org.metacsp.utility.Permutation Maven / Gradle / Ivy

Go to download

Show more of this group Show more artifacts with this name
Show all versions of meta-csp-framework Show documentation

A Java API for Meta-CSP based reasoning

There is a newer version: 1.3.5

package org.metacsp.utility;

import java.util.Arrays;

/**
 * 
 * Generates r-permutations of n integers without repetition.
 * The algorithm is from Applied Combinatorics, by Alan Tucker.
 * Based on code from koders.com.
 * 
 * @author Federico Pecora
 */
public class Permutation {
	private int n, r;
	private int[] index;
	private boolean hasNext = true;

	/**
	 * Instantiates a new permutation object for r-permutations of n integers.
	 * @param n Number of integers available.
	 * @param r Size of permutations.
	 */
	public Permutation(int n, int r) {
		this.n = n;
		this.r = r;
		index = new int[n];
		for (int i = 0; i < n; i++) index[i] = i;
		reverseAfter(r - 1);
	}

	/**
	 * Assess whether there is another permutation. 
	 * @return true iff there is another permutation.
	 */
	public boolean hasNext() { return hasNext; }

	private void moveIndex() {
		// find the index of the first element that dips
		int i = rightmostDip();
		if (i < 0) {
			hasNext = false;
			return;
		}

		// find the smallest element to the right of the dip
		int smallestToRightIndex = i+1;
		for (int j=i+2; j index[i]))
				smallestToRightIndex = j;

		// switch dip element with smallest element to its right
		swap(index,i,smallestToRightIndex);

		if (r - 1 > i) {
			// reverse the elements to the right of the dip
			reverseAfter(i);
			// reverse the elements to the right of r - 1
			reverseAfter(r - 1);
		}
	}

	/**
	 * Get the next permutation.
	 * @return The next permutation, null if none exists.
	 */
	public int[] next() {
		if (!hasNext) return null;
		int[] result = new int[r];
		for (int i=0; i=0; i--)
			if (index[i] < index[i+1])
				return i;
		return -1;
	}

	private void swap(int[] a, int i, int j) {
		int temp = a[i];
		a[i] = a[j];
		a[j] = temp;
	}
	
	public static void main(String[] args) {
		Permutation p = new Permutation(4,3);
	    while (p.hasNext()) {
	      int[] a = p.next();
	      System.out.println(Arrays.toString(a));
	    }
	}
}