Improve performance of string buffer by modifying concatAll native to allow growable array and an i…

runtime/lib/string_buffer_patch.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.
 
 patch class StringBuffer {
   static const int _BUFFER_SIZE = 64;
   static const int _PARTS_TO_COMPACT = 128;
   static const int _PARTS_TO_COMPACT_SIZE_LIMIT = _PARTS_TO_COMPACT * 8;
 
   /**
    * When strings are written to the string buffer, we add them to a
    * list of string parts.
    */
-  List _parts = null;
+  List<String> _parts = null;
 
   /**
     * Total number of code units in the string parts. Does not include
     * the code units added to the buffer.
     */
   int _partsCodeUnits = 0;
 
   /**
    * To preserve memory, we sometimes compact the parts. This combines
    * several smaller parts into a single larger part to cut down on the
    * cost that comes from the per-object memory overhead. We keep track
    * of the last index where we ended our compaction and the number of
    * code units added since the last compaction.
    */
   int _partsCompactionIndex = 0;
   int _partsCodeUnitsSinceCompaction = 0;
-  _ObjectArray _partsCompactionArray = null;
 
   /**
    * The buffer is used to build up a string from code units. It is
    * used when writing short strings or individul char codes to the
    * buffer. The buffer is allocated on demand.
    */
   Uint16List _buffer = null;
   int _bufferPosition = 0;
 
   /**
@@ skipping 52 matching lines @@
 
   /** Makes the buffer empty. */
   /* patch */ void clear() {
     _parts = null;
     _partsCodeUnits = _bufferPosition = _bufferCodeUnitMagnitude = 0;
   }
 
   /** Returns the contents of buffer as a string. */
   /* patch */ String toString() {
     _consumeBuffer();
-    if (_partsCodeUnits == 0) return "";
-
-    // TODO(kasperl): It would be nice if concatAllNative would
-    // allow me to pass in a grownable array directly, but for
-    // now we have to copy the contents to a non-growable array.
-    int length = _parts.length;
-    _ObjectArray array = new _ObjectArray(length);
-    for (int i = 0; i < length; i++) array[i] = _parts[i];
-    return _StringBase._concatAllNative(array);
+    return (_partsCodeUnits == 0) ?
+        "" :
+        _StringBase._concatAllNative(_parts, 0, _parts.length);
   }
 
   /** Ensures that the buffer has enough capacity to add n code units. */
   void _ensureCapacity(int n) {
     if (_buffer == null) {
       _buffer = new Uint16List(_BUFFER_SIZE);
     } else if (_bufferPosition + n > _buffer.length) {
       _consumeBuffer();
     }
   }
@@ skipping 30 matching lines @@
       }
     }
   }
 
   /**
    * Compacts the last N parts if their average size allows us to save a
    * lot of memory by turning them all into a single part.
    */
   void _compact() {
     if (_partsCodeUnitsSinceCompaction < _PARTS_TO_COMPACT_SIZE_LIMIT) {
-      if (_partsCompactionArray == null) {
-        _partsCompactionArray = new _ObjectArray(_PARTS_TO_COMPACT);
-      }
-      for (int i = 0; i < _PARTS_TO_COMPACT; i++) {
-        _partsCompactionArray[i] = _parts[i + _partsCompactionIndex];
-      }
-      String compacted = _StringBase._concatAllNative(_partsCompactionArray);
+      String compacted = _StringBase._concatAllNative(
+          _parts,
+          _partsCompactionIndex,                     // Start
+          _partsCompactionIndex + _PARTS_TO_COMPACT  // End
+      );
       _parts.length = _parts.length - _PARTS_TO_COMPACT;
       _parts.add(compacted);
-      for (int i = 0; i < _PARTS_TO_COMPACT; i++) {
-        _partsCompactionArray[i] = null;
-      }
     }
     _partsCodeUnitsSinceCompaction = 0;
     _partsCompactionIndex = _parts.length;
   }
 
   /**
    * Create a [String] from the UFT-16 code units in buffer.
    */
   static String _create(Uint16List buffer, int length, bool isLatin1)
       native "StringBuffer_createStringFromUint16Array";
 }