#!/bin/sh
#
# git-merge-ff-only, Copyright © 2012 Matteo Cypriani
#
# This program is free software. It comes without any warranty, to
# the extent permitted by applicable law. You can redistribute it
# and/or modify it under the terms of the Do What The Fuck You Want
# To Public License, Version 2, as published by Sam Hocevar. See
# http://sam.zoy.org/wtfpl/COPYING for more details.
#
# This script fast-forwards a branch (main_branch) to another branch
# (topic_branch), if possible. It is equivalent to git checkout
# main_branch && git merge --ff-only topic_branch, but the effort of
# checking out the main branch is saved (git branch -f is used to update
# the reference directly).

set -e

USAGE=$(cat <<EOF
Usage:
  $0 <main_branch> <topic_branch>"

<main_branch> will be fast-forwarded to <topic_branch> if possible."
EOF
)

if [ $# -ne 2 ] ; then
	printf '%s\n' "$USAGE" >&2
	exit 1
fi

MAIN_BRANCH="$1"
TOPIC_BRANCH="$2"

echo "Trying to merge $TOPIC_BRANCH into $MAIN_BRANCH..."

DIFF="$(git rev-list "$TOPIC_BRANCH".."$MAIN_BRANCH")"
if [ -z "$DIFF" ] ; then
	echo -- "$TOPIC_BRANCH contains all the commits of $MAIN_BRANCH."
	echo "Fast-forwarding $MAIN_BRANCH to $TOPIC_BRANCH."
	git branch -f "$MAIN_BRANCH" "$TOPIC_BRANCH"
else
	echo -- "$TOPIC_BRANCH does not contain all the commits of $MAIN_BRANCH."
	echo "Fast-forward impossible, aborting."
fi