Shell.java

Below is the syntax highlighted version of Shell.java from §2.1 Elementary Sorts.

/******************************************************************************
 *  Compilation:  javac Shell.java
 *  Execution:    java Shell < input.txt
 *  Dependencies: StdOut.java StdIn.java
 *  Data files:   https://algs4.cs.princeton.edu/21elementary/tiny.txt
 *                https://algs4.cs.princeton.edu/21elementary/words3.txt
 *
 *  Sorts a sequence of strings from standard input using shellsort.
 *
 *  % more tiny.txt
 *  S O R T E X A M P L E
 *
 *  % java Shell < 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 Shell < words3.txt
 *  all bad bed bug dad ... yes yet zoo    [ one string per line ]
 *
 *
 ******************************************************************************/

/**
 *  The {@code Shell} class provides static methods for sorting an
 *  array using <em>Shellsort</em> with
 *  <a href = "https://oeis.org/A003462"> Knuth's increment sequence</a>
 *  (1, 4, 13, 40, ...). In the worst case, this implementation makes
 *  &Theta;(<em>n</em><sup>3/2</sup>) compares and exchanges to sort
 *  an array of length <em>n</em>.
 *  <p>
 *  This sorting algorithm is not stable.
 *  It uses &Theta;(1) extra memory (not including the input array).
 *  <p>
 *  For additional documentation, see
 *  <a href="https://algs4.cs.princeton.edu/21elementary">Section 2.1</a> of
 *  <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
 *
 *  @author Robert Sedgewick
 *  @author Kevin Wayne
 */
public class Shell {

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

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

        // 3x+1 increment sequence:  1, 4, 13, 40, 121, 364, 1093, ...
        int h = 1;
        while (h < n/3) h = 3*h + 1;

        while (h >= 1) {
            // h-sort the array
            for (int i = h; i < n; i++) {
                for (int j = i; j >= h && less(a[j], a[j-h]); j -= h) {
                    exch(a, j, j-h);
                }
            }
            assert isHsorted(a, h);
            h /= 3;
        }
        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;
    }

    // is the array h-sorted?
    private static boolean isHsorted(Comparable[] a, int h) {
        for (int i = h; i < a.length; i++)
            if (less(a[i], a[i-h])) 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; Shellsorts them;
     * and prints them to standard output in ascending order.
     *
     * @param args the command-line arguments
     */
    public static void main(String[] args) {
        String[] a = StdIn.readAllStrings();
        Shell.sort(a);
        show(a);
    }

}