Map<S, List<T>> groupBy<S, T>(Iterable<T> values, S Function(T) key) {
  var map = <S, List<T>>{};
  for (var element in values) {
    (map[key(element)] ??= []).add(element);
  }
  return map;
}

Map<String, int> groupCount<S, T>(Map<S, List<T>> values) {
  var map = <String, int>{};
  for (var element in values.keys) {
    map["$element"] = values[element]?.length ?? 0;
  }
  return map;
}

Map<String, T> groupItem<S, T>(Map<S, List<T>> values) {
  var map = <String, T>{};
  for (var element in values.keys) {
    if (values[element] == null) {
      continue;
    }
    map["$element"] = values[element].first;
  }
  return map;
}


extension ListExtension<S, T> on Iterable<T> {

  Map<S, List<T>> lGroupBy(S Function(T) key) {
    return groupBy(this, key);
  }

}

extension MapExtension<S, T> on Map<S, List<T>> {

  Map<String, int> get mGroupCount => groupCount(this);

  Map<String, T> get mGroupItem => groupItem(this);

}