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// Copyright (c) 2017, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.
import 'dart:collection';
import 'combined_iterable.dart';
/// Returns a new map that represents maps flattened into a single map.
/// All methods and accessors treat the new map as-if it were a single
/// concatenated map, but the underlying implementation is based on lazily
/// accessing individual map instances. In the occasion where a key occurs in
/// multiple maps the first value is returned.
/// The resulting map has an index operator (`[]`) and `length` property that
/// are both `O(maps)`, rather than `O(1)`, and the map is unmodifiable - but
/// underlying changes to these maps are still accessible from the resulting
/// map.
class CombinedMapView<K, V> extends UnmodifiableMapBase<K, V> {
final Iterable<Map<K, V>> _maps;
/// Create a new combined view into multiple maps.
/// The iterable is accessed lazily so it should be collection type like
/// [List] or [Set] rather than a lazy iterable produced by `map()` et al.
V operator [](Object key) {
for (var map in _maps) {
// Avoid two hash lookups on a positive hit.
var value = map[key];
if (value != null || map.containsKey(value)) {
return value;
return null;
/// The keys of [this].
/// The returned iterable has efficient `length` and `contains` operations,
/// based on [length] and [containsKey] of the individual maps.
/// The order of iteration is defined by the individual `Map` implementations,
/// but must be consistent between changes to the maps.
/// Unlike most [Map] implementations, modifying an individual map while
/// iterating the keys will _sometimes_ throw. This behavior may change in
/// the future.
Iterable<K> get keys => new CombinedIterableView<K>( => m.keys));