split.sh

#!/usr/bin/env bash

# usage: split.sh <source_branch> <target_branch> [<main_branch>]
# example: split.sh origin/wip-feature-dirty feature-clean origin/main

# I often work on long-living dirty feature branches, saving garbage "wip"
# checkpoints, before I fully understand what am I even doing.

# When the feature is finished and tested, I end up with the garbage log that's
# mostly meaningless, but not quite. I find it hard to separate sensible commit
# messages from those that served only as a snapshot of some intermittent state.

# Usually the main units I can reason about are files/directories/namespaces etc.
# So before submitting a proper PR, I need to, not only rebase against the main branch,
# but also tidy up the log.

# This script aims to help with that, provided I know which _paths_ should
# eventually end up in the clean, public branch annotated with descriptive commits
# others could then review.

# The script will open a new fzf multi picker window with all the files
# changed between <source_branch> and <main_branch>.
# After selecting one or more files, it will switch to <target_branch>,
# creating it, if necessary, and then restore selected file(s) into it.

# The commit message will be pre-populated with a comment containing all
# the commit messages associated with selected file(s), as seen in <source_branch>.

# In case multiple authors were involved, the Co-authored-by header(s) will be
# automatically added.

myself=$(git config user.email)
src="$1"

if [ -z "$2" ]; then
	target=$(git rev-parse --abbrev-ref HEAD)
else
	target="$2"
fi

if [ -z "$src" ] || [ -z "$target" ]; then
	echo "usage: split.sh <source_branch> <target_branch>"
	exit 1
fi

if [ -z "$3" ]; then
	main="origin/master"
else
	main="$3"
fi

if ! git rev-parse --verify "$main" >/dev/null 2>&1; then
	echo "error: main branch '$main' does not exist"
	exit 1
fi

if ! git rev-parse --verify "$src" >/dev/null 2>&1; then
	echo "error: source branch '$src' does not exist"
	exit 1
fi

base=$(git merge-base "$src" "$main")
preview="git diff $base $src --color=always -- {-1}"
selection=$(git diff --name-only "$src" "$base" | fzf --multi --ansi --preview "$preview")

if [ -z "${selection}" ]; then
	echo "Nothing to split"
	exit 1
else
	git switch -c "$target"
	tmp=$(mktemp)
	printf "\n\n" >"$tmp"

	coauth=$(git log --format="Co-authored-by: %aN <%ae>" "$main..$src" -- $selection | sort | uniq | grep -v "$myself")

	for arg in $selection; do
		messages=$(git log --format="%h %s" "$main..$src" -- "$arg")
		git restore --source "$src" --staged --worktree -- "$arg"

		printf "%s - restored from:\n\n%s\n\n" "$arg" "$messages" >>"$tmp"
	done

	echo "$coauth" >>"$tmp"
	git commit -t "$tmp"
fi