/**
Copyright (C) SYSTAP, LLC DBA Blazegraph 2006-2016. All rights reserved.
Contact:
SYSTAP, LLC DBA Blazegraph
2501 Calvert ST NW #106
Washington, DC 20008
licenses@blazegraph.com
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; version 2 of the License.
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., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
*/
/*
* Created on Oct 1, 2010
*/
package com.bigdata.rdf.spo;
import com.bigdata.btree.IRangeQuery;
import com.bigdata.btree.ITuple;
import com.bigdata.btree.ITupleCursor;
import com.bigdata.btree.filter.Advancer;
import com.bigdata.btree.keys.IKeyBuilder;
import com.bigdata.btree.keys.KeyBuilder;
import com.bigdata.btree.keys.SuccessorUtil;
import com.bigdata.rdf.internal.IV;
import com.bigdata.rdf.internal.IVUtility;
/**
* Advancer for a quads index whose last key component is the "context position
* (such as SPOC or SOPC). The advancer will skip to first possible key for the
* next distinct triple for each quad which it visits. This is a cheap way to
* impose a "DISTINCT" filter using an index scan and works well for both local
* and scale-out indices.
* <p>
* You have to use {@link IRangeQuery#CURSOR} to request an {@link ITupleCursor}
* when using an {@link Advancer} pattern.
*
* @author <a href="mailto:thompsonbry@users.sourceforge.net">Bryan Thompson</a>
* @version $Id$
*/
public class ContextAdvancer extends Advancer<SPO> {
private static final long serialVersionUID = 1L;
private transient IKeyBuilder keyBuilder;
public ContextAdvancer() {
}
@Override
protected void advance(final ITuple<SPO> tuple) {
if (keyBuilder == null) {
/*
* Note: It appears that you can not set this either implicitly or
* explicitly during ctor initialization if you want it to exist
* during de-serialization. Hence it is initialized lazily here.
* This is Ok since the iterator pattern is single threaded.
*/
keyBuilder = KeyBuilder.newInstance();
}
// extract the key.
final byte[] key = tuple.getKey();
// decode the first three components of the key.
final IV[] terms = IVUtility.decode(key, 3/*nterms*/);
// reset the buffer.
keyBuilder.reset();
// encode the first three components of the key.
IVUtility.encode(keyBuilder,terms[0]);
IVUtility.encode(keyBuilder,terms[1]);
IVUtility.encode(keyBuilder,terms[2]);
// obtain the key.
final byte[] fromKey = keyBuilder.getKey();
// obtain the successor of the key.
final byte[] toKey = SuccessorUtil.successor(fromKey.clone());
// seek to that successor.
src.seek(toKey);
}
}