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Sesame is an open source Java framework for storing, querying and reasoning with RDF.
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/*  Sesame - Storage and Querying architecture for RDF and RDF Schema
 *  Copyright (C) 2001-2006 Aduna
 *
 *  Contact:
 *  	Aduna
 *  	Prinses Julianaplein 14 b
 *  	3817 CS Amersfoort
 *  	The Netherlands
 *  	tel. +33 (0)33 465 99 87
 *  	fax. +33 (0)33 465 99 87
 *
 *  	http://aduna-software.com/
 *  	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 intersection of two sets of values that are produced
 * by queries.
 **/
public class Intersect extends SetOperator {

	/**
	 * Creates a new Intersect object that only reports query answers that are
	 * present in the query answer sets of both the left- and right argument.
	 **/
	public Intersect(Query leftArg, Query rightArg, boolean all) {
		super(leftArg, rightArg, all);
	}

	// overrides 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 IntersectionFilter(qac.getQueryAnswers(), listener);

		_leftArg.evaluate(rdfSource, listener);
	}

	public String toString() {
		return
			"(" + _leftArg.toString() +
			" INTERSECT " + (_all ? "ALL " : "") +
			_rightArg.toString() + ")";
	}

/*-------------------------------+
| inner class IntersectionFilter |
+-------------------------------*/

	/**
	 * A QueryAnswerListener that filters out the answers that are not in
	 * the includeSet (an interect set-operation)
	 **/
	private static class IntersectionFilter implements QueryAnswerListener {

		/**
		 * Set of QueryAnswer objects which should not be filtered from the result.
		 **/
		private Set _includeSet;

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

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

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

			return true;
		}

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