Add MapKeySet and MapValueSet to the collection package.

pkg/collection/lib/wrappers.dart

 // Copyright (c) 2013, 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.
 
 /**
  * Delegating wrappers for [Iterable], [List], [Set], [Queue] and [Map].
  *
  * Also adds unmodifiable views for `Set` and `Map`, and a fixed length
  * view for `List`. The unmodifable list view from `dart:collection` is exported
  * as well, just for completeness.
  */
 library dart.pkg.collection.wrappers;
 
 import "dart:collection";
 import "dart:math" show Random;
 
 export "dart:collection" show UnmodifiableListView;
 
 part "src/unmodifiable_wrappers.dart";
 
 /**
- * Creates an [Iterable] that delegates all operations to a base iterable.
+ * A base class for delegating iterables.
  *
- * This class can be used hide non-`Iterable` methods of an iterable object,
- * or it can be extended to add extra functionality on top of an existing
- * iterable object.
+ * Subclasses can provide a [_base] that should be delegated to. Unlike
+ * [DelegatingIterable], this allows the base to be created on demand.
  */
-class DelegatingIterable<E> implements Iterable<E> {
-  final Iterable<E> _base;
+abstract class _DelegatingIterableBase<E> implements Iterable<E> {
+  Iterable<E> get _base;
 
-  /**
-   * Create a wrapper that forwards operations to [base].
-   */
-  const DelegatingIterable(Iterable<E> base) : _base = base;
+  const _DelegatingIterableBase();
 
   bool any(bool test(E element)) => _base.any(test);
 
   bool contains(Object element) => _base.contains(element);
 
   E elementAt(int index) => _base.elementAt(index);
 
   bool every(bool test(E element)) => _base.every(test);
 
   Iterable expand(Iterable f(E element)) => _base.expand(f);
@@ skipping 41 matching lines @@
 
   List<E> toList({bool growable: true}) => _base.toList(growable: growable);
 
   Set<E> toSet() => _base.toSet();
 
   Iterable<E> where(bool test(E element)) => _base.where(test);
 
   String toString() => _base.toString();
 }
 
+/**
+ * Creates an [Iterable] that delegates all operations to a base iterable.
+ *
+ * This class can be used hide non-`Iterable` methods of an iterable object,
+ * or it can be extended to add extra functionality on top of an existing
+ * iterable object.
+ */
+class DelegatingIterable<E> extends _DelegatingIterableBase<E> {
+  final Iterable<E> _base;
+
+  /**
+   * Create a wrapper that forwards operations to [base].
+   */
+  const DelegatingIterable(Iterable<E> base) : _base = base;
+}
+
 
 /**
  * Creates a [List] that delegates all operations to a base list.
  *
  * This class can be used hide non-`List` methods of a list object,
  * or it can be extended to add extra functionality on top of an existing
  * list object.
  */
 class DelegatingList<E> extends DelegatingIterable<E> implements List<E> {
   const DelegatingList(List<E> base) : super(base);
@@ skipping 224 matching lines @@
   int get length => _base.length;
 
   V putIfAbsent(K key, V ifAbsent()) => _base.putIfAbsent(key, ifAbsent);
 
   V remove(Object key) => _base.remove(key);
 
   Iterable<V> get values => _base.values;
 
   String toString() => _base.toString();
 }
+
+/**
+ * Creates an unmodifiable [Set] that delegates all operations to a base map's
+ * keys.
+ *
+ * This class can be used to make a `Map` look like a `Set`. Note that [lookup]
+ * is not `O(1)` for this set.
+ */
+class MapKeySet<E> extends _DelegatingIterableBase<E>
+    with UnmodifiableSetMixin<E> {
+  final Map<E, dynamic> _baseMap;
+
+  Iterable<E> get _base => _baseMap.keys;
+
+  MapKeySet(Map<E, dynamic> base) : _baseMap = base;
+
+  bool contains(Object element) => _baseMap.containsKey(element);
+
+  bool get isEmpty => _baseMap.isEmpty;
+
+  bool get isNotEmpty => _baseMap.isNotEmpty;
+
+  int get length => _baseMap.length;
+
+  String toString() => toSet().toString();
+
+  bool containsAll(Iterable<Object> other) => other.every(contains);
+
+  Set<E> difference(Set<E> other) =>
+      where((element) => !other.contains(element)).toSet();
+
+  Set<E> intersection(Set<Object> other) =>
+      where((element) => other.contains(element)).toSet();
+
+  E lookup(E element) => firstWhere((key) => element == key,
+      orElse: () => null);
+
+  Set<E> union(Set<E> other) => toSet()..addAll(other);
+}
+
+/**
+ * Creates a [Set] that delegates all operations to a base map's values.
+ *
+ * This class can be used to make a `Map` whose keys are determinable from its

[kevmoo, 2014/05/08 20:40:15]

I'm a bit confused by this sentence. What's the use case?

[nweiz, 2014/05/08 21:14:56]

See my comment on the issue:
https://code.google.com/p/dart/issues/detail?id=18705#c3

[kevmoo, 2014/05/08 23:01:53]

Your comment makes the usage clear, but the docs here are a bit confusing IMHO.
It might be worth an example.

[nweiz, 2014/05/09 02:48:50]

Done.

+ * values look like a `Set`.
+ */
+class MapValueSet<K, V> extends _DelegatingIterableBase<V> implements Set<V> {
+  final Map<K, V> _baseMap;
+  final Function _keyForValue;
+
+  Iterable<V> get _base => _baseMap.values;
+
+  /**
+   * Creates a new [MapValueSet] based on [base].
+   *
+   * [keyForValue] returns the key in the map that should be associated with the
+   * given value. The set's notion of equality is identical to the equality of
+   * the return values of [keyForValue].
+   */
+  MapValueSet(Map<K, V> base, K keyForValue(V value))
+      : _baseMap = base,
+        _keyForValue = keyForValue;
+
+  bool contains(Object element) {
+    if (element is! V) return false;
+    return _baseMap.containsKey(_keyForValue(element));
+  }
+
+  bool get isEmpty => _baseMap.isEmpty;
+
+  bool get isNotEmpty => _baseMap.isNotEmpty;
+
+  int get length => _baseMap.length;
+
+  String toString() => toSet().toString();
+
+  bool add(V value) {
+    var key = _keyForValue(value);
+    if (_baseMap.containsKey(key)) return false;
+    _baseMap[key] = value;
+    return true;
+  }
+
+  void addAll(Iterable<V> elements) => elements.forEach(add);
+
+  void clear() => _baseMap.clear();
+
+  bool containsAll(Iterable<Object> other) => other.every(contains);
+
+  Set<V> difference(Set<V> other) =>
+      where((element) => !other.contains(element)).toSet();
+
+  Set<V> intersection(Set<Object> other) =>
+      where((element) => other.contains(element)).toSet();
+
+  V lookup(V element) => _baseMap[_keyForValue(element)];
+
+  bool remove(Object value) {
+    if (value is! V) return false;
+    var key = _keyForValue(value);
+    if (!_baseMap.containsKey(key)) return false;
+    _baseMap.remove(key);
+    return true;
+  }
+
+  void removeAll(Iterable<Object> elements) => elements.forEach(remove);
+
+  void removeWhere(bool test(V element)) {
+    new Map.from(_baseMap).forEach((key, value) {
+      if (test(value)) remove(key);
+    });
+  }
+
+  void retainAll(Iterable<Object> elements) {
+    var retainKeys = elements.where((element) => element is V)
+        .map((element) => _keyForValue(element)).toSet();
+    new Map.from(_baseMap).forEach((key, value) {
+      if (!retainKeys.contains(key)) remove(key);
+    });
+  }
+
+  void retainWhere(bool test(V element)) =>
+      removeWhere((element) => !test(element));
+
+  Set<V> union(Set<V> other) => toSet()..addAll(other);
+}