Fast Merge Sort

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

import java.util.Comparator;

/**
 * Faster merge sort. When original JDK routine runs 5s for sorting 1 million
 * objects this one runs for 3.5s.
 * <p>
 * reference: Arrays.mergeSort (private method).
 */
public class FastMergeSort {

  @SuppressWarnings( { "unchecked" })
  private static void mergeSort(Object src[], Object dest[], int low, int high, int off,
      Comparator c) {
    int length = high - low;

    // use insertion sort on smallest arrays
    if (length < 7) {
      for (int i = low; i < high; i++) {
        for (int j = i; j > low && c.compare(dest[j - 1], dest[j]) > 0; j--) {
          Object temp = dest[j];
          dest[j] = dest[j - 1];
          dest[j - 1] = temp;
        }
      }
      return;
    }

    // recursively sort halves of dest into src
    int destLow = low;
    int destHigh = high;
    low += off;
    high += off;
    int mid = (low + high) >> 1;
    mergeSort(dest, src, low, mid, -off, c);
    mergeSort(dest, src, mid, high, -off, c);

    // is list already sorted?
    if (c.compare(src[mid - 1], src[mid]) <= 0) {
      System.arraycopy(src, low, dest, destLow, length);
      return;
    }

    // merge sorted halves from src into dest
    for (int i = destLow, p = low, q = mid; i < destHigh; i++) {
      if (q >= high || p < mid && c.compare(src[p], src[q]) <= 0) {
        dest[i] = src[p++];
      } else {
        dest[i] = src[q++];
      }
    }
  }

  public void sort(Object[] a, Comparator c) {
    Object aux[] = a.clone();
    mergeSort(aux, a, 0, a.length, 0, c);
  }

  public void sort(Comparable[] a) {
    Object aux[] = a.clone();
    mergeSort(aux, a, 0, a.length, 0, ComparableComparator.INSTANCE);
  }

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

  public static void doSort(Object[] a, Comparator c) {
    Object aux[] = a.clone();
    mergeSort(aux, a, 0, a.length, 0, c);
  }

  public static void doSort(Comparable[] a) {
    Object aux[] = a.clone();
    mergeSort(aux, a, 0, a.length, 0, 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);
  }

}