[es6] Optimize String{Starts, Ends}With

src/string.js

Index: src/string.js
diff --git a/src/string.js b/src/string.js
index 1db65bb0bad25b677c7fa38144976c6a39f4ca18..97ed3cdb6e7c2b502f19d16b60528522f25c77e2 100644
--- a/src/string.js
+++ b/src/string.js
@@ -15,8 +15,6 @@ var GlobalRegExp = global.RegExp;
 var GlobalString = global.String;
 var InternalArray = utils.InternalArray;
 var InternalPackedArray = utils.InternalPackedArray;
-var MathMax;
-var MathMin;
 var RegExpExec;
 var RegExpExecNoTests;
 var RegExpLastMatchInfo;
@@ -27,8 +25,6 @@ var ToString;
 utils.Import(function(from) {
   ArrayIndexOf = from.ArrayIndexOf;
   ArrayJoin = from.ArrayJoin;
-  MathMax = from.MathMax;
-  MathMin = from.MathMin;
   RegExpExec = from.RegExpExec;
   RegExpExecNoTests = from.RegExpExecNoTests;
   RegExpLastMatchInfo = from.RegExpLastMatchInfo;
@@ -986,18 +982,28 @@ function StringStartsWith(searchString /* position */) {  // length == 1
   var ss = TO_STRING_INLINE(searchString);
   var pos = 0;
   if (%_ArgumentsLength() > 1) {
-    pos = %_Arguments(1);  // position
-    pos = $toInteger(pos);
+    var arg = %_Arguments(1);  // position
+    if (!IS_UNDEFINED(arg)) {
+      pos = $toInteger(arg);
+    }
   }
 
   var s_len = s.length;
-  var start = MathMin(MathMax(pos, 0), s_len);
+  if (pos < 0) pos = 0;
+  if (pos > s_len) pos = s_len;

[Lasse Reichstein Nielsen, 2015/09/08 10:24:57]

DBC: Would it be useful to make this an "else" of the first test since both
can't be true?
And move the tests inside the second if above (no need to do them if pos is 0)?

   var ss_len = ss.length;
-  if (ss_len + start > s_len) {
+
+  if (ss_len + pos > s_len) {
     return false;
   }
 
-  return %_SubString(s, start, start + ss_len) === ss;
+  for (var i = 0; i < ss_len; i++) {
+    if (%_StringCharCodeAt(s, pos + i) !== %_StringCharCodeAt(ss, i)) {
+      return false;
+    }
+  }
+
+  return true;
 }
 
 
@@ -1021,14 +1027,22 @@ function StringEndsWith(searchString /* position */) {  // length == 1
     }
   }
 
-  var end = MathMin(MathMax(pos, 0), s_len);
+  if (pos < 0) pos = 0;
+  if (pos > s_len) pos = s_len;
   var ss_len = ss.length;
-  var start = end - ss_len;
-  if (start < 0) {
+  pos = pos - ss_len;
+
+  if (pos < 0) {
     return false;
   }

[Lasse Reichstein Nielsen, 2015/09/08 10:24:57]

The spec is really overspecifying here - if searchString is the empty string, a
lot of this work is unnecessary.

Maybe just::

   ... 
   pos = $toInteger(arg);
   if (pos > s_len) pos = s_len;
  }
 }
 var ss_len = ss.length;
 if (pos < ss_len) return ss_len == 0;

 
-  return %_SubString(s, start, start + ss_len) === ss;
+  for (var i = 0; i < ss_len; i++) {
+    if (%_StringCharCodeAt(s, pos + i) !== %_StringCharCodeAt(ss, i)) {
+      return false;
+    }
+  }
+
+  return true;
 }