Add a CanonicalizedMap class to pkg/collection.

pkg/collection/lib/src/canonicalized_map.dart

+// Copyright (c) 2014, 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.
+
+library dart.pkg.collection.canonicalized_map;
+
+import 'utils.dart';
+
+/**
+ * A map whose keys are converted to canonical values of type `C`.
+ *
+ * This is useful for using case-insensitive String keys, for example. It's more
+ * efficient than a [LinkedHashMap] with a custom equality operator because it
+ * only canonicalizes each once, rather than doing so for each comparison.

[Lasse Reichstein Nielsen, 2014/07/01 09:18:15]

each once -> each key once

[nweiz, 2014/07/01 21:21:44]

Done.

+ */

[Lasse Reichstein Nielsen, 2014/07/01 09:18:15]

Document whether the Map allows null as a key (afaics it does).

[nweiz, 2014/07/01 21:21:44]

Done.

+class CanonicalizedMap<C, K, V> implements Map<K, V> {
+  final Function _canonicalize;
+
+  final _base = new Map<C, Pair<K, V>>();
+
+  /**
+   * Creates an empty canonicalized map.
+   *
+   * The [canonicalize] function should return the canonical value for the given
+   * key. Keys with the same canonical value are considered equivalent.
+   */
+  CanonicalizedMap(C canonicalize(K key))
+      : _canonicalize = canonicalize;
+
+  /**
+   * Creates a canonicalized map that is initialized with the key/value pairs of
+   * [other].
+   *
+   * The [canonicalize] function should return the canonical value for the given
+   * key. Keys with the same canonical value are considered equivalent.
+   */
+  CanonicalizedMap.from(Map<K, V> other, C canonicalize(K key))
+      : _canonicalize = canonicalize {
+    addAll(other);
+  }
+
+  V operator [](Object key) {
+    if (key != null && key is! K) return null;
+    var pair = _base[_canonicalize(key)];
+    return pair == null ? null : pair.last;
+  }
+
+  void operator []=(K key, V value) {
+    _base[_canonicalize(key)] = new Pair(key, value);
+  }
+
+  void addAll(Map<K, V> other) {
+    other.forEach((key, value) => this[key] = value);
+  }
+
+  void clear() => _base.clear();

[Lasse Reichstein Nielsen, 2014/07/01 09:18:15]

I'd prefer "{...}" to "=>" for a void function.

[nweiz, 2014/07/01 21:21:44]

Done.

+
+  bool containsKey(Object key) {
+    if (key != null && key is! K) return null;
+    return _base.containsKey(_canonicalize(key));
+  }
+
+  bool containsValue(Object value) =>
+      _base.values.any((pair) => pair.last == value);
+
+  void forEach(void f(K key, V value)) {
+    _base.forEach((key, pair) => f(pair.first, pair.last));
+  }
+
+  bool get isEmpty => _base.isEmpty;
+
+  bool get isNotEmpty => _base.isNotEmpty;
+
+  Iterable<K> get keys => _base.values.map((pair) => pair.first);
+
+  int get length => _base.length;
+
+  V putIfAbsent(K key, V ifAbsent()) {
+    return _base.putIfAbsent(_canonicalize(key),
+        () => new Pair(key, ifAbsent())).last;
+  }
+
+  V remove(Object key) {
+    if (key != null && key is! K) return null;
+    var pair = _base.remove(_canonicalize(key));
+    return pair == null ? null : pair.last;
+  }
+
+  Iterable<K> get values => _base.values.map((pair) => pair.last);
+
+  String toString() => "{" +
+      _base.values.map((pair) => "${pair.first}: ${pair.last}").join(", ") +
+      "}";

[Lasse Reichstein Nielsen, 2014/07/01 09:18:15]

Just use:
   String toString() => Maps.mapToString(this);
from dart:collection. It also handles cyclic structures.

[nweiz, 2014/07/01 21:21:44]

Done.

+}