[es6] Optimize String{Starts, Ends}With

src/string.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 (function(global, utils) {
 
 %CheckIsBootstrapping();
 
 // -------------------------------------------------------------------
 // Imports
 
 var ArrayIndexOf;
 var ArrayJoin;
 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;
 var SymbolToString;
 var ToNumber;
 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;
   SymbolToString = from.SymbolToString;
   ToNumber = from.ToNumber;
   ToString = from.ToString;
 });
 
 //-------------------------------------------------------------------
 
@@ skipping 937 matching lines @@
 
   var s = TO_STRING_INLINE(this);
 
   if (IS_REGEXP(searchString)) {
     throw MakeTypeError(kFirstArgumentNotRegExp, "String.prototype.startsWith");
   }
 
   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;
 }
 
 
 // ES6 draft 04-05-14, section 21.1.3.7
 function StringEndsWith(searchString /* position */) {  // length == 1
   CHECK_OBJECT_COERCIBLE(this, "String.prototype.endsWith");
 
   var s = TO_STRING_INLINE(this);
 
   if (IS_REGEXP(searchString)) {
     throw MakeTypeError(kFirstArgumentNotRegExp, "String.prototype.endsWith");
   }
 
   var ss = TO_STRING_INLINE(searchString);
   var s_len = s.length;
   var pos = s_len;
   if (%_ArgumentsLength() > 1) {
     var arg = %_Arguments(1);  // position
     if (!IS_UNDEFINED(arg)) {
       pos = $toInteger(arg);
     }
   }
 
-  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;
 }
 
 
 // ES6 draft 04-05-14, section 21.1.3.6
 function StringIncludes(searchString /* position */) {  // length == 1
   CHECK_OBJECT_COERCIBLE(this, "String.prototype.includes");
 
   var string = TO_STRING_INLINE(this);
 
   if (IS_REGEXP(searchString)) {
@@ skipping 164 matching lines @@
   to.StringLastIndexOf = StringLastIndexOfJS;
   to.StringMatch = StringMatchJS;
   to.StringReplace = StringReplace;
   to.StringSlice = StringSlice;
   to.StringSplit = StringSplitJS;
   to.StringSubstr = StringSubstr;
   to.StringSubstring = StringSubstring;
 });
 
 })