#!/bin/bash -euE

# when $gitmode is true, $ibranch's commits are used as IDs
gitmode=true
tag='git-rewrite-id'

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

usage() {
	echo 'malformed call to internal function'
}

verbose() {
	: "$*"
	#echo "$*"
}

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

id2commit() {
	[[ $# = 2 ]] || { usage; return 1; }
	local branch=$1
	local id=$2
	if [[ $branch == $ibranch ]] && $gitmode; then
		printf '%s\n' "$id"
	else
		git log "$branch" --pretty=format:'%H' --grep "${tag}: ${id}"
	fi
}

commit2id() {
	[[ $# = 2 ]] || { usage; return 1; }
	local branch=$1
	local commit=$2
	if [[ $branch == $ibranch ]] && $gitmode; then
		printf '%s\n' "$commit"
	else
		git log "$branch" -n1 --pretty=formtat:'%B' "$commit" | sed -n "s|^\s*${tag}: ||p"
	fi
}

# commit2commit
c2c() {
	[[ $# = 3 ]] || { usage; return 1; }
	local from=$1
	local to=$2
	local commit=$3
	if [[ "$from" == "$to" ]]; then
		# optimization
		# also, properly normalizes $ibranch when $gitmode=true
		git log "$commit" -n1 --pretty=format:'%H'
	else
		id2commit "$to" "$(commit2id "$from" "$commit")"
	fi
}

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

main() {
	# Parse command line arguments #########################################
	if [[ $1 = '--svn' ]]; then
		gitmode=false
		tag='git-svn-id'
		shift
	fi

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

	ibranch=$1
	obranch=$2
	shift 2

	local filters=();
	if $gitmode; then
		filters=(--msg-filter "git-rewrite-branch--appendid '${tag}'")
	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 $obranch"
		verbose '          :'
		verbose "  o---o---o---o---o $ibranch"
		verbose '       \  :'
		verbose '        `-C---o---o '"$wbranch"
		verbose
		verbose "  C = $commonish"
		verbose
		verbose "  ${cmd[*]}"
		verbose

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

main "$@"