#!/bin/bash
#
# Copyright (c) 2012-2013 Luke Shumaker <lukeshu@sbcglobal.net>
#

set -eE

USAGE='[-h|-v|--no-tag|--tag=tag] <in-branch> <out-branch> <filters>...'
LONG_USAGE='Like filter-branch, but can be used to update branches.

This creates or updates <out-branch> from <in-branch>.  If <in-branch> already
exists, it only runs new commits through the filter.  This way, this can be
used to pull from a remote repository with a different layout than yours.

New commits are detected by examining a "tag" in the commit messages of
<out-branch> that can be used to tie those commits to corresponding commits in
<in-branch>.  By default, the tag is "git-rewrite-id: " followed by the commit
hash of the corresponding commit; similar to the "git-svn-id" appended by
`git-svn`.  However, if <in-branch> already has suitable tags in the commit
messages (such as "git-svn-id"), then that can be used, and the commit message
does not need to be edited.

    -h               Show this text
    -v               Be more verbose
    --svn            Use an existing "git-svn-id" instead of "git-rewrite-id"
                     (in-branch was generated by git-svn)
    --tag=<tag>      Use the exiting <tag> as found in <in-branch>, instead of
                     the commit hash.
    --no-tag         Disregard a previous "--tag="; set "git-rewrite-id" to
                     the commit hash.
    --svn            An alias for "--tag=git-svn-id"

<filters> is passed directly to `git filter-branch`, and therefore has the same
format.
'
SUBDIRECTORY_OK=Yes
OPTIONS_SPEC=
. git-sh-setup
. git-sh-i18n
require_work_tree_exists

# I $hastag is false, $ibranch's commit hashes are used as IDs
hastag=false # whether ibranch already has tags
tag='git-rewrite-id'

ibranch=''
obranch=''
wbranch=''

panic() {
	echo 'panic: malformed call to internal function' >&2
	exit 1
}

# Usage: verbose format arg1 arg2...
# Print the printf string, but only if in verbose mode.
# Verbose mode works by redefining this function during option parsing if -v is
# encountered.
verbose() {
	:
}

################################################################################

# Usage: id2commit $branch $id
# Returns the commit on $branch with $id.
id2commit() {
	[[ $# == 2 ]] || panic
	local branch=$1
	local id=$2
	if [[ $branch == $ibranch ]] && ! $hastag; then
		printf '%s\n' "$id"
	else
		git log "$branch" --pretty=format:'%H' --grep "${tag}: ${id}"
	fi
}

# Usage: commit2id $branch $commit
# Returns the id of $commit, which is on $branch.
commit2id() {
	[[ $# == 2 ]] || panic
	local branch=$1
	local commit=$2
	if [[ $branch == $ibranch ]] && ! $hastag; then
		printf '%s\n' "$commit"
	else
		git log "$branch" -n1 --pretty=formtat:'%B' "$commit" | sed -n "s|^\s*${tag}: ||p"
	fi
}

# commit2commit
# Usage: c2c $from $to $commit
# Returns the commit on branch $to that has the same id as $commit, which is
# on branch $from.
c2c() {
	[[ $# == 3 ]] || panic
	local from=$1
	local to=$2
	local commit=$3
	if [[ "$from" == "$to" ]]; then
		# optimization
		# also, properly normalizes $ibranch when $hastag=false
		git log "$commit" -n1 --pretty=format:'%H'
	else
		id2commit "$to" "$(commit2id "$from" "$commit")"
	fi
}

################################################################################

main() {
	# Parse command line arguments #########################################
	while true; do
		case "${1:-}" in
			--svn)    hastag=true;  tag='git-svn-id'    ; shift 1 ;;
			--tag=*)  hastag=true;  tag="${1#*=}"       ; shift 1 ;;
			--tag)    hastag=true;  tag=$2              ; shift 2 ;;
			--no-tag) hastag=false; tag='git-rewrite-id'; shift 1 ;;
			-h) usage; return 0;;
			-v)
				verbose() {
					local fmt=$1
					shift
					printf "${fmt}\n" "$@"
				}
				shift
				;;
			*) break;;
		esac
	done

	if [[ $# < 3 ]]; then
		usage
		return 1
	fi

	ibranch=$1
	obranch=$2
	shift 2

	local filters=();
	if ! $hastag; then
		# Add a filter to append the id to the commit message.
		# This is a little confusing to read because of double quoting.
		filters=(--msg-filter "sed '\$a'\"${tag}: \${GIT_COMMIT}\"")
		# Here it is annotated; "@" indicates characters to be taken
		# literally, and "^" indicates string interpolation.
		#
		# Argument to git-filter-branch:
		#    sed '$a'"${tag}: ${GIT_COMMIT}"
		#    @@@@@@@@@^^^^^^@@@@@@@@@@@@@@@@
		# Argument to sed:
		#    $a${tag}: ${GIT_COMMIT}
		#    @@^^^^^^@@^^^^^^^^^^^^^
	fi
	filters+=("$@")

	# Main #################################################################

	wbranch="$obranch.tmp"

	local revlist
	local rebase
	if git checkout "$obranch" -- 2>/dev/null; then
		# obranch exists, update it
		echo 'Updating existing rewritten branch...'

		local icommit="$(c2c "$ibranch" "$ibranch" "$ibranch")"
		local ocommit="$(c2c "$obranch" "$ibranch" "$obranch")"
		if [[ "$icommit" == "$ocommit" ]]; then
			echo "Nothing to do"
			return 0
		fi

		local common="$(c2c "$obranch" "$ibranch" "$obranch")"
		if c2c "$ibranch" "$ibranch" "${common}^" &>/dev/null; then
			revlist="$(c2c "$obranch" "$ibranch" "$obranch")^..${wbranch}"
		else
			# There is only one commit from $ibranch in $obranch
			revlist="$wbranch"
		fi
		rebase=true
	else
		# obranch does not exist, create it
		echo 'Creating new rewritten branch...'
		revlist=$wbranch
		rebase=false
	fi

	git checkout "$ibranch"
	git branch -D "$wbranch" 2>/dev/null || true
	git checkout -b "$wbranch"
	git filter-branch -f "${filters[@]}" "$revlist"

	if $rebase; then
		# rebase the changes in wbranch onto obranch
		echo 'Rebasing rewrites onto existing branch...'
		local wcommit="$(c2c "$wbranch" "$ibranch" "$wbranch")"
		if [[ "$wcommit" == "$ocommit" ]]; then
			echo "Nothing to do"
			return 0
		fi

		local commonish="$(c2c "$obranch" "$wbranch" "$obranch")"
		cmd=(git rebase --onto "$obranch" "$commonish" "$wbranch")

		verbose ''
		verbose '  o---o---o %s' "$obranch"
		verbose '          :'
		verbose '  o---o---o---o---o %s' "$ibranch"
		verbose '       \  :'
		verbose '        `-C---o---o %s' "$wbranch"
		verbose ''
		verbose '  C = %s' "$commonish"
		verbose ''
		verbose '  %s' "$(printf '%q ' "${cmd[@]}")"
		verbose ''

		"${cmd[@]}"
		git checkout "$obranch"
		git branch -d "$wbranch"
	else
		git branch -m "$wbranch" "$obranch"
	fi
}

main "$@"