Merge.java

Below is the syntax highlighted version of Merge.java from §4.2 Sorting and Searching.

/*************************************************************************
 *  Compilation:  javac Merge.java
 *  Execution:    java Merge N
 *
 *  A bare-bones N log N implementation of mergesort.
 *
 *  See MergeSortX.java for an optimized version that avoids
 *  memory allocation in the inner loop and cuts of to
 *  insertionsort for small N.
 *
 *************************************************************************/

public class Merge {

    public static void sort(Comparable[] a) {
        sort(a, 0, a.length);
    } 

    // Sort a[lo, hi). 
    public static void sort(Comparable[] a, int lo, int hi) {
        int N = hi - lo;        // number of elements to sort

        // 0- or 1-element file, so we're done
        if (N <= 1) return; 

        // recursively sort left and right halves
        int mid = lo + N/2; 
        sort(a, lo, mid); 
        sort(a, mid, hi); 

        // merge two sorted subarrays
        Comparable[] aux = new Comparable[N];
        int i = lo, j = mid;
        for (int k = 0; k < N; k++) {
            if      (i == mid)  aux[k] = a[j++];
            else if (j == hi)   aux[k] = a[i++];
            else if (a[j].compareTo(a[i]) < 0) aux[k] = a[j++];
            else                               aux[k] = a[i++];
        }

        // copy back
        for (int k = 0; k < N; k++) {
            a[lo + k] = aux[k]; 
        }
    } 



   /***********************************************************************
    *  Check if array is sorted - useful for debugging
    ***********************************************************************/
    private static boolean isSorted(Comparable[] a) {
        for (int i = 1; i < a.length; i++)
            if (a[i].compareTo(a[i-1]) < 0) return false;
        return true;
    }

   /***********************************************************************
    *  Show results
    ***********************************************************************/
    public static void show(Comparable[] a) {
        for (int i = 0; i < a.length; i++)
            System.out.println(a[i]);
    }





    // test client
    public static void main(String[] args) {
        int N = Integer.parseInt(args[0]);

        // generate N random real numbers between 0 and 1
        long start = System.currentTimeMillis();
        Double[] a = new Double[N];
        for (int i = 0; i < N; i++)
            a[i] = Math.random();
        long stop = System.currentTimeMillis();
        double elapsed = (stop - start) / 1000.0;
        System.out.println("Generating input:  " + elapsed + " seconds");

        // sort them
        start = System.currentTimeMillis();
        sort(a);
        stop = System.currentTimeMillis();
        elapsed = (stop - start) / 1000.0;
        System.out.println("Mergesort: " + elapsed + " seconds");
        System.out.println(isSorted(a));

        if (N < 100) show(a);
    }
}