#!/usr/bin/env bash

# Copyright 2016 Google Inc.
#
# 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; version 2 of the License.
#
# 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.

# $0 from-branch to-branch
#
# applies all commits that from-branch has over to-branch,
# based on a common ancestor and gerrit meta-data
from=$1
to=$2

# match string: this is the git commit line that is used to
# identify commits that were already copied over.
#
# Must not contain spaces except for leading and trailing.
#
# The first pick was Change-Id, but it was lost too often,
# so go for Reviewed-on instead. It's also unique because it
# contains the gerrit instance's host name and the change's number
# on that system.
match_string='[-A-Za-z]*[Rr]eviewed-on:'

# Custom root: allow a certain CL (identified by matching either side's
# match_string) to be the new root, instead of using git's common history only.
# This allows cutting down on commits that are re-evaluated on every run.
#
# Use:
# To the commit message of a commit on the "to" side, add
# $custom_root: match_string (the part coming after $match_string)
#
# For a $match_string of ~ "Reviewed-on: " this might
# be "$custom_root: https://example.com/12345"
#
# On traversal, the commit with "$match_string: https://example.com/12345"
# is then considered a base commit.
custom_root='^Gerrit-Rebase-Ignore-CLs-Before:'

# fetch common ancestor
common_base=$(git merge-base ${from} ${to} 2>/dev/null)

if [ -z "${common_base}" ]; then
	echo \"${from}\" or \"${to}\" is not a valid branch name.
	exit 1
fi

from_base=$common_base

# fetch custom root ID
croot_marker=$(git log --pretty=%b -1 --grep "${custom_root}" \
	${common_base}..${to} | \
	grep "${custom_root}" | cut -d: -f2-)
if [ -n "${croot_marker}" ]; then
	from_base=$( ( \
		git log --pretty=%H -1 \
		--grep "^${match_string}${croot_marker}" \
		${from_base}..${from}; echo ${from_base} )| head -1)
fi

# collect matches that are present on the target side
to_matches="$(git log ${common_base}..${to} | \
	grep "^    ${match_string}" | \
	cut -d: -f2-)"

# start rebase process, but fail immediately by enforcing an invalid todo
GIT_SEQUENCE_EDITOR="echo foo >" \
	git rebase -i --onto ${to} ${from} ${to} 2>/dev/null

# write new rebase todo
# the appended "commit" line triggers handling of the last log entry
commit=""
(git log --reverse ${from_base}..${from} | \
	grep -E "(^commit [0-9a-f]{40}\$|^    ${match_string})"; \
	echo "commit") | \
while read key value; do
	if [ "${key}" = "commit" ]; then
		if [ -n "${commit}" ]; then
			git log -n 1 --pretty="pick %h %s" ${commit}
		fi
		commit="${value}"
	else
		# if value was already found on the "to" side, skip this
		# commit
		if [[ ${to_matches} == *"${value}"* ]]; then
			commit=""
		fi
	fi
done | GIT_SEQUENCE_EDITOR="cat >" git rebase --edit-todo

# allow user to edit todo
git rebase --edit-todo

# start processing todo to mimick git rebase -i behavior
git rebase --continue