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, {int Function(T) key}) {
  var map = <String, T>{};
  for (var element in values.keys) {
    if (values[element] == null) {
      continue;
    }
    map["$element"] = key == null ? values[element].first : values[element].lMax(key);
  }
  return map;
}

T max<T>(Iterable<T> list, int Function(T) key) {
  T tt;
  for (T t in list) {
    if (tt == null) {
      tt = t;
    }
    if (key(tt) < key(t)) {
      tt = t;
    }
  }
  return tt;
}



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

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

  T lMax(int Function(T) key) {
    return max(this, key);
  }

}

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

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

  Map<String, T> mGroupItem({int Function(T) key}) {
    return groupItem(this, key: key);
  }

}