MergeX.java

Below is the syntax highlighted version of MergeX.java from §2.2 Mergesort.

/*************************************************************************
 *  Compilation:  javac MergeX.java
 *  Execution:    java MergeX < input.txt
 *  Dependencies: StdOut.java StdIn.java
 *  Data files:   http://algs4.cs.princeton.edu/22mergesort/tiny.txt
 *                http://algs4.cs.princeton.edu/22mergesort/words3.txt
 *   
 *  Sorts a sequence of strings from standard input using an
 *  optimized version of mergesort.
 *   
 *  % more tiny.txt
 *  S O R T E X A M P L E
 *
 *  % java MergeX < tiny.txt
 *  S O R T E X A M P L E A               [ one string per line ]
 *    
 *  % more words3.txt
 *  bed bug dad yes zoo ... all bad yet
 *  
 *  % java MergeX < words3.txt
 *  all bad bed bug dad ... yes yet zoo    [ one string per line ]
 *
 *************************************************************************/

public class MergeX {
    private static final int CUTOFF = 7;  // cutoff to insertion sort


    private static void merge(Comparable[] src, Comparable[] dst, int lo, int mid, int hi) {

        // precondition: src[lo .. mid] and src[mid+1 .. hi] are sorted subarrays
        assert isSorted(src, lo, mid);
        assert isSorted(src, mid+1, hi);

        int i = lo, j = mid+1;
        for (int k = lo; k <= hi; k++) {
            if      (i > mid)              dst[k] = src[j++];
            else if (j > hi)               dst[k] = src[i++];
            else if (less(src[j], src[i])) dst[k] = src[j++];   // to ensure stability
            else                           dst[k] = src[i++];
        }

        // postcondition: dst[lo .. hi] is sorted subarray
        assert isSorted(dst, lo, hi);
    }

    private static void sort(Comparable[] src, Comparable[] dst, int lo, int hi) {
        if (hi <= lo + CUTOFF) { 
            insertionSort(src, lo, hi);
            return;
        }
        int mid = lo + (hi - lo) / 2;
        sort(dst, src, lo, mid);
        sort(dst, src, mid+1, hi);

/*
        if (!less(dst[mid+1], dst[mid])) {
            for (int i = lo; i <= hi; i++) src[i] = dst[i];
            return;
        }
*/
        // a bit faster
        if (!less(dst[mid+1], dst[mid])) {
            System.arraycopy(dst, lo, src, lo, hi - lo + 1);
            return;
        }

        merge(dst, src, lo, mid, hi);
    }

    public static void sort(Comparable[] a) {
/*
        Comparable[] aux = new Comparable[a.length];
        for (int i = 0; i < a.length; i++)
            aux[i] = a[i];
*/
        // a bit faster
        Comparable[] aux = a.clone();
        sort(a, aux, 0, a.length-1);  

        assert isSorted(a);
    }


    // sort from a[lo] to a[hi] using insertion sort
    private static void insertionSort(Comparable[] a, int lo, int hi) {
        for (int i = lo; i <= hi; i++)
            for (int j = i; j > lo && less(a[j], a[j-1]); j--)
                exch(a, j, j-1);
    }


    // exchange a[i] and a[j]
    private static void exch(Comparable[] a, int i, int j) {
        Comparable swap = a[i];
        a[i] = a[j];
        a[j] = swap;
    }

    // is a[i] < a[j]?
    private static boolean less(Comparable a, Comparable b) {
        return (a.compareTo(b) < 0);
    }

   /***********************************************************************
    *  Check if array is sorted - useful for debugging
    ***********************************************************************/
    private static boolean isSorted(Comparable[] a) {
        return isSorted(a, 0, a.length - 1);
    }

    private static boolean isSorted(Comparable[] a, int lo, int hi) {
        for (int i = lo + 1; i <= hi; i++)
            if (less(a[i], a[i-1])) return false;
        return true;
    }

    // print array to standard output
    private static void show(Comparable[] a) {
        for (int i = 0; i < a.length; i++) {
            StdOut.println(a[i]);
        }
    }

    // Read strings from standard input, sort them, and print.
    public static void main(String[] args) {
        String[] a = StdIn.readStrings();
        MergeX.sort(a);
        show(a);
    }
}