MapReduce中的排序之 -- 归并排序

排序是大数据的核心精华.

快排和归并排序更是MapReduce中的精华部分. 

以下是归并排序部分.

一、百度百科: 

        归并排序（MERGE-SORT）是建立在归并操作上的一种有效的排序算法,该算法是采用分治法（Divide and Conquer）的一个非常典型的应用。将已有序的子序列合并，得到完全有序的序列；即先使每个子序列有序，再使子序列段间有序。若将两个有序表合并成一个有序表，称为二路归并。归并排序是一种稳定的排序方法。

二、通俗解释:

三、Show you code:       

四、MapReduce中的归并排序源码

package org.apache.hadoop.util;

import java.util.Comparator;

import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;
import org.apache.hadoop.io.IntWritable;

/** An implementation of the core algorithm of MergeSort. */
@InterfaceAudience.LimitedPrivate({"MapReduce"})
@InterfaceStability.Unstable
public class MergeSort {
  //Reusable IntWritables
  IntWritable I = new IntWritable(0);
  IntWritable J = new IntWritable(0);
  
  //the comparator that the algo should use
  private Comparator comparator;
  
  public MergeSort(Comparator comparator) {
    this.comparator = comparator;
  }
  
  public void mergeSort(int src[], int dest[], int low, int high) {
    int length = high - low;

    // Insertion sort on smallest arrays
    if (length < 7) {
      for (int i=low; i low; j--) {
          I.set(dest[j-1]);
          J.set(dest[j]);
          if (comparator.compare(I, J)>0)
            swap(dest, j, j-1);
        }
      }
      return;
    }

    // Recursively sort halves of dest into src
    int mid = (low + high) >>> 1;
    mergeSort(dest, src, low, mid);
    mergeSort(dest, src, mid, high);

    I.set(src[mid-1]);
    J.set(src[mid]);
    // If list is already sorted, just copy from src to dest.  This is an
    // optimization that results in faster sorts for nearly ordered lists.
    if (comparator.compare(I, J) <= 0) {
      System.arraycopy(src, low, dest, low, length);
      return;
    }

    // Merge sorted halves (now in src) into dest
    for (int i = low, p = low, q = mid; i < high; i++) {
      if (q < high && p < mid) {
        I.set(src[p]);
        J.set(src[q]);
      }
      if (q>=high || p