Quick3string.java

Below is the syntax highlighted version of Quick3string.java from §5.1 String Sorts.

/******************************************************************************
 *  Compilation:  javac Quick3string.java
 *  Execution:    java Quick3string < input.txt
 *  Dependencies: StdIn.java StdOut.java
 *  Data files:   https://algs4.cs.princeton.edu/51radix/words3.txt
 *                https://algs4.cs.princeton.edu/51radix/shells.txt
 *
 *  Reads string from standard input and 3-way string quicksort them.
 *
 *  % java Quick3string < shell.txt
 *  are
 *  by
 *  sea
 *  seashells
 *  seashells
 *  sells
 *  sells
 *  she
 *  she
 *  shells
 *  shore
 *  surely
 *  the
 *  the
 *
 *
 ******************************************************************************/

/**
 *  The {@code Quick3string} class provides static methods for sorting an
 *  array of strings using 3-way radix quicksort.
 *  <p>
 *  For additional documentation,
 *  see <a href="https://algs4.cs.princeton.edu/51radix">Section 5.1</a> of
 *  <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
 *
 *  @author Robert Sedgewick
 *  @author Kevin Wayne
 */
public class Quick3string {
    private static final int CUTOFF =  15;   // cutoff to insertion sort

    // do not instantiate
    private Quick3string() { }

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

    // return the dth character of s, -1 if d = length of s
    private static int charAt(String s, int d) {
        assert d >= 0 && d <= s.length();
        if (d == s.length()) return -1;
        return s.charAt(d);
    }


    // 3-way string quicksort a[lo..hi] starting at dth character
    private static void sort(String[] a, int lo, int hi, int d) {

        // cutoff to insertion sort for small subarrays
        if (hi <= lo + CUTOFF) {
            insertion(a, lo, hi, d);
            return;
        }

        int lt = lo, gt = hi;
        int v = charAt(a[lo], d);
        int i = lo + 1;
        while (i <= gt) {
            int t = charAt(a[i], d);
            if      (t < v) exch(a, lt++, i++);
            else if (t > v) exch(a, i, gt--);
            else              i++;
        }

        // a[lo..lt-1] < v = a[lt..gt] < a[gt+1..hi].
        sort(a, lo, lt-1, d);
        if (v >= 0) sort(a, lt, gt, d+1);
        sort(a, gt+1, hi, d);
    }

    // sort from a[lo] to a[hi], starting at the dth character
    private static void insertion(String[] a, int lo, int hi, int d) {
        for (int i = lo; i <= hi; i++)
            for (int j = i; j > lo && less(a[j], a[j-1], d); j--)
                exch(a, j, j-1);
    }

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

    // is v less than w, starting at character d
    // DEPRECATED BECAUSE OF SLOW SUBSTRING EXTRACTION IN JAVA 7
    // private static boolean less(String v, String w, int d) {
    //    assert v.substring(0, d).equals(w.substring(0, d));
    //    return v.substring(d).compareTo(w.substring(d)) < 0;
    // }

    // is v less than w, starting at character d
    private static boolean less(String v, String w, int d) {
        assert v.substring(0, d).equals(w.substring(0, d));
        for (int i = d; i < Math.min(v.length(), w.length()); i++) {
            if (v.charAt(i) < w.charAt(i)) return true;
            if (v.charAt(i) > w.charAt(i)) return false;
        }
        return v.length() < w.length();
    }

    // is the array sorted
    private static boolean isSorted(String[] a) {
        for (int i = 1; i < a.length; i++)
            if (a[i].compareTo(a[i-1]) < 0) return false;
        return true;
    }


    /**
     * Reads in a sequence of fixed-length strings from standard input;
     * 3-way radix quicksorts them;
     * and prints them to standard output in ascending order.
     *
     * @param args the command-line arguments
     */
    public static void main(String[] args) {

        // read in the strings from standard input
        String[] a = StdIn.readAllStrings();
        int n = a.length;

        // sort the strings
        sort(a);

        // print the results
        for (int i = 0; i < n; i++)
            StdOut.println(a[i]);
    }
}