minus.java

这是外国一个开源推理机

/*  Sesame - Storage and Querying architecture for RDF and RDF Schema
 *  Copyright (C) 2001-2005 Aduna
 *
 *  Contact: 
 *  	Aduna
 *  	Prinses Julianaplein 14 b
 *  	3817 CS Amersfoort
 *  	The Netherlands
 *  	tel. [phone]
 *  	fax. [phone]
 *
 *  	http://aduna.biz/
 *  	http://www.openrdf.org/
 *  
 *  This library is free software; you can redistribute it and/or
 *  modify it under the terms of the GNU Lesser General Public
 *  License as published by the Free Software Foundation; either
 *  version 2.1 of the License, or (at your option) any later version.
 *
 *  This library 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 this library; if not, write to the Free Software
 *  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 */

package org.openrdf.sesame.sail.query;

import java.io.IOException;
import java.util.Set;

import org.openrdf.sesame.sail.RdfSource;

/**
 * Determines the difference between two sets of values that are produced
 * by queries.
 *
 * @author Jeen Broekstra
 **/
public class Minus extends SetOperator {

	/**
	 * Creates a new Minus object that filters the query answers from the left
	 * argument if they are not in the query answer set from the right argument.
	 **/
	public Minus(Query leftArg, Query rightArg, boolean all) {
		super(leftArg, rightArg, all);
	}
	
	// implements Query.evaluate()
	public void evaluate(RdfSource rdfSource, QueryAnswerListener listener)
		throws SailQueryException
	{
		QueryAnswerCollector qac = new QueryAnswerCollector();
		_rightArg.evaluate(rdfSource, qac);
		
		if (!_all) {
			listener = new DuplicatesFilter(listener);
		}
		
		listener = new MinusFilter(qac.getQueryAnswers(), listener);

		_leftArg.evaluate(rdfSource, listener);
	}
	
	public String toString() {
		return
			"(" + _leftArg.toString() +
			" MINUS " + (_all ? "ALL " : "") +
			_rightArg.toString() + ")";
	}

/*------------------------+
| inner class MinusFilter |
+------------------------*/

	/**
	 * A QueryAnswerListener that filters out the answers that are in the
	 * excludeSet (a minus set-operation)
	 **/
	private static class MinusFilter implements QueryAnswerListener {

		/** Set of answers to filter from result. **/
		private Set _excludeSet;

		/** The listener to forward the filtered query results to. **/
		private QueryAnswerListener _listener;

		/**
		 * Creates a new MinusFilter.
		 *
		 * @param excludeSet A Set of QueryAnswer objects.
		 * @param l The QueryAnswerListener that will receive the filtered query results.
		 **/
		public MinusFilter(Set excludeSet, QueryAnswerListener l) {
			_excludeSet = excludeSet;
			_listener = l;
		}

		// implements QueryAnswerListener.queryAnswer(QueryAnswer)
		public boolean queryAnswer(QueryAnswer qa)
			throws IOException
		{
			if (!_excludeSet.contains(qa)) {
				return _listener.queryAnswer(qa);
			}

			return true;
		}

		// implements QueryAnswerListener.clear()
		public void clear() {
			_listener.clear();
		}
	}
}