Shrink growable arrays

runtime/lib/growable_array.dart

 // Copyright (c) 2012, 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.
 
 class _GrowableList<T> extends ListBase<T> {
 
   void insert(int index, T element) {
     if ((index < 0) || (index > length)) {
       throw new RangeError.range(index, 0, length);
     }
@@ skipping 95 matching lines @@
 
   factory _GrowableList.from(Iterable<T> other) {
     List<T> result = new _GrowableList<T>();
     result.addAll(other);
     return result;
   }
 
   factory _GrowableList.withData(_List data)
     native "GrowableList_allocate";
 
+  int get _capacity native "GrowableList_getCapacity";
+
   int get length native "GrowableList_getLength";
 
-  int get _capacity native "GrowableList_getCapacity";
-
   void set length(int new_length) {
-    if (new_length > _capacity) {
-      _grow(new_length);
+    int new_capacity = (new_length == 0) ? _kDefaultCapacity : new_length;
+    if (new_capacity > _capacity) {
+      _grow(new_capacity);
+      _setLength(new_length);
+      return;
+    }
+    // We are shrinking. Pick the method which has fewer writes.
+    // In the shrink-to-fit path, we write |new_capacity + new_length| words
+    // (null init + copy).
+    // In the non-shrink-to-fit path, we write |length - new_length| words
+    // (null overwrite).
+    final bool shouldShrinkToFit =
+        (new_capacity + new_length) < (length - new_length);
+    if (shouldShrinkToFit) {
+      _shrink(new_capacity, new_length);
     } else {
       for (int i = new_length; i < length; i++) {
         this[i] = null;
       }
     }
     _setLength(new_length);
   }
 
   void _setLength(int new_length) native "GrowableList_setLength";
 
@@ skipping 77 matching lines @@
     if (length > 0) return this[length - 1];
     throw IterableElementError.noElement();
   }
 
   T get single {
     if (length == 1) return this[0];
     if (length == 0) throw IterableElementError.noElement();
     throw IterableElementError.tooMany();;
   }
 
-  void _grow(int new_length) {
-    var new_data = new _List(new_length);
+  void _grow(int new_capacity) {
+    var new_data = new _List(new_capacity);
     for (int i = 0; i < length; i++) {
       new_data[i] = this[i];
     }
     _setData(new_data);
   }
 
+  void _shrink(int new_capacity, int new_length) {
+    var new_data = new _List(new_capacity);
+    for (int i = 0; i < new_length; i++) {
+      new_data[i] = this[i];
+    }
+    _setData(new_data);
+  }
+
   // Iterable interface.
 
   void forEach(f(T element)) {
     int initialLength = length;
     for (int i = 0; i < length; i++) {
       f(this[i]);
       if (length != initialLength) throw new ConcurrentModificationError(this);
     }
   }
 
@@ skipping 97 matching lines @@
       result._setLength(length);
       return result;
     }
     return growable ? <T>[] : new List<T>(0);
   }
 
   Set<T> toSet() {
     return new Set<T>.from(this);
   }
 }