Handle join of sparse arrays with non-empty separator more efficiently.

src/array.js

 // Copyright 2010 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 49 matching lines @@
           keys.push(key);
         }
       }
     }
   }
   keys.sort(function(a, b) { return a - b; });
   return keys;
 }
 
 
+function SparseJoinWithSeparator(array, len, convert, separator) {
+  var keys = GetSortedArrayKeys(array, %GetArrayKeys(array, len));
+  var totalLength = 0;
+  var elements = new InternalArray(keys.length * 2);
+  var previousKey = -1;
+  for (var i = 0; i < keys.length; i++) {
+    var key = keys[i];
+    if (key != previousKey) {  // keys may contain duplicates.
+      var e = array[key];
+      if (!IS_STRING(e)) e = convert(e);
+      elements[i * 2] = key;
+      elements[i * 2 + 1] = e;
+      previousKey = key;
+    }
+  }
+  return %SparseJoinWithSeparator(elements, len, separator);
+}
+
+
 // Optimized for sparse arrays if separator is ''.
 function SparseJoin(array, len, convert) {
   var keys = GetSortedArrayKeys(array, %GetArrayKeys(array, len));
   var last_key = -1;
   var keys_length = keys.length;
 
   var elements = new InternalArray(keys_length);
   var elements_length = 0;
 
   for (var i = 0; i < keys_length; i++) {
@@ skipping 23 matching lines @@
   var is_array = IS_ARRAY(array);
 
   if (is_array) {
     // If the array is cyclic, return the empty string for already
     // visited arrays.
     if (!%PushIfAbsent(visited_arrays, array)) return '';
   }
 
   // Attempt to convert the elements.
   try {
-    if (UseSparseVariant(array, length, is_array) && (separator.length == 0)) {
-      return SparseJoin(array, length, convert);
+    if (UseSparseVariant(array, length, is_array)) {
+      if (separator.length == 0) {
+        return SparseJoin(array, length, convert);
+      } else {
+        return SparseJoinWithSeparator(array, length, convert, separator);
+      }
     }
 
     // Fast case for one-element arrays.
     if (length == 1) {
       var e = array[0];
       if (IS_STRING(e)) return e;
       return convert(e);
     }
 
     // Construct an array for the elements.
     var elements = new InternalArray(length);
 
     // We pull the empty separator check outside the loop for speed!
     if (separator.length == 0) {
       var elements_length = 0;
       for (var i = 0; i < length; i++) {
         var e = array[i];
-        if (!IS_UNDEFINED(e)) {
-          if (!IS_STRING(e)) e = convert(e);
-          elements[elements_length++] = e;
-        }
+        if (!IS_STRING(e)) e = convert(e);
+        elements[elements_length++] = e;
       }
       elements.length = elements_length;
       var result = %_FastAsciiArrayJoin(elements, '');
       if (!IS_UNDEFINED(result)) return result;
       return %StringBuilderConcat(elements, elements_length, '');
     }
     // Non-empty separator case.
     // If the first element is a number then use the heuristic that the
     // remaining elements are also likely to be numbers.
     if (!IS_NUMBER(array[0])) {
       for (var i = 0; i < length; i++) {
         var e = array[i];
         if (!IS_STRING(e)) e = convert(e);
         elements[i] = e;
       }
     } else {
       for (var i = 0; i < length; i++) {
         var e = array[i];
-        if (IS_NUMBER(e)) elements[i] = %_NumberToString(e);
-        else {
-          if (!IS_STRING(e)) e = convert(e);
+          if (IS_NUMBER(e)) {
+            e = %_NumberToString(e);
+          } else if (!IS_STRING(e)) {
+            e = convert(e);
+          }
           elements[i] = e;
-        }
       }
     }
     var result = %_FastAsciiArrayJoin(elements, separator);
     if (!IS_UNDEFINED(result)) return result;
 
     return %StringBuilderJoin(elements, length, separator);
   } finally {
     // Make sure to remove the last element of the visited array no
     // matter what happens.
     if (is_array) visited_arrays.length = visited_arrays.length - 1;
@@ skipping 1071 matching lines @@
   InternalArray.prototype.join = getFunction("join", ArrayJoin);
   InternalArray.prototype.pop = getFunction("pop", ArrayPop);
   InternalArray.prototype.push = getFunction("push", ArrayPush);
   InternalArray.prototype.toString = function() {
     return "Internal Array, length " + this.length;
   };
 }
 
 
 SetupArray();