MergeBU.java

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

/*************************************************************************
 *  Compilation:  javac MergeBU.java
 *  Execution:    java MergeBU < 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
 *  bottom-up mergesort.
 *   
 *  % more tiny.txt
 *  S O R T E X A M P L E
 *
 *  % java MergeBU < 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 MergeBU < words3.txt
 *  all bad bed bug dad ... yes yet zoo    [ one string per line ]
 *
 *************************************************************************/

public class MergeBU {



    // stably merge a[lo..m] with a[m+1..hi] using aux[lo..hi]
    private static void merge(Comparable[] a, Comparable[] aux, int lo, int m, int hi) {

        // copy to aux[]
        for (int k = lo; k <= hi; k++) {
            aux[k] = a[k]; 
        }

        // merge back to a[]
        int i = lo, j = m+1;
        for (int k = lo; k <= hi; k++) {
            if      (i > m)                a[k] = aux[j++];
            else if (j > hi)               a[k] = aux[i++];
            else if (less(aux[j], aux[i])) a[k] = aux[j++];
            else                           a[k] = aux[i++];
        }

    }

    // bottom-up mergesort
    public static void sort(Comparable[] a) {
        int N = a.length;
        Comparable[] aux = new Comparable[N];
        for (int n = 1; n < N; n = n+n) {
            for (int i = 0; i < N-n; i += n+n) {
                int lo = i;
                int m  = i+n-1;
                int hi = Math.min(i+n+n-1, N-1);
                merge(a, aux, lo, m, hi);
            }
        }
        assert isSorted(a);
    }

  /***********************************************************************
    *  Helper sorting functions
    ***********************************************************************/
    
    // is v < w ?
    private static boolean less(Comparable v, Comparable w) {
        return (v.compareTo(w) < 0);
    }

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


   /***********************************************************************
    *  Check if array is sorted - useful for debugging
    ***********************************************************************/
    private static boolean isSorted(Comparable[] a) {
        for (int i = 1; i < a.length; 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();
        MergeBU.sort(a);
        show(a);
    }
}