conventional-merge-commit

#!/bin/sh
# prepare-commit-msg:
# rewrite auto-generated merge/squash messages to conventional commit format

# shellcheck source=lib/commit-msg.sh disable=SC1091
. "$(dirname "$0")/lib/commit-msg.sh"

commit_msg_file="$1"
commit_source=${2-}

# Short-circuit: if git explicitly tells us it's not a merge/squash/revert, exit early
# This avoids reading/parsing the message for regular commits
case "$commit_source" in
merge | squash | revert | message | "") ;;
*) exit 0 ;;
esac

# Read and parse commit message
read_commit_msg "$commit_msg_file"

# Strip leading whitespace (POSIX parameter expansion)
stripped="${COMMIT_FIRST_LINE#"${COMMIT_FIRST_LINE%%[![:space:]]*}"}"

# Detect message type from content (single source of truth for trigger patterns)
# Word boundary: keyword alone or followed by non-word character
# Ordered by frequency: most common patterns first (Git defaults: "Merge", "Revert", "Squash")
#   ^\s*([Mm]erge[ds]?|[Ss]quash(e[ds])?|[Rr]evert|[Rr]eapply)\b.*$
msg_type=""
case "$stripped" in
[Mm]erge | [Mm]erge[ds] | [Mm]erge[![:alnum:]_]* | [Mm]erge[ds][![:alnum:]_]*) msg_type="merge" ;;
[Rr]evert | [Rr]evert[![:alnum:]_]* | [Rr]eapply | [Rr]eapply[![:alnum:]_]*) msg_type="revert" ;;
[Ss]quash | [Ss]quashe[ds] | [Ss]quash[![:alnum:]_]* | [Ss]quashe[ds][![:alnum:]_]*) msg_type="squash" ;;
esac

# Only process if we detected a merge/squash/revert pattern
if [ -z "$msg_type" ]; then
	exit 0
fi

# Transform based on message type
if [ "$msg_type" = "revert" ]; then
	# For revert/reapply: "revert: revert <rest>" or "revert: reapply <rest>"
	# This reuses the same logic as for merge/squash: lowercase the first letter.
	# e.g. "Revert" -> "r" + "evert", "Reapply" -> "r" + "eapply"
	rest="${stripped#?}"
	new_first_line="revert: r$rest"
else
	# For merge/squash: "chore: " + lowercase first char + rest
	# Note: Detection ensures first char is M/m (merge) or S/s (squash)
	first_char="${stripped%"${stripped#?}"}"
	rest="${stripped#?}"
	case "$first_char" in
	[Mm]) new_first_line="chore: m$rest" ;;
	*) new_first_line="chore: s$rest" ;; # Default: S/s (squash)
	esac
fi

write_commit_msg "$commit_msg_file" "$new_first_line" "$COMMIT_REST"