Quicksort: Choose pivot with recursive sort of pivot candidates on large...

Quicksort: Choose pivot with recursive sort of pivot candidates on large arrays to avoid patholgical cases.
Review URL: http://codereview.chromium.org/10532193

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11873 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/array.js

@@ -777,18 +777,36 @@ function ArraySort(comparefn) {
     }
   };
 
+  var GetThirdIndex = function(a, from, to) {
+    var t_array = [];
+    // Use both 'from' and 'to' to determine the pivot candidates.
+    var increment = 200 + ((to - from) & 15);
+    for (var i = from + 1; i < to - 1; i += increment) {
+      t_array.push([i, a[i]]);
+    }
+    t_array.sort(function(a, b) {
+        return %_CallFunction(receiver, a[1], b[1], comparefn) } );
+    var third_index = t_array[t_array.length >> 1][0];
+    return third_index;
+  }
+
   var QuickSort = function QuickSort(a, from, to) {
+    var third_index = 0;
     while (true) {
       // Insertion sort is faster for short arrays.
       if (to - from <= 10) {
         InsertionSort(a, from, to);
         return;
       }
+      if (to - from > 1000) {
+        third_index = GetThirdIndex(a, from, to);
+      } else {
+        third_index = from + ((to - from) >> 1);
+      }
       // 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 v2 = a[third_index];
       var c01 = %_CallFunction(receiver, v0, v1, comparefn);
       if (c01 > 0) {
         // v1 < v0, so swap them.
@@ -819,7 +837,7 @@ function ArraySort(comparefn) {
       var pivot = v1;
       var low_end = from + 1;   // Upper bound of elements lower than pivot.
       var high_start = to - 1;  // Lower bound of elements greater than pivot.
-      a[middle_index] = a[low_end];
+      a[third_index] = a[low_end];
       a[low_end] = pivot;
 
       // From low_end to i are elements equal to pivot.

test/mjsunit/regress/regress-2185-2.js

+// Copyright 2012 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
+//       with the distribution.
+//     * Neither the name of Google Inc. nor the names of its
+//       contributors may be used to endorse or promote products derived
+//       from this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+// These tests used to time out before this was fixed.
+
+var LEN = 1e5;
+
+function short() {
+  var sum = 0;
+  for (var i = 0; i < 10000; i++) {
+    var a = [1, 4, 34, 23, 6, 123, 3, 2, 11, 515, 4, 33, 22, 2, 2, 1, 0, 123,
+            23, 42, 43, 1002, 44, 43, 101, 23, 55, 11, 101, 102, 45, 11, 404,
+            31415, 34, 53, 453, 45, 34, 5, 2, 35, 5, 345, 36, 45, 345, 3, 45,
+            3, 5, 5, 2, 2342344, 2234, 23, 2718, 1500, 2, 19, 22, 43, 41, 0,
+            -1, 33, 45, 78];
+    a.sort(function(a, b) { return a - b; });
+    sum += a[0];
+  }
+  return sum;
+}
+
+function short_bench(name, array) {
+  var start = new Date();
+  short();
+  var end = new Date();
+  var ms = end - start;
+  print("Short " + Math.floor(ms) + "ms");
+}
+
+function sawseq(a, tooth) {
+  var count = 0;
+  while (true) {
+    for (var i = 0; i < tooth; i++) {
+      a.push(i);
+      if (++count >= LEN) return a;
+    }
+  }
+}
+
+function sawseq2(a, tooth) {
+  var count = 0;
+  while (true) {
+    for (var i = 0; i < tooth; i++) {
+      a.push(i);
+      if (++count >= LEN) return a;
+    }
+    for (var i = 0; i < tooth; i++) {
+      a.push(tooth - i);
+      if (++count >= LEN) return a;
+    }
+  }
+}
+
+function sawseq3(a, tooth) {
+  var count = 0;
+  while (true) {
+    for (var i = 0; i < tooth; i++) {
+      a.push(tooth - i);
+      if (++count >= LEN) return a;
+    }
+  }
+}
+
+function up(a) {
+  for (var i = 0; i < LEN; i++) {
+    a.push(i);
+  }
+  return a;
+}
+
+function down(a) {
+  for (var i = 0; i < LEN; i++) {
+    a.push(LEN - i);
+  }
+  return a;
+}
+
+function ran(a) {
+  for (var i = 0; i < LEN; i++) {
+    a.push(Math.floor(Math.random() * LEN));
+  }
+  return a;
+}
+
+var random = ran([]);
+var asc = up([]);
+var desc = down([]);
+var asc_desc = down(up([]));
+var desc_asc = up(down([]));
+var asc_asc = up(up([]));
+var desc_desc = down(down([]));
+var saw1 = sawseq([], 1000);
+var saw2 = sawseq([], 500);
+var saw3 = sawseq([], 200);
+var saw4 = sawseq2([], 200);
+var saw5 = sawseq3([], 200);
+
+function bench(name, array) {
+  var start = new Date();
+  array.sort(function(a, b) { return a - b; });
+  var end = new Date();
+  for (var i = 0; i < array.length - 1; i++) {
+    if (array[i] > array[i + 1]) throw name + " " + i;
+  }
+  var ms = end - start;
+  print(name + " " + Math.floor(ms) + "ms");
+}
+
+short_bench();
+bench("random", random);
+bench("up", asc);
+bench("down", desc);
+bench("saw 1000", saw1);
+bench("saw 500", saw2);
+bench("saw 200", saw3);
+bench("saw 200 symmetric", saw4);
+bench("saw 200 down", saw4);
+bench("up, down", asc_desc);
+bench("up, up", asc_asc);
+bench("down, down", desc_desc);
+bench("down, up", desc_asc);