#!/usr/bin/env bash
#
# Run treefmt on every commit and abort if some files have changed.
#
# To install, copy this file to .git/hooks/pre-commit and make sure it's
# executable.
#
set -euo pipefail

# Redirect stdout to stderr
exec 1>&2

# Get list of files that will be committed
mapfile -t commit_files < <(git diff --name-only --cached)

log() {
    echo "treefmt pre-commit: $*"
}

# If the commit has no files, skip everything as there is nothing to format
if [[ -z ${commit_files+x} ]] || [[ ${#commit_files} = 0 ]]; then
    log "no files to format"
    exit 0
fi

# Will be called at the end
restore_stash() {
    # Store exit status
    local ret=$?
    # Don't fail on error from now on
    set +e
    # Put bash the staged files
    git stash pop -q

    if [[ $ret -gt 0 ]]; then
        log "aborting commit, detected unformatted files"
    fi
    exit "$ret"
}

# Stash index and work dir, keeping only the to-be-committed changes in
# the working directory.
git stash push --quiet --keep-index --message "treefmt pre-commit"

# Install the callback to restore the stash on script exit
trap restore_stash EXIT

# Run treefmt on the files in the index and record the result.
nix fmt -- --no-cache "${commit_files[@]}"

# Check if there is a diff
git diff --name-only --exit-code