#!/usr/bin/perl -w
# git-debrebase
# Script helping make fast-forwarding histories while still rebasing
# upstream deltas when working on Debian packaging
#
# Copyright (C)2017 Ian Jackson
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.

# usages:
#    git-debrebase status
#    git-debrebase start       # like ffqrebase start + debrebase launder
#    git-debrebase new-upstream [stuff]  # see below
#    git-debrebase <git-rebase options>  # does debrebase start if necessary
#
#    git-debrebase analyse
#    git-debrebase launder     # prints breakwater tip
#    git-debrebase create-new-upstream-breakwater [-f] <upstreaminfo>...
#
# <upstreaminfo> is
#    [,][<subdir>:][+]<commitid>[,...]
#
# if initial comma is supplied, entries are not positional.  Unspecified
# <subdir> means root (and there may be only one).
# xxx want auto branch names
# xxx too complicated
# how about for now
#    [+]<commit> [<subdir/> [+]<commit>...]
# ?  plus options
#     --new-upstream-different-subtrees
#
#  automatic case
#       git-debrebase new-upstream
#             - previous breakwater merge must be gdr-generated
#             - orig set is the same as before
#             - implicitly uses upstream branches according to orig set
#             - not all upstream branches need be updated
#             - insists on fast-forward of each branch, unless
#                  --force (or --force=<subdir>[/])
#  branch set adjustments
#       git-debrebase new-upstream --add <subdir>/
#       git-debrebase new-upstream --rm <subdir>/
#       git-debrebase new-upstream / [<subdir>/ ...]
#             - orig set is adjusted
#             - otherwise like auto (--add is not checked for ffness, obv)
#             - multiple --add and --rm may be specified
#             - --add makes new upstream the last contributor
#  explicit
#       git-debrebase / [<rootcommitid>] [<subdir>/ [<commitid>] ...]
#             - orig set is precisely as specified now
#             - previous breakwater merge is irrelevant
#             - no fast forward checks
#  for now only explicit with commitids

    # implicitly uses `upstream'
#                                     # (or multiple other branches)
#       git-debrebase new-upstream \
#             [<subdir>/]=<commitid>

#    UPSTREAM[,[[SUBDIR:]SUBUPSTREAM]
#    default for SUBDIR: is from previous upstream merge[xxx terminology]
#    
#
#xxx
# when starting must record original start (for ff)
# and new rebase basis
#
#    git-ffqrebase start [BASE]
#                # records previous HEAD so it can be overwritten
#                # records base for future git-ffqrebase
#    git-ffqrebase set-base BASE
#    git-ffqrebase <git-rebase options>
#    git-ffqrebase finish
#    git-ffqrebase status [BRANCH]
#
#  refs/ffqrebase-prev/BRANCH    BRANCH may be refs/...; if not it means
#  refs/ffqrebase-base/BRANCH      refs/heads/BRANCH
#                               zero, one, or both of these may exist

use strict;

use Memoize;
use Data::Dumper;

use Debian::Dgit qw(:DEFAULT $wa);

sub cfg ($) {
    my ($k) = @_;
    $/ = "\0";
    my @cmd = qw(git config -z);
    push @cmd, qw(--get-all) if wantarray;
    push @cmd, $k;
    my $out = cmdoutput @cmd;
    return split /\0/, $out;
}

memoize('cfg');

sub get_commit ($) {
    my ($objid) = @_;
    my ($type,$data) = git_cat_file $objid;
    die unless $type eq 'commit';
    $data =~ m/(?<=\n)\n/;
    return ($`,$');
}

sub D_DEB ()     { return 0x1; }
sub D_UPS ()     { return 0x2; }
sub D_PAT_ADD () { return 0x4; }
sub D_PAT_OTH () { return 0x8; }

our $rd = ".git/git-debrebase";
our $ud = "$rd/work";

sub commit_pr_info ($) {
    my ($r) = @_;
    return Data::Dumper->dump([$r], [qw(commit)]);
}

sub calculate_committer_authline () {
    my $c = cmdoutput @git, qw(commit-tree --no-gpg-sign -m),
	'XXX DUMMY COMMIT (git-debrebase)', "$basis:";
    my ($h,$m) = get_commit $c;
    $h =~ m/^committer .*$/m or confess "($h) ?";
    return $&;
}

sub classify ($) {
    my ($objid) = @_;

    my ($h,$m) = get_commit $objid;

    my ($t) = $h =~ m/^tree (\w+)$/m or die $cur;
    my (@ph) = $h =~ m/^parent (\w+)$/m;
    my @p;

    my $r = {
	CommitId => $objid,
	Hdr => $hdr,
	Msg => $m,
        Tree => $t,
	Parents => \@p,
    };

    foreach my $ph (@ph) {
	push @p, {
            Ix => $#p,
            CommitId => $ph,
            Differs => (get_differs $t, $ph),
        };
    }

    my $classify = sub {
	my ($type, @rest) = @_;
	$r = { %r, Type => $type, @rest };
	return $r;
    };
    my $unknown = sub {
	my ($why) = @_;
	$r = { %r, Type => Unknown };
	return $r;
    }

    if (@p == 1) {
	my $d = $r->{Parents}[0]{Differs};
	if ($d == D_DPAT_ADD) {
	    return $classify->(qw(AddPatches));
	} elsif ($d & (D_DPAT_ADD|D_DPAT_OTH)) {
	    return $unknown->("edits debian/patches");
	} elsif ($d == D_DEB) {
	    return $classify->(qw(Packaging));
	} elsif ($d == D_UPS) {
	    return $classify->(qw(Upstream));
	} elsif ($d == D_DEB|D_UPS) {
	    return $classify->(qw(Mixed));
	} elsif ($d == 0) {
	    return $unknown->("no changes");
	} else {
	    confess "internal error $objid ?";
	}
    }
    if (!@p) {
	return $unknown->("origin commit");
    }

    my @identical = grep { !$_->{Differs} } @p;
    if (@p == 2 && @identical == 1) {
	my @overwritten = grep { $_->{Differs} } @p;
	confess "internal error $objid ?" unless @overwritten==1;
	return $classify->(qw(Pseudomerge),
			   Overwritten => $overwritten[0],
			   Contributor => $identical[0]);
    }
    if (@p == 2 && @identical == 2) {
	my @bytime = nsort_by {
	    my ($ph,$pm) = get_commit $_->{CommitId};
	    $ph =~ m/^committer .* (\d+) [-+]\d+$/m or die "$_->{CommitId} ?";
	    $1;
	} @p;
	return $classify->(qw(Pseudomerge),
			   SubType => qw(Ambiguous),
			   Overwritten => $bytime[0],
			   Contributor => $bytime[1]);
    }!
    foreach my $p (@p) {
	my ($p_h, $p_m) = get_commit $p;
	$p->{IsOrigin} = $p_h !~ m/^parent \w+$/m;
	($p->{IsDgitImport},) = $p_m =~ m/^\[dgit import ([0-9a-z]+) .*\]$/m;
    }
    my @orig_ps = grep { ($_->{IsDgitImport}//'X') eq 'orig' };
    my $m2 = $m;
    if (!(grep { !$_->{IsOrigin} } @p) and
	(@origs >= @p - 1) and
	$m2 =~ s{^\[(dgit import unpatched .*)\]$}{[was: $1]}m) {
	$r->{NewMsg} = $m2;
	return $classify->(qw(DgitImportUnpatched),
			   OrigParents => \@orig_ps);
    }

    my ($stype, $series) = git_cat_file "$t:debian/patches/series";
    my $haspatches = $stype ne 'missing' && $series =~ m/^\s*[^#\n\t ]/m;

    # How to decide about l/r ordering of breakwater merges ?  git
    # --topo-order prefers to expand 2nd parent first.  There's
    # already an easy rune to look for debian/ history anyway (git log
    # debian/) so debian breakwater branch should be 1st parent; that
    # way also there's also an easy rune to look for the upstream
    # patches (--topo-order).
    if (@p == 2 &&
	!$haspatches &&
	!$p[0]{IsOrigin} && # breakwater merge never starts with an origin
	!($p[0]{Differs} & ~D_DEB) &&
	!($p[1]{Differs} & ~D_UPS)) {
	return $classify->(qw(BreakwaterUpstreamMerge),
			   Upstream => $p[1]);
    }

    return $unknown->("complex merge");
}

sub launder ($$$) {
    my ($input, $pseudos_must_overwrite_this, $wantdebonly) = @_;
    # go through commits backwards
    # we generate two lists of commits to apply
    my (@deb_cl, @ups_cl, @processed);
    my %found;
    my @pseudomerges;

    my $cl;
    my $xmsg = sub {
	my ($appendinfo) = @_;
	my $ms = $cl->{Msg};
	chomp $ms;
	$ms .= "\n\n[git-debrebase $appendinfo]\n";
	return (Msg => $ms);
    };
    my $rewrite_from_here = sub {
	push @processed, { SpecialMethod => 'StartRewrite' };
    };

    my $cur = $input;

    for (;;) {
	$cl = classify $cur;
	my $ty = $cl->{Type};
	my $st = $cl->{SubType};
	$found{$ty. ( defined($st) ? "-$st" : '' )}++;
	my $p0 = $cl->{Parents}[0]{CommitId};
	if ($ty eq 'AddPatches') {
	    $cur = $p0;
	    $rewrite_from_here->();
	    next;
	} elsif ($ty eq 'Packaging') {
	    push @deb_cl, $cl;
	    push @processed, $cl;
	    $cur = $p0;
	    next;
	} elsif ($ty eq 'Upstream') {
	    push @ups_cl, $cl;
	    push @processed, $cl;
	    $cur = $p0;
	    next;
	} elsif ($ty eq 'Mixed') {
	    my $queue = sub {
		my ($q, $wh) = @_;
		my $cls = { $cl, $xmsg->("split mixed commit: $wh part") };
		push @$q, $cls;
	    };
	    $queue->(\@deb_cl, "debian");
	    $queue->(\@ups_cl, "upstream");
	    $rewrite_from_here->();
	    next;
	} elsif ($ty eq 'Pseudomerge') {
	    if (defined $pseudos_must_overwrite_this) {
		confess 'xxx actually check this';
	    }
	    push @pseudomerges, $cl;
	    $rewrite_from_here->();
	    $cur = $ty->{Contributor};
	    next;
	} elsif ($ty eq 'BreakwaterUpstreamMerge') {
	    $basis = $cur;
	    last;
	} elsif ($ty eq 'DgitImportUnpatched' &&
		 @pseudomerges == 1) {
	    # This import has a tree which is just like a breakwater
	    # tree, but it has the wrong history.  Its ought to have
	    # the previous breakwater (which dgit ought to have
	    # generated a pseudomerge to overwrite) as an ancestor.
	    # That will make the history of the debian/ files correct.
	    # As for the upstream version: either it's the same upstream
	    # as the previous breakwater, in which case that history is
	    # precisely right.  Otherwise, it was a non-gitish upload
	    # of a new upstream version.  We can tell these apart
	    # by looking at the tree of the supposed upstream.
	    if ($differs & D_UPS) {
		push @deb_cl, {
                    %r,
                    SpecialMethod => 'DgitImportUpstreamUpdate',
                    $xmsg->("convert dgit import: debian changes")
                };
	    }
	    push @deb_cl, {
		%r,
		SpecialMethod => 'DgitImportDebianUpdate',
                $xmsg->("convert dgit import: upstream changes")
	    };
	    my $differs = get_differs $previous_breakwater, $cl->{Tree};
	    $basis = launder $pseudomerges[0]{Overwritten}, undef, 1;
	    $rewrite_from_here->();
	    last;
	} else {
	    die "Reached difficult commit $cur: ".Dumper($cl);
	}
    }
    # Now we build it back up again

    workarea_fresh();
    in_workarea sub { xxx attributes xxx };

    my $rewriting = 1;

    my $build = $basis;

    my $rm_tree_cached = sub {
	my ($subdir) = @_;
	runcmd @git, qw(rm --quiet -rf --cached), $subdir;
    };
    my $read_tree_debian = sub {
	my ($treeish) = @_;
	$rm_tree_cached->(qw(debian));
	runcmd @git, qw(read-tree --prefix=debian/), "$treeish:debian";
    };
    my $read_tree_upstream = sub {
	my ($treeish) = @_;
	runcmd @git, qw(read-tree), $treeish;
	$read_tree_debian->($build);
    };
 
    my $committer_authline = calculate_committer_authline();

    in_workarea sub {
	mkdir $rd or $!==EEXIST or die $!;
	my $current_method;
	foreach my $cl (qw(Debian), (reverse @deb_cl),
			{ SpecialMethod => 'RecordBreakwaterTip' },
			qw(Upstream), (reverse @ups_cl)) {
	    if (!ref $cl) {
		$current_method = $cl;
		next;
	    }
	    $method = $cl->{SpecialMethod} // $current_method;
	    my @parents = ($build);
	    my $cltree = $cl->{CommitId}
	    if ($method eq 'Debian') {
		$read_tree_debian->($cltree);
	    } elsif ($method eq 'Upstream') {
		$read_tree_upstream->($cltree);
	    } elsif ($method eq 'StartRewrite') {
		$rewriting = 1;
		next;
	    } elsif ($method eq 'RecordBreakwaterTip') {
		last if $wantdebonly;
		$breakwater = $build;
		next;
	    } elsif ($method eq 'DgitImportDebianUpdate') {
		$read_tree_debian->($cltree);
		$rm_tree_cached(qw(debian/patches));
	    } elsif ($method eq 'DgitImportUpstreamUpdate') {
		$read_tree_upstream->($cltree);
		push @parents, map { $_->{CommitId} } @{ $cl->{OrigParents} };
	    } else {
		confess "$method ?";
	    }
	    $rewriting ||= $cl ne pop @processed;
	    my $newtree = cmdoutput @git, qw(write-tree);
	    my $ch = $cl->{Hdr};
	    $ch =~ s{^tree .*}{tree $newtree}m or confess "$ch ?";
	    $ch =~ s{^parent .*\n}{}m;
	    $ch =~ s{(?=^author}{
		map { "parent $_\n" } @parents
	    }me or confess "$ch ?";
	    if ($rewrite) {
		$ch =~ s{^committer .*$}{$committer_authline}m
		    or confess "$ch ?";
	    }
	    my $cf = "$rd/m$rewrite"
		open CD, ">", $cf or die $!;
	    print CD $ch, "\n", $cl->{Msg}; or die $!;
	    close CD or die $!;
	    my @cmd = (@git, qw(hash-object));
	    push @cmd, qw(-w) if $rewrite;
	    push @cmd, qw(-t commit), $cf;
	    my $newcommit = cmdoutput @cmd;
	    confess "$ch ?" unless $rewrite or $newcommit eq $cl->{CommitId};
	    $build = $newcommit;
	}
    };

    runcmd @git, qw(diff-tree --quiet),
	map { $wantdebonly ? "$_:debian" : $_ },
	$input, $build;

    return ($build, $breakwater);
}

sub get_head () { return git_rev_parse qw(HEAD); }

sub update_head ($$) {
    my ($old, $new, $mrest) = @_;
    runcmd @git, qw(update-ref -m), "git-debrebase $mrest", $new, $old;
}

sub cmd_launder () {
    my $old = get_head();
    my ($tip,$breakwater) = launder $old, 0, undef, 0;
    update_head $old, $tip, 'launder'; # no tree changes! xxx debian/patches
    printf "# breakwater tip:\n%s\n", $breakwater;
}

my $toplevel = runcmd @git, qw(rev-parse --show-toplevel);
chdir $toplevel or die "chdir $toplevel: $!";

my $cmd = shift @ARGV;
my $cmdfn = $cmd;
$cmdfn =~ y/-/_/;
$cmdfn = ${*::}{"cmd_$cmdfn"};

$cmdfn or badusage "unknown git-debrebase sub-operation $cmd";
$cmdfn->();