documentiteratorvisitor.java

MG4J (Managing Gigabytes for Java) is a free full-text search engine for large document collections

package it.unimi.dsi.mg4j.search.visitor;

/*		 
 * MG4J: Managing Gigabytes for Java
 *
 * Copyright (C) 2006-2007 Sebastiano Vigna 
 *
 *  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 program; if not, write to the Free Software
 *  Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 *
 */

import java.io.IOException;

import it.unimi.dsi.mg4j.index.IndexIterator;
import it.unimi.dsi.mg4j.search.DocumentIterator;

/** A visitor for the tree defined by a {@link DocumentIterator}.
 *
 * <p>Implementations of this interface must be reusable. The user must
 * invoke {@link #prepare()} before a visit so that the internal state
 * of the visitor can be suitably set up.
 * 
 * <p>For maximum flexibility, there is just {@linkplain #visit(IndexIterator) one visit method}
 * for leaves, but <em>two</em> visits methods for internal nodes,
 * which should be used for {@linkplain #visitPre(DocumentIterator) preorder}
 * and {@linkplain #visitPost(DocumentIterator) postorder} visits, respectively.
 *
 * <p>Note that this visitor interface and that defined in {@link it.unimi.dsi.mg4j.query.nodes.QueryBuilderVisitor}
 * are based on different principles: in the latter case, the action of the visitor will likely
 * be different for each type of internal node, so we have specific visit methods for each type of such nodes. 
 * In our case, the visit will most likely behave differently just for internal nodes and leaves, so we
 * prefer a simpler interface thatalso let us implement more easily visitor acceptance methods
 * ({@link it.unimi.dsi.mg4j.search.DocumentIterator#accept(DocumentIteratorVisitor)}
 * and {@link it.unimi.dsi.mg4j.search.DocumentIterator#acceptOnTruePaths(DocumentIteratorVisitor)}).
 *
 * @author Sebastiano Vigna
 */ 

public interface DocumentIteratorVisitor {

	/** Prepares the internal state of this visitor for a(nother) visit. 
	 * 
	 * <p>By specification, it must be safe to call this method any number of times.
	 * @return this visitor.
	 */
	DocumentIteratorVisitor prepare();

	/** Visits an internal node <em>before</em> recursing into the corresponding subtree.
	 * 
	 * @param documentIterator the internal node to be visited.
	 * @return true if the visit should continue.
	 */
	boolean visitPre( DocumentIterator documentIterator );

	/** Visits an internal node <em>after</em> recursing into the corresponding subtree.
	 * 
	 * @param documentIterator the internal node to be visited.
	 * @return true if the visit should continue.
	 */
	boolean visitPost( DocumentIterator documentIterator );

	/** Visits a leaf.
	 * 
	 * @param indexIterator the leaf to be visited.
	 * @return true if the visit should continue.
	 */
	boolean visit( IndexIterator indexIterator ) throws IOException;

}