#!/usr/bin/env bash
#
# Check a commit message against Conventional Commits, and its scope against
# the epics this repo's work belongs to.
#
# Two differences from headquarters' hook of the same name, both deliberate.
#
# It enforces the *form* as well as the scope. headquarters lets an
# unconventional subject through, because that repo has years of them; this
# one has had the convention from its second commit and there is no reason to
# let it slip.
#
# And the epic ids are written down here rather than read from plan/, because
# helm has no plan/ - its epics live in headquarters, which is not guaranteed
# to be checked out beside this repo and is certainly not guaranteed to be at
# any particular revision. A hardcoded list that is occasionally stale is
# better than a hook that passes silently whenever a sibling directory is
# missing.
#
# Run by prek at the commit-msg stage, with the message file as $1.

set -euo pipefail

# Conventional Commits types.
TYPES="feat fix docs style refactor perf test build ci chore revert"

# Epic ids from headquarters' plan/. Add one here when an epic that helm does
# work for is added there.
EPIC_SCOPES="unit-library unit-search unit-rules forces"

# Scopes that are deliberately not epics. Keep this short: every entry is a
# commit that cannot be found from an epic.
NON_EPIC_SCOPES="deps scripts bridge ci repo"

msg_file="${1:?usage: check-commit-scope.sh <commit-msg-file>}"

# First line that is not a comment or blank; git strips the comments later.
subject="$(grep -v '^#' "$msg_file" | grep -m1 . || true)"

# An empty message aborts the commit on git's side, so it is not this hook's
# to complain about.
[ -n "$subject" ] || exit 0

# A revert or a merge git wrote itself is not the author's prose to police.
case "$subject" in
Revert\ * | Merge\ *) exit 0 ;;
esac

# type(scope)!: subject, where the scope is optional. The pattern is a
# variable because bash's [[ ]] parser trips on an unquoted ")".
full_re='^([a-z]+)(\(([^)]*)\))?!?: .+'
if [[ ! "$subject" =~ $full_re ]]; then
  {
    echo "commit subject is not Conventional Commits:"
    echo "  $subject"
    echo
    echo "Write:  type(scope): what changed"
    echo "        type: what changed"
    echo
    # shellcheck disable=SC2086
    echo "Types:  $(printf '%s, ' $TYPES | sed 's/, $//')"
    echo "Scopes: $EPIC_SCOPES $NON_EPIC_SCOPES"
  } >&2
  exit 1
fi

type="${BASH_REMATCH[1]}"
scopes="${BASH_REMATCH[3]}"

case " $TYPES " in
*" $type "*) ;;
*)
  {
    echo "unknown commit type: $type"
    echo
    # shellcheck disable=SC2086
    printf '  %s\n' $TYPES | sort
  } >&2
  exit 1
  ;;
esac

# No scope is allowed: not every commit belongs to an epic.
[ -n "$scopes" ] || exit 0

known="$EPIC_SCOPES $NON_EPIC_SCOPES"
bad=""
IFS=',' read -ra parts <<<"$scopes"
for scope in "${parts[@]}"; do
  scope="$(printf '%s' "$scope" | tr -d '[:space:]')"
  [ -n "$scope" ] || continue
  case " $known " in
  *" $scope "*) ;;
  *) bad="$bad $scope" ;;
  esac
done

[ -n "$bad" ] || exit 0

{
  echo "commit scope is not an epic or a known non-epic:$bad"
  echo
  echo "A scope names the epic in headquarters' plan/ that the work belongs"
  echo "to, so a commit is findable from it. Use one of:"
  echo
  # shellcheck disable=SC2086
  printf '  %s\n' $known | sort -u
  echo
  echo "or drop the scope. If a new epic exists in headquarters, add its id"
  echo "to EPIC_SCOPES in this script."
} >&2
exit 1