programming-examples/java/Data_Structures/MergeX.java

 

import edu.princeton.cs.introcs.StdIn;
import edu.princeton.cs.introcs.StdOut;

/*************************************************************************
 *  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
 *  A E E L M O P R S T X                 [ 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 ]
 *
 *************************************************************************/

/**
 *  The  MergeX  class provides static methods for sorting an
 *  array using an optimized version of mergesort.
 *   
 *  For additional documentation, see <a href="http://algs4.cs.princeton.edu/22mergesort">Section 2.2</a> of
 *   Algorithms, 4th Edition  by Robert Sedgewick and Kevin Wayne.
 *
 *  @author Robert Sedgewick
 *  @author Kevin Wayne
 */
public class MergeX {
    private static final int CUTOFF = 7;  // cutoff to insertion sort

    // This class should not be instantiated.
    private MergeX() { }

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

        // if (!less(src[mid+1], src[mid])) {
        //    for (int i = lo; i <= hi; i++) dst[i] = src[i];
        //    return;
        // }

        // using System.arraycopy() is a bit faster than the above loop
        if (!less(src[mid+1], src[mid])) {
            System.arraycopy(src, lo, dst, lo, hi - lo + 1);
            return;
        }

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

    /**
     * Rearranges the array in ascending order, using the natural order.
     * @param a the array to be sorted
     */
    public static void sort(Comparable[] a) {
        Comparable[] aux = a.clone();
        sort(aux, a, 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]);
        }
    }

    /**
     * Reads in a sequence of strings from standard input; mergesorts them
     * (using an optimized version of mergesort); 
     * and prints them to standard output in ascending order. 
     */
    public static void main(String[] args) {
        String[] a = StdIn.readAllStrings();
        MergeX.sort(a);
        show(a);
    }
}