Optimize algorithm for String.prototype.repeat(), fix value caching in others

src/harmony-string.js

Index: src/harmony-string.js
diff --git a/src/harmony-string.js b/src/harmony-string.js
index 4cd8e6687edaeacb51079801b74480a21cc352b0..debdc7e6600aaf968e444aeb728efd34d1e780b9 100644
--- a/src/harmony-string.js
+++ b/src/harmony-string.js
@@ -21,12 +21,17 @@ function StringRepeat(count) {
     throw MakeRangeError("invalid_count_value", []);
   }
 
-  var elements = new InternalArray(n);
-  for (var i = 0; i < n; i++) {
-    elements[i] = s;
-  }
+  if (n < 1) return '';

[Michael Starzinger, 2014/07/28 11:13:52]

This special case is not needed if comment below is addressed.

+
+  // Optimized O(log N) algorithm, without the added overhead of an extra array

[Michael Starzinger, 2014/07/28 11:13:51]

nit: Let's drop (at least) the second half of the comment. Not sure it's worth
referring to an array that no longer exists.

+  var res = '';
 
-  return %StringBuilderConcat(elements, n, "");
+  while (n > 1) {

[Michael Starzinger, 2014/07/28 11:13:51]

If we change the predicate to be "n > 0" most special cases resolve naturally.

+      if (n & 1) res += s;
+      n >>= 1;
+      s += s;
+  }
+  return res + s;

[Michael Starzinger, 2014/07/28 11:13:51]

This addition is not needed if comment above is addressed.

 }
 
 
@@ -50,8 +55,7 @@ function StringStartsWith(searchString /* position */) {  // length == 1
 
   var s_len = s.length;
   var start = MathMin(MathMax(pos, 0), s_len);
-  var ss_len = ss.length;
-  if (ss_len + start > s_len) {
+  if (ss.length + start > s_len) {
     return false;
   }
 
@@ -81,8 +85,7 @@ function StringEndsWith(searchString /* position */) {  // length == 1
   }
 
   var end = MathMin(MathMax(pos, 0), s_len);
-  var ss_len = ss.length;
-  var start = end - ss_len;
+  var start = end - ss.length;
   if (start < 0) {
     return false;
   }
@@ -111,8 +114,7 @@ function StringContains(searchString /* position */) {  // length == 1
 
   var s_len = s.length;
   var start = MathMin(MathMax(pos, 0), s_len);
-  var ss_len = ss.length;
-  if (ss_len + start > s_len) {
+  if (ss.length + start > s_len) {
     return false;
   }