Add Map constructors taking iterables.

sdk/lib/collection/hash_map.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.
 
 part of dart.collection;
 
 /**
  * A hash-table based implementation of [Map].
  *
  * The keys of a `HashMap` must have consistent [Object.operator==]
  * and [Object.hashCode] implementations. This means that the `==` operator
  * must define a stable equivalence relation on the keys (reflexive,
  * anti-symmetric, transitive, and consistent over time), and that `hashCode`
  * must be the same for objects that are considered equal by `==`.
  *
  * The map allows `null` as a key.
  */
 class HashMap<K, V> implements Map<K, V> {
   external HashMap();
 
+
   factory HashMap.from(Map<K, V> other) {
     return new HashMap<K, V>()..addAll(other);
   }
 
+  factory HashMap.fromIterable(Iterable iterable,
+      {K key(element), V value(element)}) {
+    HashMap<K, V> map = new HashMap<K, V>();
+
+    var keyFun = (key != null) ? key : (x) => x;

[floitsch, 2013/06/27 11:45:04]

In "dart:" libraries we type local variables too. (yes this is not following the
style guide).

reuse the argument. This way you don't need to come up with a new name.
if (key == null) key = ...;

Use a static identity function. Otherwise the VM will probably allocate a new
function at every constructor call. (In theory it could eventually optimize this
away, but it won't happen in the near future).

[zarah, 2013/06/27 13:02:43]

Done.

+    var valueFun = (value != null) ? value : (x) => x;
+
+    iterable.forEach((e) {

[floitsch, 2013/06/27 11:45:04]

personally I prefer

for (var element in iterable) {
  map[key(element)] = value(element);
}

but that's just, because I expect it to be easier to get optimized.

[zarah, 2013/06/27 13:02:43]

Done.

+      map[keyFun(e)] = valueFun(e);
+    });
+
+    return map;
+  }
+
+  factory HashMap.fromIterables(Iterable<K> keys, Iterable<V> values) {
+    HashMap<K, V> map = new HashMap<K, V>();
+
+    var keyIterator = keys.iterator;

[floitsch, 2013/06/27 11:45:04]

Use type.

[zarah, 2013/06/27 13:02:43]

Done.

+    var valueIterator = values.iterator;
+
+    bool hasNextKey = keyIterator.moveNext();
+    bool hasNextValue = valueIterator.moveNext();
+
+    while (hasNextKey && hasNextValue) {
+      map[keyIterator.current] = valueIterator.current;
+      hasNextKey = keyIterator.moveNext();
+      hasNextValue = valueIterator.moveNext();
+    }
+
+    if (hasNextKey != hasNextValue) {

[floitsch, 2013/06/27 11:45:04]

if (hasNextKey || hasNextValue) {

It has the same effect, but, to me, it makes it more clear what happens.
hasNextKey != hasNextValue could (in if implemented naively) be more efficient,
though.

[zarah, 2013/06/27 13:02:43]

Done.

+      throw new ArgumentError("Iterables do not have same length.");
+    }
+
+    return map;
+  }
+
   external int get length;
   external bool get isEmpty;
   external bool get isNotEmpty;
 
   external Iterable<K> get keys;
   external Iterable<V> get values;
 
   external bool containsKey(Object key);
   external bool containsValue(Object value);
 
   external void addAll(Map<K, V> other);
 
   external V operator [](Object key);
   external void operator []=(K key, V value);
 
   external V putIfAbsent(K key, V ifAbsent());
 
   external V remove(Object key);
   external void clear();
 
   external void forEach(void action(K key, V value));
 
   String toString() => Maps.mapToString(this);
 }