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Yaroslavsky Sort: 2-pivot quicksort + Insertion sort for smaller chunks
// See original announcement at http://permalink.gmane.org/gmane.comp.java.openjdk.core-libs.devel/2628
/**
* <at> author Vladimir Yaroslavskiy
* <at> version 2009.09.10 m765
*/
public class DualPivotQuicksort {
public static void sort(int[] a) {
sort(a, 0, a.length);
}
public static void sort(int[] a, int fromIndex, int toIndex) {
rangeCheck(a.length, fromIndex, toIndex);
dualPivotQuicksort(a, fromIndex, toIndex - 1, 3);
}
private static void rangeCheck(int length, int fromIndex, int toIndex) {
if (fromIndex > toIndex) {
throw new IllegalArgumentException("fromIndex(" + fromIndex + ") > toIndex(" + toIndex + ")");
}
if (fromIndex < 0) {
throw new ArrayIndexOutOfBoundsException(fromIndex);
}
if (toIndex > length) {
throw new ArrayIndexOutOfBoundsException(toIndex);
}
}
private static void swap(int[] a, int i, int j) {
int temp = a[i];
a[i] = a[j];
a[j] = temp;
}
private static void dualPivotQuicksort(int[] a, int left, int right, int div) {
int len = right - left;
if (len < 27) { // insertion sort for tiny array
for (int i = left + 1; i <= right; i++) {
for (int j = i; j > left && a[j] < a[j - 1]; j--) {
swap(a, j, j - 1);
}
}
return;
}
int third = len / div;
// "medians"
int m1 = left + third;
int m2 = right - third;
if (m1 <= left) {
m1 = left + 1;
}
if (m2 >= right) {
m2 = right - 1;
}
if (a[m1] < a[m2]) {
swap(a, m1, left);
swap(a, m2, right);
}
else {
swap(a, m1, right);
swap(a, m2, left);
}
// pivots
int pivot1 = a[left];
int pivot2 = a[right];
// pointers
int less = left + 1;
int great = right - 1;
// sorting
for (int k = less; k <= great; k++) {
if (a[k] < pivot1) {
swap(a, k, less++);
}
else if (a[k] > pivot2) {
while (k < great && a[great] > pivot2) {
great--;
}
swap(a, k, great--);
if (a[k] < pivot1) {
swap(a, k, less++);
}
}
}
// swaps
int dist = great - less;
if (dist < 13) {
div++;
}
swap(a, less - 1, left);
swap(a, great + 1, right);
// subarrays
dualPivotQuicksort(a, left, less - 2, div);
dualPivotQuicksort(a, great + 2, right, div);
// equal elements
if (dist > len - 13 && pivot1 != pivot2) {
for (int k = less; k <= great; k++) {
if (a[k] == pivot1) {
swap(a, k, less++);
}
else if (a[k] == pivot2) {
swap(a, k, great--);
if (a[k] == pivot1) {
swap(a, k, less++);
}
}
}
}
// subarray
if (pivot1 < pivot2) {
dualPivotQuicksort(a, less, great, div);
}
}
}
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