Make String.fromCharCodes take start/end.

runtime/lib/string_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 String {
-  /* patch */ factory String.fromCharCodes(Iterable<int> charCodes) {
-    return _StringBase.createFromCharCodes(charCodes);
+  /* patch */ factory String.fromCharCodes(Iterable<int> charCodes,
+                                           [int start = 0, int end]) {
+    return _StringBase.createFromCharCodes(charCodes, start, end);

[srdjan, 2014/09/17 15:31:58]

Adding optional arguments will slow down this factory; please verify performance
of tracked benchmarks.

[Lasse Reichstein Nielsen, 2014/09/18 09:03:00]

Will do. 

Alternatively, we could have a
  String.fromCharCodeRange(Iterable<int> charCodes, int start, int end).

   }
 
   /* patch */ factory String.fromCharCode(int charCode) {
     if (charCode >= 0) {
       if (charCode <= 0xff) {
         return _OneByteString._allocate(1).._setAt(0, charCode);
       }
       if (charCode <= 0xffff) {
-        return _StringBase._createFromCodePoints(new _List(1)..[0] = charCode);
+        return _StringBase._createFromCodePoints(new _List(1)..[0] = charCode,
+                                                 0, 1);
       }
       if (charCode <= 0x10ffff) {
         var low = 0xDC00 | (charCode & 0x3ff);
         int bits = charCode - 0x10000;
         var high = 0xD800 | (bits >> 10);
         return  _StringBase._createFromCodePoints(new _List(2)..[0] = high
-                                                              ..[1] = low);
+                                                              ..[1] = low,
+                                                  0, 2);
       }
     }
     throw new RangeError.range(charCode, 0, 0x10ffff);
   }
 
   /* patch */ const factory String.fromEnvironment(String name,
                                                    {String defaultValue})
       native "String_fromEnvironment";
 }
 
@@ skipping 10 matching lines @@
   }
 
   Type get runtimeType => String;
 
   int get hashCode native "String_getHashCode";
 
   /**
    *  Create the most efficient string representation for specified
    *  [codePoints].
    */
-  static String createFromCharCodes(Iterable<int> charCodes) {
+  static String createFromCharCodes(Iterable<int> charCodes,
+                                    int start, int end) {
+    if (start < 0) throw new RangeError.value(start);
     if (charCodes != null) {
       // TODO(srdjan): Also skip copying of wide typed arrays.
       final ccid = ClassID.getID(charCodes);
       bool isOneByteString = false;
       if ((ccid != ClassID.cidArray) &&
           (ccid != ClassID.cidGrowableObjectArray) &&
           (ccid != ClassID.cidImmutableArray)) {
         if ((charCodes is Uint8List) || (charCodes is Int8List)) {
           isOneByteString = true;
         } else {
-          charCodes = new List<int>.from(charCodes, growable: false);
+          charCodes = new List.from(charCodes, growable: false);
         }
       }
-      final len = charCodes.length;
+      if (end == null || end > charCodes.length) {

[srdjan, 2014/09/17 15:31:58]

Please use parentheses

[Lasse Reichstein Nielsen, 2014/09/18 09:03:00]

Done.

+        end = charCodes.length;
+      }
+      if (end <= start) return "";
+      final len = end - start;
       if (!isOneByteString) {
-        for (int i = 0; i < len; i++) {
+        for (int i = start; i < end; i++) {
           int e = charCodes[i];
           if (e is! _Smi) throw new ArgumentError(e);
           // Is e Latin1?
           if ((e < 0) || (e > 0xFF)) {
-            return _createFromCodePoints(charCodes);
+            return _createFromCodePoints(charCodes, start, end);
           }
         }
       }
       // Allocate a one byte string. When the list is 128 entries or longer,
       // it's faster to perform a runtime-call.
       if (len >= 128) {
-        return _OneByteString._allocateFromOneByteList(charCodes);
+        return _OneByteString._allocateFromOneByteList(charCodes, start, end);
       }
       var s = _OneByteString._allocate(len);
       for (int i = 0; i < len; i++) {
-        s._setAt(i, charCodes[i]);
+        s._setAt(i, charCodes[start + i]);
       }
       return s;
     }
-    return _createFromCodePoints(charCodes);
+    // charCodes is null.
+    throw new ArgumentError(charCodes);

[srdjan, 2014/09/17 15:31:58]

Why not in the start:
if (charCodes == null) throw new ArgumentError(charCodes);

and avoid the nested main body.

[Lasse Reichstein Nielsen, 2014/09/18 09:03:00]

A naive attempt at putting the common branch first. Probably irrelevant in
practice. I'll rewrite without the nesting.

   }
 
-  static String _createFromCodePoints(List<int> codePoints)
+  static String _createFromCodePoints(List<int> codePoints, int start, int end)
       native "StringBase_createFromCodePoints";
 
   String operator [](int index) native "String_charAt";
 
   int codeUnitAt(int index) native "String_codeUnitAt";
 
   int get length native "String_getLength";
 
   bool get isEmpty {
     return this.length == 0;
@@ skipping 805 matching lines @@
       return result;
     }
     return this;
   }
 
   // Allocates a string of given length, expecting its content to be
   // set using _setAt.
   static _OneByteString _allocate(int length) native "OneByteString_allocate";
 
 
-  static _OneByteString _allocateFromOneByteList(List<int> list)
+  static _OneByteString _allocateFromOneByteList(List<int> list,
+                                                 int start, int end)
       native "OneByteString_allocateFromOneByteList";
 
   // This is internal helper method. Code point value must be a valid
   // Latin1 value (0..0xFF), index must be valid.
   void _setAt(int index, int codePoint) native "OneByteString_setAt";
 }
 
 
 class _TwoByteString extends _StringBase implements String {
   factory _TwoByteString._uninstantiable() {
@@ skipping 80 matching lines @@
 class _CodeUnits extends Object with ListMixin<int>,
                                      UnmodifiableListMixin<int> {
   /** The string that this is the code units of. */
   String _string;
 
   _CodeUnits(this._string);
 
   int get length => _string.length;
   int operator[](int i) => _string.codeUnitAt(i);
 }