edu.princeton.cs.algs4.MSD Maven / Gradle / Ivy
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/******************************************************************************
* Compilation: javac MSD.java
* Execution: java MSD < 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
*
* Sort an array of strings or integers using MSD radix sort.
*
* % java MSD < shells.txt
* are
* by
* sea
* seashells
* seashells
* sells
* sells
* she
* she
* shells
* shore
* surely
* the
* the
*
******************************************************************************/
package edu.princeton.cs.algs4;
/**
* The {@code MSD} class provides static methods for sorting an
* array of extended ASCII strings or integers using MSD radix sort.
*
* For additional documentation,
* see Section 5.1 of
* Algorithms, 4th Edition by Robert Sedgewick and Kevin Wayne.
*
* @author Robert Sedgewick
* @author Kevin Wayne
*/
public class MSD {
private static final int BITS_PER_BYTE = 8;
private static final int BITS_PER_INT = 32; // each Java int is 32 bits
private static final int R = 256; // extended ASCII alphabet size
private static final int CUTOFF = 15; // cutoff to insertion sort
// do not instantiate
private MSD() { }
/**
* Rearranges the array of extended ASCII strings in ascending order.
*
* @param a the array to be sorted
*/
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 (excludes sentinel -1)
for (int r = 0; r < R; r++)
sort(a, lo + count[r], lo + count[r+1] - 1, d+1, aux);
}
// insertion sort a[lo..hi], starting at 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
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();
}
/**
* Rearranges the array of 32-bit integers in ascending order.
* Currently assumes that the integers are nonnegative.
*
* @param a the array to be sorted
*/
public static void sort(int[] a) {
int n = a.length;
int[] aux = new int[n];
sort(a, 0, n-1, 0, aux);
}
// MSD sort from a[lo] to a[hi], starting at the dth byte
private static void sort(int[] a, int lo, int hi, int d, int[] aux) {
// cutoff to insertion sort for small subarrays
if (hi <= lo + CUTOFF) {
insertion(a, lo, hi, d);
return;
}
// compute frequency counts (need R = 256)
int[] count = new int[R+1];
int mask = R - 1; // 0xFF;
int shift = BITS_PER_INT - BITS_PER_BYTE*d - BITS_PER_BYTE;
for (int i = lo; i <= hi; i++) {
int c = (a[i] >> shift) & mask;
count[c + 1]++;
}
// transform counts to indicies
for (int r = 0; r < R; r++)
count[r+1] += count[r];
/************* BUGGGY CODE.
// for most significant byte, 0x80-0xFF comes before 0x00-0x7F
if (d == 0) {
int shift1 = count[R] - count[R/2];
int shift2 = count[R/2];
for (int r = 0; r < R/2; r++)
count[r] += shift1;
for (int r = R/2; r < R; r++)
count[r] -= shift2;
}
************************************/
// distribute
for (int i = lo; i <= hi; i++) {
int c = (a[i] >> shift) & mask;
aux[count[c]++] = a[i];
}
// copy back
for (int i = lo; i <= hi; i++)
a[i] = aux[i - lo];
// no more bits
if (d == 4) return;
// recursively sort for each character
if (count[0] > 0)
sort(a, lo, lo + count[0] - 1, d+1, aux);
for (int r = 0; r < R; r++)
if (count[r+1] > count[r])
sort(a, lo + count[r], lo + count[r+1] - 1, d+1, aux);
}
// TODO: insertion sort a[lo..hi], starting at dth character
private static void insertion(int[] a, int lo, int hi, int d) {
for (int i = lo; i <= hi; i++)
for (int j = i; j > lo && a[j] < a[j-1]; j--)
exch(a, j, j-1);
}
// exchange a[i] and a[j]
private static void exch(int[] a, int i, int j) {
int temp = a[i];
a[i] = a[j];
a[j] = temp;
}
/**
* Reads in a sequence of extended ASCII strings from standard input;
* MSD radix sorts 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();
int n = a.length;
sort(a);
for (int i = 0; i < n; i++)
StdOut.println(a[i]);
}
}
/******************************************************************************
* Copyright 2002-2018, Robert Sedgewick and Kevin Wayne.
*
* This file is part of algs4.jar, which accompanies the textbook
*
* Algorithms, 4th edition by Robert Sedgewick and Kevin Wayne,
* Addison-Wesley Professional, 2011, ISBN 0-321-57351-X.
* http://algs4.cs.princeton.edu
*
*
* algs4.jar is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* algs4.jar is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with algs4.jar. If not, see http://www.gnu.org/licenses.
******************************************************************************/