Change quicksort pivot from random to median-of-three.

src/array.js

Index: src/array.js
diff --git a/src/array.js b/src/array.js
index a805157b13b516977189bd7c67b83654e448c36a..73c761e8ac05361244bd7cf68cb806db4bbe8ff1 100644
--- a/src/array.js
+++ b/src/array.js
@@ -641,6 +641,23 @@ function ArraySplice(start, delete_count) {
 }
 
 
+function InsertionSort(a, from, to, comparefn, global_receiver) {
+  for (var i = from + 1; i < to; i++) {
+    var element = a[i];
+    for (var j = i - 1; j >= from; j--) {
+      var tmp = a[j];
+      var order = %_CallFunction(global_receiver, tmp, element, comparefn);
+      if (order > 0) {
+        a[j + 1] = tmp;
+      } else {
+        break;
+      }
+    }
+    a[j + 1] = element;
+  }

[Karl Klose, 2010/12/17 09:55:37]

Consider putting InsertionSort back into ArraySort, so that it is on the same
level as QuickSort

[Lasse Reichstein, 2010/12/17 10:04:22]

Done.
It was an attempt at a micro-optimization, but its impact, if there is one, is
below the level of measurability.

+}
+
+
 function ArraySort(comparefn) {
   // In-place QuickSort algorithm.
   // For short (length <= 22) arrays, insertion sort is used for efficiency.
@@ -659,38 +676,54 @@ function ArraySort(comparefn) {
   }
   var global_receiver = %GetGlobalReceiver();
 
-  function InsertionSort(a, from, to) {
-    for (var i = from + 1; i < to; i++) {
-      var element = a[i];
-      for (var j = i - 1; j >= from; j--) {
-        var tmp = a[j];
-        var order = %_CallFunction(global_receiver, tmp, element, comparefn);
-        if (order > 0) {
-          a[j + 1] = tmp;
-        } else {
-          break;
-        }
-      }
-      a[j + 1] = element;
-    }
-  }
-
   function QuickSort(a, from, to) {
     // Insertion sort is faster for short arrays.
-    if (to - from <= 22) {
-      InsertionSort(a, from, to);
+    if (to - from <= 13) {
+      InsertionSort(a, from, to, comparefn, global_receiver);
       return;
     }
-    var pivot_index = $floor($random() * (to - from)) + from;
-    var pivot = a[pivot_index];
-    // Issue 95: Keep the pivot element out of the comparisons to avoid
-    // infinite recursion if comparefn(pivot, pivot) != 0.
-    %_SwapElements(a, from, pivot_index);
-    var low_end = from;   // Upper bound of the elements lower than pivot.
-    var high_start = to;  // Lower bound of the elements greater than pivot.
+
+    // Find a pivot as the median of first, last and middle element.
+    var v0 = a[from];
+    var v1 = a[to - 1];
+    var middle_index = from + ((to - from) >> 1);
+    var v2 = a[middle_index];
+    var c01 = comparefn(v0, v1);
+    if (c01 > 0) {
+      // v1 < v0, so swap them.
+      var tmp = v0;
+      v0 = v1;
+      v1 = tmp;
+    } // v0 <= v1.
+    var c02 = comparefn(v0, v2);
+    if (c02 >= 0) {
+      // v2 <= v0 <= v1.
+      var tmp = v0;
+      v0 = v2;
+      v2 = v1;
+      v1 = tmp;
+    } else {
+      // v0 <= v1 && v0 < v2
+      var c12 = comparefn(v1, v2);
+      if (c12 > 0) {
+        // v0 <= v2 < v1
+        var tmp = v1;
+        v1 = v2;
+        v2 = tmp;
+      }
+    }
+    // v0 <= v1 <= v2
+    a[from] = v0;
+    a[to - 1] = v2;
+    var pivot = v1;
+    var low_end = from + 1;   // Upper bound of the elements lower than pivot.
+    var high_start = to - 1;  // Lower bound of the elements greater than pivot.
+    a[middle_index] = a[low_end];
+    a[low_end] = pivot;
+
     // From low_end to i are elements equal to pivot.
     // From i to high_start are elements that haven't been compared yet.
-    for (var i = from + 1; i < high_start; ) {
+    for (var i = low_end + 1; i < high_start; ) {
       var element = a[i];
       var order = %_CallFunction(global_receiver, element, pivot, comparefn);
       if (order < 0) {