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AIMA-Java Core Algorithms from the book Artificial Intelligence a Modern Approach 3rd Ed.
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package aima.core.logic.fol.domain;

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

/**
 * @author Ravi Mohan
 * @author Ciaran O'Reilly
 */
public class FOLDomain {
	private Set constants, functions, predicates;
	private int skolemConstantIndexical = 0;
	private int skolemFunctionIndexical = 0;
	private int answerLiteralIndexical = 0;
	private List listeners = new ArrayList();

	public FOLDomain() {
		this.constants = new HashSet();
		this.functions = new HashSet();
		this.predicates = new HashSet();
	}

	public FOLDomain(FOLDomain toCopy) {
		this(toCopy.getConstants(), toCopy.getFunctions(), toCopy
				.getPredicates());
	}

	public FOLDomain(Set constants, Set functions,
			Set predicates) {
		this.constants = new HashSet(constants);
		this.functions = new HashSet(functions);
		this.predicates = new HashSet(predicates);
	}

	public Set getConstants() {
		return constants;
	}

	public Set getFunctions() {
		return functions;
	}

	public Set getPredicates() {
		return predicates;
	}

	public void addConstant(String constant) {
		constants.add(constant);
	}

	public String addSkolemConstant() {

		String sc = null;
		do {
			sc = "SC" + (skolemConstantIndexical++);
		} while (constants.contains(sc) || functions.contains(sc)
				|| predicates.contains(sc));

		addConstant(sc);
		notifyFOLDomainListeners(new FOLDomainSkolemConstantAddedEvent(this, sc));

		return sc;
	}

	public void addFunction(String function) {
		functions.add(function);
	}

	public String addSkolemFunction() {
		String sf = null;
		do {
			sf = "SF" + (skolemFunctionIndexical++);
		} while (constants.contains(sf) || functions.contains(sf)
				|| predicates.contains(sf));

		addFunction(sf);
		notifyFOLDomainListeners(new FOLDomainSkolemFunctionAddedEvent(this, sf));

		return sf;
	}

	public void addPredicate(String predicate) {
		predicates.add(predicate);
	}

	public String addAnswerLiteral() {
		String al = null;
		do {
			al = "Answer" + (answerLiteralIndexical++);
		} while (constants.contains(al) || functions.contains(al)
				|| predicates.contains(al));

		addPredicate(al);
		notifyFOLDomainListeners(new FOLDomainAnswerLiteralAddedEvent(this, al));

		return al;
	}

	public void addFOLDomainListener(FOLDomainListener listener) {
		synchronized (listeners) {
			if (!listeners.contains(listener)) {
				listeners.add(listener);
			}
		}
	}

	public void removeFOLDomainListener(FOLDomainListener listener) {
		synchronized (listeners) {
			listeners.remove(listener);
		}
	}

	//
	// PRIVATE METHODS
	//
	private void notifyFOLDomainListeners(FOLDomainEvent event) {
		synchronized (listeners) {
			for (FOLDomainListener l : listeners) {
				event.notifyListener(l);
			}
		}
	}
}