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

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Tweak quicksort bailout parameter to minimize time.

Review URL: http://codereview.chromium.org/5962003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@6063 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
This commit is contained in:
lrn@chromium.org 2010-12-17 10:06:11 +00:00
parent 7b6d7f5ba4
commit 9f1bcdf8ac
1 changed files with 43 additions and 11 deletions
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54
src/array.js
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@ -677,20 +677,52 @@ function ArraySort(comparefn) {
function QuickSort(a, from, to) {
// Insertion sort is faster for short arrays.
if (to - from <= 22) {
if (to - from <= 13) {
InsertionSort(a, from, to);
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) {
@ -708,8 +740,8 @@ function ArraySort(comparefn) {
QuickSort(a, high_start, to);
}
// Copies elements in the range 0..length from obj's prototype chain
// to obj itself, if obj has holes. Returns one more than the maximal index
// Copy elements in the range 0..length from obj's prototype chain
// to obj itself, if obj has holes. Return one more than the maximal index
// of a prototype property.
function CopyFromPrototype(obj, length) {
var max = 0;