Java Data Structure How to - Implement a Fast Quick Sort

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Question

We would like to know how to implement a Fast Quick Sort.

Answer

//  ww w. ja  v a  2s.com


     
// Copyright (c) 2003-2009, Jodd Team (jodd.org). All Rights Reserved.


import java.util.Comparator;


/**
 * Maybe the fastest implementation of famous Quick-Sort
 * algorithm. It is even faster than Denisa Ahrensa implementation that
 * performs 7.5s for sorting million objects, this implementation
 * sorts for 6.8s. However, {@link FastMergeSort} is much faster.     
 */
public class FastQuickSort  {
  
  @SuppressWarnings({"unchecked"})
  public static void qsort(Object[] c, Comparator comparator) {
    int i, j, left = 0, right = c.length - 1, stack_pointer = -1;
    int[] stack = new int[128];
    Object swap, temp;
    while (true) {
      if (right - left <= 7) {
        for (j = left + 1; j <= right; j++) {
          swap = c[j];
          i = j - 1;
          while (i >= left && comparator.compare(c[i], swap) > 0) {
            c[i + 1] = c[i--];
          }
          c[i + 1] = swap;
        }
        if (stack_pointer == -1) {
          break;
        }
        right = stack[stack_pointer--];
        left = stack[stack_pointer--];
      } else {
        int median = (left + right) >> 1;
        i = left + 1;
        j = right;
        swap = c[median]; c[median] = c[i]; c[i] = swap;
        if (comparator.compare(c[left], c[right]) > 0) {
          swap = c[left]; c[left] = c[right]; c[right] = swap;
        }
        if (comparator.compare(c[i], c[right]) > 0) {
          swap = c[i]; c[i] = c[right]; c[right] = swap;
        }
        if (comparator.compare(c[left], c[i]) > 0) {
          swap = c[left]; c[left] = c[i]; c[i] = swap;
        }
        temp = c[i];
        while (true) {
          //noinspection ControlFlowStatementWithoutBraces,StatementWithEmptyBody
          while (comparator.compare(c[++i], temp) < 0);
          //noinspection ControlFlowStatementWithoutBraces,StatementWithEmptyBody
          while (comparator.compare(c[--j], temp) > 0);
          if (j < i) {
            break;
          }
          swap = c[i]; c[i] = c[j]; c[j] = swap;
        }
        c[left + 1] = c[j];
        c[j] = temp;
        if (right - i + 1 >= j - left) {
          stack[++stack_pointer] = i;
          stack[++stack_pointer] = right;
          right = j - 1;
        } else {
          stack[++stack_pointer] = left;
          stack[++stack_pointer] = j - 1;
          left = i;
        }
      }
    }
  } 

  public void sort(Object a[], Comparator comparator) {
    qsort(a, comparator);
  }
  
  public void sort(Comparable a[]) {
    qsort(a, new ComparableComparator());
  }

  // ---------------------------------------------------------------- static

  public static void doSort(Object a[], Comparator comparator) {
    qsort(a, comparator);
  }

  public static void doSort(Comparable a[]) {
    qsort(a, ComparableComparator.INSTANCE);
  }

}

// Copyright (c) 2003-2009, Jodd Team (jodd.org). All Rights Reserved.

/**
 * Comparator that adapts <code>Comparables to the
 * <code>Comparator interface.
 */
class ComparableComparator<T extends Comparable<T>> implements Comparator<T> {

  /**
   * Cached instance.
   */
  public static final ComparableComparator INSTANCE = new ComparableComparator();

  public int compare(T o1, T o2) {
    return o1.compareTo(o2);
  }

}

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