programming-examples/java/Data_Structures/MSD.java

 

import edu.princeton.cs.introcs.StdIn;
import edu.princeton.cs.introcs.StdOut;

/***********************************************************************************
 *  Compilation: javac MSD.java
 *  Execution:   java MSD < input.txt
 *
 *  Reads extended ASCII string from standard input and MSD radix sorts them.
 *
 *  % java MSD < shells.txt 
 *  are
 *  by
 *  sea
 *  seashells
 *  seashells
 *  sells
 *  sells
 *  she
 *  she
 *  shells
 *  shore
 *  surely
 *  the
 *  the
 *
 ***********************************************************************************/

public class MSD {
    private static final int R      = 256;   // extended ASCII alphabet size
    private static final int CUTOFF =  15;   // cutoff to insertion sort

    // sort array of strings
    public static void sort(String[] a) {
        int N = a.length;
        String[] aux = new String[N];
        sort(a, 0, N-1, 0, aux);
    }

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

    // sort from a[lo] to a[hi], starting at the dth character
    private static void sort(String[] a, int lo, int hi, int d, String[] aux) {

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

        // compute frequency counts
        int[] count = new int[R+2];
        for (int i = lo; i <= hi; i++) {
            int c = charAt(a[i], d);
            count[c+2]++;
        }

        // transform counts to indicies
        for (int r = 0; r < R+1; r++)
            count[r+1] += count[r];

        // distribute
        for (int i = lo; i <= hi; i++) {
            int c = charAt(a[i], d);
            aux[count[c+1]++] = a[i];
        }

        // copy back
        for (int i = lo; i <= hi; i++) 
            a[i] = aux[i - lo];


        // recursively sort for each character
        for (int r = 0; r < R; r++)
            sort(a, lo + count[r], lo + count[r+1] - 1, d+1, aux);
    }


    // return dth character of s, -1 if d = length of string
    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();
    }


    public static void main(String[] args) {
        String[] a = StdIn.readAllStrings();
        int N = a.length;
        sort(a);
        for (int i = 0; i < N; i++)
            StdOut.println(a[i]);
    }
}