Make Utf8Decoder and Utf8Encoder's convert methods take start and end too.

sdk/lib/convert/ascii.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.convert;
 
 /**
  * An instance of the default implementation of the [AsciiCodec].
  *
  * This instance provides a convenient access to the most common ASCII
@@ skipping 42 matching lines @@
    */
   String decode(List<int> bytes, { bool allowInvalid }) {
     if (allowInvalid == null) allowInvalid = _allowInvalid;
     if (allowInvalid) {
       return const AsciiDecoder(allowInvalid: true).convert(bytes);
     } else {
       return const AsciiDecoder(allowInvalid: false).convert(bytes);
     }
   }
 
-  Converter<String, List<int>> get encoder => const AsciiEncoder();
+  AsciiEncoder get encoder => const AsciiEncoder();
 
-  Converter<List<int>, String> get decoder =>
+  AsciiDecoder get decoder =>
       _allowInvalid ? const AsciiDecoder(allowInvalid: true)
                     : const AsciiDecoder(allowInvalid: false);
 }
 
 // Superclass for [AsciiEncoder] and [Latin1Encoder].
 // Generalizes common operations that only differ by a mask;
 class _UnicodeSubsetEncoder extends Converter<String, List<int>> {
   final int _subsetMask;
 
   const _UnicodeSubsetEncoder(this._subsetMask);
 
-  List<int> convert(String string) {
-    List result = new Uint8List(string.length);
-    for (int i = 0; i < string.length; i++) {
-      var codeUnit = string.codeUnitAt(i);
+  List<int> convert(String string, [int start = 0, int end]) {

[Søren Gjesse, 2014/11/20 10:41:41]

Where are the new arguments documented?

[Lasse Reichstein Nielsen, 2014/11/20 12:46:22]

Ack, documenting.

+    int stringLength = string.length;

[Søren Gjesse, 2014/11/20 10:41:41]

Maybe factor out argument checking.

[Lasse Reichstein Nielsen, 2014/11/20 12:46:22]

Funny you should say that - I'm already doing that in another CL :)

+    if (start < 0 || start > stringLength) {
+      throw new RangeError.range(start, 0, stringLength, "start");
+    }
+    if (end == null) {
+      end = stringLength;
+    } else {
+      if (end < start || end > stringLength) {
+        throw new RangeError.range(end, start, stringLength, "end");
+      }
+    }
+    int length = end - start;
+    List result = new Uint8List(length);
+    for (int i = 0; i < length; i++) {
+      var codeUnit = string.codeUnitAt(start + i);
       if ((codeUnit & ~_subsetMask) != 0) {
         throw new ArgumentError("String contains invalid characters.");
       }
       result[i] = codeUnit;
     }
     return result;
   }
 
   /**
    * Starts a chunked conversion.
@@ skipping 73 matching lines @@
    * [_ASCII_MASK] for ASCII (7-bit).
    *
    * If [_allowInvalid] is `true`, [convert] replaces invalid bytes with the
    * Unicode Replacement character `U+FFFD` (�).
    * Otherwise it throws a [FormatException].
    */
   const _UnicodeSubsetDecoder(this._allowInvalid, this._subsetMask);
 
   /**
    * Converts the [bytes] (a list of unsigned 7- or 8-bit integers) to the
    * corresponding string.

[Søren Gjesse, 2014/11/20 10:41:41]

Add documentation.

    */
-  String convert(List<int> bytes) {
-    for (int i = 0; i < bytes.length; i++) {
+  String convert(List<int> bytes, [int start = 0, int end]) {
+    int byteCount = bytes.length;
+    if (start < 0 || start > byteCount) {
+      throw new RangeError.range(start, 0, byteCount, "start");
+    }
+    if (end == null) {
+      end = byteCount;
+    } else {
+      if (end < start || end > byteCount) {
+        throw new RangeError.range(end, start, byteCount, "end");
+      }
+    }
+    int length = end - start;
+
+    for (int i = start; i < end; i++) {
       int byte = bytes[i];
       if ((byte & ~_subsetMask) != 0) {
         if (!_allowInvalid) {
           throw new FormatException("Invalid value in input: $byte");
         }
-        return _convertInvalid(bytes);
+        return _convertInvalid(bytes, start, end);
       }
     }
-    return new String.fromCharCodes(bytes);
+    return new String.fromCharCodes(bytes, start, end);
   }
 
-  String _convertInvalid(List<int> bytes) {
+  String _convertInvalid(List<int> bytes, int start, int end) {
     StringBuffer buffer = new StringBuffer();
-    for (int i = 0; i < bytes.length; i++) {
+    for (int i = start; i < end; i++) {
       int value = bytes[i];
       if ((value & ~_subsetMask) != 0) value = 0xFFFD;
       buffer.writeCharCode(value);
     }
     return buffer.toString();
   }
 
   /**
    * Starts a chunked conversion.
    *
@@ skipping 97 matching lines @@
     }
     if (start < end) {
       if (start != 0 || end != length) {
         source = source.sublist(start, end);
       }
       add(source);
     }
     if (isLast) close();
   }
 }