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Basic utility library containing several useful classes (e.g., a Pair class) to ease everyday programming.

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/* Copyright (C) 2013 TU Dortmund
 * This file is part of AutomataLib, http://www.automatalib.net/.
 * 
 * AutomataLib is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License version 3.0 as published by the Free Software Foundation.
 * 
 * AutomataLib 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
 * Lesser General Public License for more details.
 * 
 * You should have received a copy of the GNU Lesser General Public
 * License along with AutomataLib; if not, see
 * http://www.gnu.de/documents/lgpl.en.html.
 */
package net.automatalib.commons.util;

/**
 * The well-known disjoint-set forest data structure for dealing with partitions
 * on a fixed-range integer domain.
 * 
 * @author fhowar
 * @author Malte Isberner 
 */
public final class UnionFind {

	private final int[] p;
	private final int[] rank;

	/**
	 * Initializes the disjoint-set data structure.
	 * @param n the overall size of the domain
	 */
	public UnionFind(int n) {
		p = new int[n];
		rank = new int[n];

		for(int i = 0; i < n; i++) {
			// primitive arrays are always zero initialized
			// rank[i] = 0;
			p[i] = i;
		}
	}

	/**
	 * Unites the sets containing the two given elements.
	 * @param p the first element
	 * @param q the second element
	 * @return the identifier of the resulting set
	 */
	public int union(int p, int q) {
		return link(find(p), find(q));
	}

	/**
	 * Unites two given sets. Note that the behavior of this method is not specified
	 * if the given parameters are normal elements and no set identifiers.
	 * @param x the first set
	 * @param y the second set
	 * @return the identifier of the resulting set (either x or y)
	 */
	public int link(int x, int y) {
		if(rank[x] > rank[y]) {
			p[y] = x;
			return x;
		}
		p[x] = y;
		if(rank[x] == rank[y])
			rank[y]++;
		return y;
	}

	/**
	 * Finds the set of a given element, and compresses the path to the root node.
	 * @param x the element
	 * @return the identifier of the set which contains the given element
	 */
	public int find(int x) {
		int r = p[x];
		if(x != r)
			p[x] = r = find(r);

		return r;
	}

	// public synchronized boolean findAndUnite(int x, int y)
	// {
	// int r1 = find(x);
	// int r2 = find(y);
	//
	// if (r1 == r2)
	// return false;
	//
	// union(r1, r2);
	// return true;
	// }
}