Fix performance of readLine

runtime/bin/string_stream.dart

 // Copyright (c) 2011, 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.
 
 // Interface for decoders decoding binary data into string data. The
 // decoder keeps track of line breaks during decoding.
 interface _StringDecoder {
   // Add more binary data to be decoded. The ownership of the buffer
   // is transfered to the decoder and the caller most not modify it any more.
   int write(List<int> buffer);
@@ skipping 22 matching lines @@
   String toString() => "DecoderException: $message";
   final String message;
 }
 
 
 // Utility class for decoding UTF-8 from data delivered as a stream of
 // bytes.
 class _StringDecoderBase implements _StringDecoder {
   _StringDecoderBase()
       : _bufferList = new _BufferList(),
-      _result = new List<int>(),
-      _lineBreakEnds = new Queue<int>();
+        _result = new List<int>(),
+        _lineBreakEnds = new Queue<int>();
 
   int write(List<int> buffer) {
     _bufferList.add(buffer);
     // Decode as many bytes into characters as possible.
     while (_bufferList.length > 0) {
       if (!_processNext()) {
         break;
       }
     }
     return buffer.length;
   }
 
   bool isEmpty() {
     return _result.isEmpty();
   }
 
   int get lineBreaks() => _lineBreaks;
 
   String get decoded() {
     if (isEmpty()) return null;
 
-    String result =  new String.fromCharCodes(_result);
-    _charOffset += result.length;
+    String result;
+    if (_resultOffset == 0) {
+      result = new String.fromCharCodes(_result);
+    } else {
+      result =
+          new String.fromCharCodes(
+              _result.getRange(_resultOffset, _result.length - _resultOffset));
+    }
     while (!_lineBreakEnds.isEmpty() && _lineBreakEnds.first() < _charOffset) {
       _lineBreakEnds.removeFirst();
       _lineBreaks--;
     }
-    _result = new List<int>();
+    _resetResult();
     return result;
   }
 
   String get decodedLine() {
     if (_lineBreakEnds.isEmpty()) return null;
     int lineEnd = _lineBreakEnds.removeFirst();
     int terminationSequenceLength = 1;
     if (_result[lineEnd - _charOffset] == LF &&
         lineEnd > _charOffset &&
         _result[lineEnd - _charOffset - 1] == CR) {
       terminationSequenceLength = 2;
     }
-    var lineLength = lineEnd - _charOffset - terminationSequenceLength + 1;
-    String result = new String.fromCharCodes(_result.getRange(0, lineLength));
+    var lineLength =
+        lineEnd - _charOffset - _resultOffset - terminationSequenceLength + 1;
+    String result =
+        new String.fromCharCodes(_result.getRange(_resultOffset, lineLength));
     _lineBreaks--;
-    int removeCount = lineLength + terminationSequenceLength;
-    _result = _result.getRange(removeCount, _result.length - removeCount);
-    _charOffset = lineEnd + 1;
+    _resultOffset += (lineLength + terminationSequenceLength);
+    if (_result.length == _resultOffset) _resetResult();
     return result;
   }
 
   // Add another decoded character.
   void addChar(int charCode) {
     _result.add(charCode);
     _charCount++;
     // Check for line ends (\r, \n and \r\n).
     if (charCode == LF) {
       _recordLineBreakEnd(_charCount - 1);
     } else if (_lastCharCode == CR) {
       _recordLineBreakEnd(_charCount - 2);
     }
     _lastCharCode = charCode;
   }
 
   void _recordLineBreakEnd(int charPos) {
-      _lineBreakEnds.add(charPos);
-      _lineBreaks++;
+    _lineBreakEnds.add(charPos);
+    _lineBreaks++;
+  }
+
+  void _resetResult() {
+    _charOffset += _result.length;
+    _result = new List<int>();
+    _resultOffset = 0;
   }
 
   abstract bool _processNext();
 
   _BufferList _bufferList;
+  int _resultOffset = 0;
   List<int> _result;
   int _lineBreaks = 0;  // Number of line breaks in the current list.
   // The positions of the line breaks are tracked in terms of absolute
   // character positions from the begining of the decoded data.
   Queue<int> _lineBreakEnds;  // Character position of known line breaks.
   int _charOffset = 0;  // Character number of the first character in the list.
   int _charCount = 0;  // Total number of characters decodes.
   int _lastCharCode = -1;
 
   final int LF = 10;
@@ skipping 194 matching lines @@
   InputStream _input;
   String _encoding;
   _StringDecoder _decoder;
   bool _inputClosed = false;  // Is the underlying input stream closed?
   bool _closed = false;  // Is this stream closed.
   bool _eof = false;  // Has all data been read from the decoder?
   var _clientDataHandler;
   var _clientLineHandler;
   var _clientCloseHandler;
 }