edu.princeton.cs.algs4.MergeX Maven / Gradle / Ivy
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/******************************************************************************
* Compilation: javac MergeX.java
* Execution: java MergeX < input.txt
* Dependencies: StdOut.java StdIn.java
* Data files: https://algs4.cs.princeton.edu/22mergesort/tiny.txt
* https://algs4.cs.princeton.edu/22mergesort/words3.txt
*
* Sorts a sequence of strings from standard input using an
* optimized version of mergesort.
*
* % more tiny.txt
* S O R T E X A M P L E
*
* % java MergeX < 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 MergeX < words3.txt
* all bad bed bug dad ... yes yet zoo [ one string per line ]
*
******************************************************************************/
package edu.princeton.cs.algs4;
import java.util.Comparator;
/**
* The {@code MergeX} class provides static methods for sorting an
* array using an optimized version of mergesort.
*
* For additional documentation, see Section 2.2 of
* Algorithms, 4th Edition by Robert Sedgewick and Kevin Wayne.
*
* @author Robert Sedgewick
* @author Kevin Wayne
*/
public class MergeX {
private static final int CUTOFF = 7; // cutoff to insertion sort
// This class should not be instantiated.
private MergeX() { }
private static void merge(Comparable[] src, Comparable[] dst, int lo, int mid, int hi) {
// precondition: src[lo .. mid] and src[mid+1 .. hi] are sorted subarrays
assert isSorted(src, lo, mid);
assert isSorted(src, mid+1, hi);
int i = lo, j = mid+1;
for (int k = lo; k <= hi; k++) {
if (i > mid) dst[k] = src[j++];
else if (j > hi) dst[k] = src[i++];
else if (less(src[j], src[i])) dst[k] = src[j++]; // to ensure stability
else dst[k] = src[i++];
}
// postcondition: dst[lo .. hi] is sorted subarray
assert isSorted(dst, lo, hi);
}
private static void sort(Comparable[] src, Comparable[] dst, int lo, int hi) {
// if (hi <= lo) return;
if (hi <= lo + CUTOFF) {
insertionSort(dst, lo, hi);
return;
}
int mid = lo + (hi - lo) / 2;
sort(dst, src, lo, mid);
sort(dst, src, mid+1, hi);
// if (!less(src[mid+1], src[mid])) {
// for (int i = lo; i <= hi; i++) dst[i] = src[i];
// return;
// }
// using System.arraycopy() is a bit faster than the above loop
if (!less(src[mid+1], src[mid])) {
System.arraycopy(src, lo, dst, lo, hi - lo + 1);
return;
}
merge(src, dst, lo, mid, hi);
}
/**
* Rearranges the array in ascending order, using the natural order.
* @param a the array to be sorted
*/
public static void sort(Comparable[] a) {
Comparable[] aux = a.clone();
sort(aux, a, 0, a.length-1);
assert isSorted(a);
}
// sort from a[lo] to a[hi] using insertion sort
private static void insertionSort(Comparable[] a, int lo, int hi) {
for (int i = lo; i <= hi; i++)
for (int j = i; j > lo && less(a[j], a[j-1]); j--)
exch(a, j, j-1);
}
/*******************************************************************
* Utility methods.
*******************************************************************/
// 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;
}
// is a[i] < a[j]?
private static boolean less(Comparable a, Comparable b) {
return a.compareTo(b) < 0;
}
// is a[i] < a[j]?
private static boolean less(Object a, Object b, Comparator comparator) {
return comparator.compare(a, b) < 0;
}
/*******************************************************************
* Version that takes Comparator as argument.
*******************************************************************/
/**
* Rearranges the array in ascending order, using the provided order.
*
* @param a the array to be sorted
* @param comparator the comparator that defines the total order
*/
public static void sort(Object[] a, Comparator comparator) {
Object[] aux = a.clone();
sort(aux, a, 0, a.length-1, comparator);
assert isSorted(a, comparator);
}
private static void merge(Object[] src, Object[] dst, int lo, int mid, int hi, Comparator comparator) {
// precondition: src[lo .. mid] and src[mid+1 .. hi] are sorted subarrays
assert isSorted(src, lo, mid, comparator);
assert isSorted(src, mid+1, hi, comparator);
int i = lo, j = mid+1;
for (int k = lo; k <= hi; k++) {
if (i > mid) dst[k] = src[j++];
else if (j > hi) dst[k] = src[i++];
else if (less(src[j], src[i], comparator)) dst[k] = src[j++];
else dst[k] = src[i++];
}
// postcondition: dst[lo .. hi] is sorted subarray
assert isSorted(dst, lo, hi, comparator);
}
private static void sort(Object[] src, Object[] dst, int lo, int hi, Comparator comparator) {
// if (hi <= lo) return;
if (hi <= lo + CUTOFF) {
insertionSort(dst, lo, hi, comparator);
return;
}
int mid = lo + (hi - lo) / 2;
sort(dst, src, lo, mid, comparator);
sort(dst, src, mid+1, hi, comparator);
// using System.arraycopy() is a bit faster than the above loop
if (!less(src[mid+1], src[mid], comparator)) {
System.arraycopy(src, lo, dst, lo, hi - lo + 1);
return;
}
merge(src, dst, lo, mid, hi, comparator);
}
// sort from a[lo] to a[hi] using insertion sort
private static void insertionSort(Object[] a, int lo, int hi, Comparator comparator) {
for (int i = lo; i <= hi; i++)
for (int j = i; j > lo && less(a[j], a[j-1], comparator); j--)
exch(a, j, j-1);
}
/***************************************************************************
* 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;
}
private static boolean isSorted(Object[] a, Comparator comparator) {
return isSorted(a, 0, a.length - 1, comparator);
}
private static boolean isSorted(Object[] a, int lo, int hi, Comparator comparator) {
for (int i = lo + 1; i <= hi; i++)
if (less(a[i], a[i-1], comparator)) return false;
return true;
}
// print array to standard output
private static void show(Object[] a) {
for (int i = 0; i < a.length; i++) {
StdOut.println(a[i]);
}
}
/**
* Reads in a sequence of strings from standard input; mergesorts them
* (using an optimized version of mergesort);
* 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();
MergeX.sort(a);
show(a);
}
}
/******************************************************************************
* 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.
******************************************************************************/