Proof-of-concept matchPrefix on Pattern.

sdk/lib/_internal/compiler/implementation/lib/js_string.dart

Index: sdk/lib/_internal/compiler/implementation/lib/js_string.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/js_string.dart b/sdk/lib/_internal/compiler/implementation/lib/js_string.dart
index 92d7c673a2a62bc8da0b152e3afa0a81c59d5bdb..9c9c4a38b43bed8fdd5373488e7f1898fc014f81 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/js_string.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/js_string.dart
@@ -25,6 +25,19 @@ class JSString extends Interceptor implements String, JSIndexable {
     return allMatchesInStringUnchecked(this, str);
   }
 
+  Match matchPrefix(String string, [int start = 0]) {
+    if (start < 0 || start > string.length) {
+      throw new RangeError.range(start, 0, string.length);
+    }
+    if (start + this.length > string.length) return null;
+    for (int i = 0; i < this.length; i++) {

[floitsch, 2013/06/11 12:45:24]

TODO that this could be made much faster.

[Lasse Reichstein Nielsen, 2013/06/12 09:24:05]

I'm not sure it can be made that much faster. If it's translated to similar JS
code, I think an optimizing VM like V8 should do fine on it.

[sra1, 2013/06/13 17:30:54]

The code is pretty horrible.
dart2js inlines codeUnitAt which contains bounds checks.



  matchAsPrefix$2: function(receiver, string, start) {
    var t1, t2, i, t3;
    if (start < 0 || start > string.length)
      throw $.wrapException(new $.RangeError("value " + $.S(start) + " not in
range 0.." + string.length));
    t1 = receiver.length;
    t2 = string.length;
    if (start + t1 > t2)
      return;
    for (i = 0; i < t1; ++i) {
      t3 = start + i;
      if (t3 < 0)
        $.throwExpression(new $.RangeError("value " + $.S(t3)));
      if (t3 >= t2)
        $.throwExpression(new $.RangeError("value " + $.S(t3)));
      t3 = string.charCodeAt(t3);
      if (i >= t1)
        $.throwExpression(new $.RangeError("value " + i));
      if (t3 !== receiver.charCodeAt(i))
        return;
    }
    return new $.StringMatch(start, string, receiver);
  },

Options:
1. Improve the (hand written) bounds checking elimination
2. Tweak the code by replacing  x.codeUnitAt(i) by JS('int', '#.charCodeAt(#)',
x, i)
3. Change the code to use JavaScript string level operations

I think #2 is fair game.

+      if (string.codeUnitAt(start + i) != this.codeUnitAt(i)) {
+        return null;
+      }
+    }
+    return new _StringMatch(start, string, this);
+  }
+
   String operator +(String other) {
     if (other is !String) throw new ArgumentError(other);
     return JS('String', r'# + #', this, other);