#!/usr/bin/perl

# Rebases master and everything based on master to the new trunk.  Use
# after a git-svn-fetch.

use strict;
use warnings;

use Getopt::Long;
use List::Util qw(first);

use Git;

my $dry_run;

GetOptions("dry-run|n" => \$dry_run)
  or die "usage error";

sub ref2branch {
    my $ref = shift;
    $ref =~ s,^refs/heads/,,
      or die "Not a branch: '$ref'";
    return $ref;
}

my $repo = Git->repository();

my %remotes_by_name;
my %remotes_by_hash;
my %remote_revs;

for ($repo->command('for-each-ref', 'refs/remotes')) {
    my ($hash, undef, $ref) = split;
    $remotes_by_name{$ref} = $hash;
    $remotes_by_hash{$hash} = $ref;
    $remote_revs{$ref} = [$repo->command('rev-list', $ref)];
}

my %heads_by_name;
my %heads_by_hash;

for ($repo->command('for-each-ref', 'refs/heads')) {
    my ($hash, undef, $ref) = split;
    $heads_by_name{$ref} = $hash;
    $heads_by_hash{$hash} = $ref;
}

my %roots;
my %heads_by_parent;

for my $head (sort keys %heads_by_name) {
    #print STDERR "Considering $head\n";
    my $parent;
    my $last_rev;
    for my $rev ($repo->command('rev-list', $head, '--not', keys %remotes_by_name)) {
        my $maybe_parent = $heads_by_hash{$rev};
        if ($maybe_parent && $maybe_parent ne $head) {
            #print STDERR "  found parent $maybe_parent\n";
            $parent = $maybe_parent;
            last;
        }
        $last_rev = $rev;
    }
    if ($parent) {
        push @{$heads_by_parent{$parent}}, $head;
    } elsif ($last_rev) {
        my $remote_base = $repo->command_oneline('rev-parse', "$last_rev^");
        my @remotes;
        #print STDERR "  last rev $last_rev $remote_base\n";
        for my $remote_name (sort keys %remotes_by_name) {
            my $remote = first { $_ eq $remote_base } @{$remote_revs{$remote_name}};
            if (defined($remote) && $remote eq $remote_base) {
                #print STDERR "  found remote $remote_name\n";
                push @remotes, $remote_name;
            }
        }
        if (@remotes == 1) {
            $roots{$head} = $remotes[0];
        } else {
            print STDERR "WARNING: Not exactly one candidate remote for $head: ",
                join(' ', @remotes), "\n";
        }
    }
}

for my $root (sort keys %roots) {
    my $remote = $roots{$root};
    my $short_root = ref2branch($root);
    $remote =~ s,^refs/,,;
    print "git rebase $remote $short_root\n";
    rebase_tree($root);
}

sub rebase_tree {
    my ($parent) = @_;
    for my $head (@{$heads_by_parent{$parent}}) {
        my $short_parent = ref2branch($parent);
        my $short_head = ref2branch($head);
        print "git rebase --onto $short_parent $heads_by_name{$parent} $short_head\n";
        rebase_tree($head);
    }
}