QuickHoare.java

Below is the syntax highlighted version of QuickHoare.java from §2.3 Quicksort.

/******************************************************************************
 *  Compilation:  javac QuickHoare.java
 *  Execution:    java QuickHoare < input.txt
 *  Dependencies: StdOut.java StdIn.java
 *  Data files:   https://algs4.cs.princeton.edu/23quicksort/tiny.txt
 *                https://algs4.cs.princeton.edu/23quicksort/words3.txt
 *
 *  Sorts a sequence of strings from standard input using quicksort.
 *
 *  % more tiny.txt
 *  S O R T E X A M P L E
 *
 *  % java QuickHoare < 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 QuickHoare < words3.txt
 *  all bad bed bug dad ... yes yet zoo    [ one string per line ]
 *
 *
 ******************************************************************************/

/**
 *  The {@code QuickHoare} class provides a static method for sorting an
 *  array using Hoare's original quicksort algorithm.
 *  <p>
 *  For additional documentation, see
 *  <a href="https://algs4.cs.princeton.edu/23quicksort">Section 2.3</a>
 *  of <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
 *
 *  @author Robert Sedgewick
 *  @author Kevin Wayne
 */
public class QuickHoare {

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

    /**
     * Rearranges the array in ascending order, using the natural order.
     * @param a the array to be sorted
     */
    public static void sort(Comparable[] a) {
        StdRandom.shuffle(a);
        sort(a, 0, a.length - 1);
        assert isSorted(a);
    }

    // quicksort the subarray from a[lo] to a[hi]
    private static void sort(Comparable[] a, int lo, int hi) {
        if (hi <= lo) return;
        int j = partition(a, lo, hi);
        sort(a, lo, j);
        sort(a, j+1, hi);
        assert isSorted(a, lo, hi);
    }

    private static int partition(Comparable[] a, int lo, int hi) {
        int i = lo - 1;
        int j = hi + 1;
        int mid = (hi + lo) >>> 1;
        Comparable pivot = a[mid];

        while (true) {
            do {
                i++;
            } while (less(a[i], pivot));

            do {
                j--;
            } while (less(pivot, a[j]));

            // the pivot doesn't end up at a[j], but it does still partition
            // the array into keys <= pivot and keys >= pivot
            if (i >= j) return j;

            exch(a, i, j);
        }
    }

   /***************************************************************************
    *  Helper sorting functions.
    ***************************************************************************/

    // is v < w ?
    private static boolean less(Comparable v, Comparable w) {
        if (v == w) return false;   // optimization when reference equals
        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) {
        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; quicksorts them;
     * and prints them to standard output in ascending order.
     * Shuffles the array and then prints the strings again to
     * standard output, but this time, using the select method.
     *
     * @param args the command-line arguments
     */
    public static void main(String[] args) {
        String[] a = StdIn.readAllStrings();
        QuickHoare.sort(a);
        show(a);
        assert isSorted(a);
    }

}