have been hand resolved
# in the working tree since then, but the initial run would catch all
# and register their preimages.

for my $path (keys %conflict) {
        # This path has conflict.  If it is not recorded yet,
        # record the pre-image.
        if (!exists $merge_rr{$path}) {
                my ($name, $hunk) = compute_conflict_name($path);
                next unless ($hunk);
                $merge_rr{$path} = $name;
                if (! -d "$rr_dir/$name") {
                        mkpath("$rr_dir/$name", 0, 0777);
                        print STDERR "Recorded preimage for '$path'\n";
                        record_preimage($path, "$rr_dir/$name/preimage");
                }
        }
}

# Now some of the paths that had conflicts earlier might have been
# hand resolved.  Others may be similar to a conflict already that
# was resolved before.

for my $path (keys %merge_rr) {
        my $name = $merge_rr{$path};

        # We could resolve this automatically if we have images.
        if (-f "$rr_dir/$name/preimage" &&
            -f "$rr_dir/$name/postimage") {
                if (merge($name, $path)) {
                        print STDERR "Resolved '$path' using previous resolution.\n";
                        # Then we do not have to worry about this path
                        # anymore.
                        delete $merge_rr{$path};
                        next;
                }
        }

        # Let's see if we have resolved it.
        (undef, my $hunk) = compute_conflict_name($path);
        next if ($hunk);

        print STDERR "Recorded resolution for '$path'.\n";
        copy($path, "$rr_dir/$name/postimage");
        # And we do not have to worry about this path anymore.
        delete $merge_rr{$path};
}

# Write out the rest.
write_rr();