Add MapKeySet and MapValueSet to the collection package.

pkg/collection/lib/wrappers.dart

Index: pkg/collection/lib/wrappers.dart
diff --git a/pkg/collection/lib/wrappers.dart b/pkg/collection/lib/wrappers.dart
index 0fcc8c23e80a42f6e86c84ebea3988f512cfca02..c9e919d5c24c5d5a2b29a4135e7241cd8198990a 100644
--- a/pkg/collection/lib/wrappers.dart
+++ b/pkg/collection/lib/wrappers.dart
@@ -19,19 +19,15 @@ 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);
 
@@ -93,6 +89,22 @@ class DelegatingIterable<E> implements Iterable<E> {
   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.
@@ -337,3 +349,219 @@ class DelegatingMap<K, V> implements Map<K, V> {
 
   String toString() => _base.toString();
 }
+
+/**
+ * An unmodifiable [Set] view of the keys of a [Map].
+ *
+ * The set delegates all operations to the underlying map.
+ *
+ * A `Map` can only contain each key once, so its keys can always
+ * be viewed as a `Set` without any loss, even if the [Map.keys]
+ * getter only shows an [Iterable] view of the keys.
+ *
+ * Note that [lookup] is not supported for this set.
+ */
+class MapKeySet<E> extends _DelegatingIterableBase<E>
+    with UnmodifiableSetMixin<E> {
+  final Map<E, dynamic> _baseMap;
+
+  MapKeySet(Map<E, dynamic> base) : _baseMap = base;
+
+  Iterable<E> get _base => _baseMap.keys;
+
+  bool contains(Object element) => _baseMap.containsKey(element);
+
+  bool get isEmpty => _baseMap.isEmpty;
+
+  bool get isNotEmpty => _baseMap.isNotEmpty;
+
+  int get length => _baseMap.length;
+
+  String toString() => "{${_base.join(', ')}}";
+
+  bool containsAll(Iterable<Object> other) => other.every(contains);
+
+  /**
+   * Returns a new set with the the elements of [this] that are not in [other].
+   *
+   * That is, the returned set contains all the elements of this [Set] that are
+   * not elements of [other] according to `other.contains`.
+   *
+   * Note that the returned set will use the default equality operation, which
+   * may be different than the equality operation [this] uses.
+   */
+  Set<E> difference(Set<E> other) =>
+      where((element) => !other.contains(element)).toSet();
+
+  /**
+   * Returns a new set which is the intersection between [this] and [other].
+   *
+   * That is, the returned set contains all the elements of this [Set] that are
+   * also elements of [other] according to `other.contains`.
+   *
+   * Note that the returned set will use the default equality operation, which
+   * may be different than the equality operation [this] uses.
+   */
+  Set<E> intersection(Set<Object> other) => where(other.contains).toSet();
+
+  /**
+   * Throws an [UnsupportedError] since there's no corresponding method for
+   * [Map]s.
+   */
+  E lookup(E element) => throw new UnsupportedError(
+      "MapKeySet doesn't support lookup().");
+
+  /**
+   * Returns a new set which contains all the elements of [this] and [other].
+   *
+   * That is, the returned set contains all the elements of this [Set] and all
+   * the elements of [other].
+   *
+   * Note that the returned set will use the default equality operation, which
+   * may be different than the equality operation [this] uses.
+   */
+  Set<E> union(Set<E> other) => toSet()..addAll(other);
+}
+
+/**
+ * Creates a modifiable [Set] view of the values of a [Map].
+ * 
+ * The `Set` view assumes that the keys of the `Map` can be uniquely determined
+ * from the values. The `keyForValue` function passed to the constructor finds
+ * the key for a single value. The `keyForValue` function should be consistent
+ * with equality. If `value1 == value2` then `keyForValue(value1)` and
+ * `keyForValue(value2)` should be considered equal keys by the underlying map,
+ * and vice versa.
+ *
+ * Modifying the set will modify the underlying map based on the key returned by
+ * `keyForValue`.
+ *
+ * If the `Map` contents are not compatible with the `keyForValue` function, the
+ * set will not work consistently, and may give meaningless responses or do
+ * inconsistent updates.
+ *
+ * This set can, for example, be used on a map from database record IDs to the
+ * records. It exposes the records as a set, and allows for writing both
+ * `recordSet.add(databaseRecord)` and `recordMap[id]`.
+ *
+ * Effectively, the map will act as a kind of index for the set.
+ */
+class MapValueSet<K, V> extends _DelegatingIterableBase<V> implements Set<V> {
+  final Map<K, V> _baseMap;
+  final Function _keyForValue;
+
+  /**
+   * 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;
+
+  Iterable<V> get _base => _baseMap.values;
+
+  bool contains(Object element) {
+    if (element != null && 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) {
+    K key = _keyForValue(value);
+    bool result = false;
+    _baseMap.putIfAbsent(key, () {
+      result = true;
+      return value;
+    });
+    return result;
+  }
+
+  void addAll(Iterable<V> elements) => elements.forEach(add);
+
+  void clear() => _baseMap.clear();
+
+  bool containsAll(Iterable<Object> other) => other.every(contains);
+
+  /**
+   * Returns a new set with the the elements of [this] that are not in [other].
+   *
+   * That is, the returned set contains all the elements of this [Set] that are
+   * not elements of [other] according to `other.contains`.
+   *
+   * Note that the returned set will use the default equality operation, which
+   * may be different than the equality operation [this] uses.
+   */
+  Set<V> difference(Set<V> other) =>
+      where((element) => !other.contains(element)).toSet();
+
+  /**
+   * Returns a new set which is the intersection between [this] and [other].
+   *
+   * That is, the returned set contains all the elements of this [Set] that are
+   * also elements of [other] according to `other.contains`.
+   *
+   * Note that the returned set will use the default equality operation, which
+   * may be different than the equality operation [this] uses.
+   */
+  Set<V> intersection(Set<Object> other) => where(other.contains).toSet();
+
+  V lookup(V element) => _baseMap[_keyForValue(element)];
+
+  bool remove(Object value) {
+    if (value != null && 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)) {
+    var toRemove = [];
+    _baseMap.forEach((key, value) {
+      if (test(value)) toRemove.add(key);
+    });
+    toRemove.forEach(_baseMap.remove);
+  }
+
+  void retainAll(Iterable<Object> elements) {
+    var valuesToRetain = new Set<V>.identity();
+    for (var element in elements) {
+      if (element != null && element is! V) continue;
+      var key = _keyForValue(element);
+      if (!_baseMap.containsKey(key)) continue;
+      valuesToRetain.add(_baseMap[key]);
+    }
+
+    var keysToRemove = [];
+    _baseMap.forEach((k, v) {
+      if (!valuesToRetain.contains(v)) keysToRemove.add(k);
+    });
+    keysToRemove.forEach(_baseMap.remove);
+  }
+
+  void retainWhere(bool test(V element)) =>
+      removeWhere((element) => !test(element));
+
+  /**
+   * Returns a new set which contains all the elements of [this] and [other].
+   *
+   * That is, the returned set contains all the elements of this [Set] and all
+   * the elements of [other].
+   *
+   * Note that the returned set will use the default equality operation, which
+   * may be different than the equality operation [this] uses.
+   */
+  Set<V> union(Set<V> other) => toSet()..addAll(other);
+}