SHELL希尔排序

/******************************************************************************
 *  Compilation:  javac Shell.java
 *  Execution:    java Shell < input.txt
 *  Dependencies: StdOut.java StdIn.java
 *  Data files:   http://algs4.cs.princeton.edu/21elementary/tiny.txt
 *                http://algs4.cs.princeton.edu/21elementary/words3.txt
 *   
 *  Sorts a sequence of strings from standard input using shellsort.
 *
 *  Uses increment sequence proposed by Sedgewick and Incerpi.
 *  The nth element of the sequence is the smallest integer >= 2.5^n
 *  that is relatively prime to all previous terms in the sequence.
 *  For example, incs[4] is 41 because 2.5^4 = 39.0625 and 41 is
 *  the next integer that is relatively prime to 3, 7, and 16.
 *   
 *  % 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 ]
 *
 *
 ******************************************************************************/

package edu.princeton.cs.algs4;

/**
 *  The {@code Shell} class provides static methods for sorting an
 *  array using Shellsort with Knuth's increment sequence (1, 4, 13, 40, ...).
 *  <p>
 *  For additional documentation, see <a href="http://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 {

  
    private Shell() { }

 
    public static void sort(Comparable[] a) {
        int n = a.length;

      
        int h = 1;
        while (h < n/3) h = 3*h + 1; 

        while (h >= 1) {
        
            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);
    }



    
   
    private static boolean less(Comparable v, Comparable w) {
        return v.compareTo(w) < 0;
    }
        
  
    private static void exch(Object[] a, int i, int j) {
        Object swap = a[i];
        a[i] = a[j];
        a[j] = swap;
    }


  
    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;
    }


    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;
    }

  
    private static void show(Comparable[] a) {
        for (int i = 0; i < a.length; i++) {
            StdOut.println(a[i]);
        }
    }

    
    public static void main(String[] args) {
        String[] a = StdIn.readAllStrings();
        Shell.sort(a);
        show(a);
    }

}