#!/usr/bin/env bash
set -euo pipefail

if [ "$#" -lt 2 ]; then
	echo "Usage: $0 <source-branch> <new-branch> [-m \"commit message\"]"
	exit 1
fi

source_branch="$1"
new_branch="$2"
shift 2

msg=""
while [[ $# -gt 0 ]]; do
	case "$1" in
		-m|--message)
			shift
			msg="$1"
			shift
			;;
		*)
			echo "Unknown option: $1"
			exit 1
			;;
	esac
done

if [ -z "$msg" ]; then
	echo "Commit message required (-m)"
	exit 1
fi

# check if source branch exists
if ! git rev-parse --verify "$source_branch" >/dev/null 2>&1; then
	echo "Source branch '$source_branch' does not exist"
	exit 1
fi

# check if new branch already exists
if git rev-parse --verify "$new_branch" >/dev/null 2>&1; then
	echo "Branch '$new_branch' already exists"
	exit 1
fi

# check for staged changes
if git diff --cached --quiet; then
	echo "No staged changes to commit"
	exit 1
fi

# 1. save staged diff
patchfile=$(mktemp)
git diff --cached > "$patchfile"

# 2. prepare a temporary index rooted at source branch
idx=$(mktemp)
GIT_INDEX_FILE="$idx" git read-tree "$source_branch"

# 3. apply the staged diff to that index
if ! GIT_INDEX_FILE="$idx" git apply --cached --allow-empty "$patchfile"; then
	echo "Failed to apply staged changes on top of $source_branch"
	rm -f "$patchfile" "$idx"
	exit 1
fi

# 4. create commit from that temporary index
tree=$(GIT_INDEX_FILE="$idx" git write-tree)
commit=$(echo "$msg" | GIT_INDEX_FILE="$idx" git commit-tree -p "$source_branch" "$tree")

# 5. create branch pointing at the new commit
git branch "$new_branch" "$commit"

echo "Created branch '$new_branch' from '$source_branch' at commit $commit"
rm -f "$patchfile" "$idx"

# 6. remove all the staged changes
git stash push --staged --quiet
git stash drop --quiet