lazy_map.dart

import 'dart:collection';

class LazyMap<K, V> extends MapMixin<K, V> {
  final _map = Map<K, V>();
  final V Function(K) _generator;

  LazyMap(this._generator);

  @override
  V operator [](Object key) {
    var res = _map[key];
    if (res == null) {
      res = _generator(key);
      _map[key] = res;
    }
    return res;
  }

  @override
  void operator []=(K key, V value) {
    _map[key] = value;
  }

  @override
  void clear() {
    _map.clear();
  }

  @override
  Iterable<K> get keys => _map.keys;

  @override
  V remove(Object key) {
    return _map.remove(key);
  }
}