S5nModel.java

/**
  * This program (working title: MAS Prover) is an automated tableaux prover
  * for epistemic logic (S5n).
  * Copyright (C) 2007  Elske van der Vaart and Gert van Valkenhoef

  * This program is free software; you can redistribute it and/or modify it
  * under the terms of the GNU General Public License version 2 as published
  * by the Free Software Foundation.

  * This program 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 this program; if not, write to the Free Software Foundation, Inc.,
  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */

package nl.rug.ai.mas.oops.model;

import nl.rug.ai.mas.oops.formula.AgentId;
import java.util.Set;
import java.util.LinkedList;

/**
 * An S5n Model. This is a KripkeModel with the added restriction that the
 * accessibility relation for every agent be reflexive, symmetric and
 * transitive.
 */
public class S5nModel extends KripkeModel {
	public S5nModel(Set<AgentId> agents) {
		super(agents);
	}
	
	@Override
	public S5nModel newModel() {
		return new S5nModel(d_agents);
	}

	/**
	 * Add a world to the model. This includes adding reflexive arrows for all
	 * agents.
	 */
	public boolean addWorld(World w) {
		if (!super.addWorld(w)) { // the world already existed
			return false;
		}
		for (AgentId agent : d_agents) {
			super.addArrow(agent, w, w);
		}

		return true;
	}

	/**
	 * Add an arrow. This includes closure under symmetry and transitivity.
	 */
	public boolean addArrow(Arrow r1) {
		if (!super.addArrow(r1)) { // the arrow already existed
			return false;
		}
		// close under symmetry
		Arrow r2 = new Arrow(r1.getAgent(), r1.getTarget(), r1.getSource());
		super.addArrow(r2);

		// close under transitivity (the transitive closure of a symmetric
		// relation is symmetric)
		transitiveClosure(r1, r2);

		return true;
	}

	/**
	 * Convenience function for adding new arrows.
	 */
	public boolean addArrow(AgentId agent, World source, World target) {
		return addArrow(new Arrow(agent, source, target));
	}

	/**
	 * Incrementally close relations under transitivity, given two newly added
	 * arrows to an otherwise transitively closed set.
	 */
	private void transitiveClosure(Arrow r1, Arrow r2) {
		LinkedList<Arrow> fringe = new LinkedList<Arrow>();
		fringe.add(r1);
		fringe.add(r2);

		Arrow r = null;
		while ((r = fringe.poll()) != null) {
			// get all outgoing arrows from the target
			Set<Arrow> arrows = getArrowsFrom(r.getAgent(), r.getTarget());
			// and add the merge of r and each arrow
			for (Arrow s : arrows) {
				Arrow t1 =
					new Arrow(r.getAgent(), r.getSource(), s.getTarget());
				if (super.addArrow(t1)) { // t1 is a new arrow
					fringe.offer(t1);
				}
			}
			// get all ingoing arrows to the source
			arrows = getArrowsTo(r.getAgent(), r.getSource());
			for (Arrow s : arrows) {
				Arrow t1 =
					new Arrow(r.getAgent(), s.getSource(), r.getTarget());
				if (super.addArrow(t1)) { // t1 is a new arrow
					fringe.offer(t1);
				}
			}
		}
	}
}