Make better use of the %_FastAsciiArrayJoin function.

src/array.js

Index: src/array.js
===================================================================
--- src/array.js	(revision 6227)
+++ src/array.js	(working copy)
@@ -125,12 +125,11 @@
     }
 
     // Construct an array for the elements.
-    var elements;
+    var elements = new $Array(length);
     var elements_length = 0;
 
     // We pull the empty separator check outside the loop for speed!
     if (separator.length == 0) {
-      elements = new $Array(length);
       for (var i = 0; i < length; i++) {
         var e = array[i];
         if (!IS_UNDEFINED(e) || (i in array)) {
@@ -138,21 +137,32 @@
           elements[elements_length++] = e;
         }
       }
-    } else {
-      elements = new $Array(length << 1);
-      for (var i = 0; i < length; i++) {
-        var e = array[i];
-        if (i != 0) elements[elements_length++] = separator;
-        if (!IS_UNDEFINED(e) || (i in array)) {
-          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.
+    for (var i = 0; i < length; i++) {
+      var e = array[i];
+      if (!IS_UNDEFINED(e) || (i in array)) {

[Lasse Reichstein, 2011/01/07 14:58:58]

If all actually used convert functions convert undefined to the empty string,
feel free to omit the (i in array) test, and document that convert isn't called
on undefined either.

[sandholm, 2011/01/08 12:49:52]

Done.

+        if (!IS_STRING(e)) e = convert(e);
+        elements[i] = e;
+      } else {
+        elements[i] = '';

[Lasse Reichstein, 2011/01/07 14:58:58]

Would it be slower to have a separate counter for inserts, and not insert empty
strings at all?
(And remember to set the length at the end).

[sandholm, 2011/01/08 12:49:52]

You need to know where the separator goes, hence a single counter wouldn't
suffice.

       }
     }
-    elements.length = elements_length;
-    var result = %_FastAsciiArrayJoin(elements, "");
-    if (!IS_UNDEFINED(result)) return result;
-    return %StringBuilderConcat(elements, elements_length, '');
+    var result = %_FastAsciiArrayJoin(elements, separator);
+    if (!IS_UNDEFINED(result)) return result;   
+
+    var length2 = (length << 1) - 1;
+    var elements2 = new $Array(length2);
+    elements2[0] = elements[0];
+    for (var i = 1, j = 1; i < length; i++) {
+      elements2[j++] = separator;
+      elements2[j++] = elements[i];
+    }

[Lasse Reichstein, 2011/01/07 14:58:58]

If you run backwards, you could expand the array in-place, and not allocate a
second array.
If the array had twice the size from the start, you wouldn't even need to expand
it - but you would waste some memory in some cases.

[sandholm, 2011/01/08 12:49:52]

Done.

+    return %StringBuilderConcat(elements2, length2, '');    
   } finally {
     // Make sure to pop the visited array no matter what happens.
     if (is_array) visited_arrays.pop();