BindLeftJoinIteration.java
/*******************************************************************************
* Copyright (c) 2024 Eclipse RDF4J contributors.
*
* All rights reserved. This program and the accompanying materials
* are made available under the terms of the Eclipse Distribution License v1.0
* which accompanies this distribution, and is available at
* http://www.eclipse.org/org/documents/edl-v10.php.
*
* SPDX-License-Identifier: BSD-3-Clause
*******************************************************************************/
package org.eclipse.rdf4j.federated.evaluation.iterator;
import java.util.List;
import java.util.ListIterator;
import org.eclipse.collections.impl.set.mutable.primitive.IntHashSet;
import org.eclipse.rdf4j.common.iteration.CloseableIteration;
import org.eclipse.rdf4j.common.iteration.LookAheadIteration;
import org.eclipse.rdf4j.model.Literal;
import org.eclipse.rdf4j.query.Binding;
import org.eclipse.rdf4j.query.BindingSet;
import org.eclipse.rdf4j.query.QueryEvaluationException;
import org.eclipse.rdf4j.query.algebra.evaluation.QueryBindingSet;
/**
* A {@link LookAheadIteration} for processing bind left join results (i.e., result of joining OPTIONAL clauses)
*
* Algorithm:
*
* <ul>
* <li>execute left bind join using regular bound join query</li>
* <li>process result iteration similar to {@link BoundJoinVALUESConversionIteration}</li>
* <li>remember seen set of bindings (using index) and add original bindings to those, i.e. put to result return all
* non-seen bindings directly from the input</li>
*
*
* @author Andreas Schwarte
*/
public class BindLeftJoinIteration extends LookAheadIteration<BindingSet> {
protected final CloseableIteration<BindingSet> iter;
protected final List<BindingSet> bindings;
protected IntHashSet seenBindingIndexes = new IntHashSet();
protected final ListIterator<BindingSet> bindingsIterator;
public BindLeftJoinIteration(CloseableIteration<BindingSet> iter,
List<BindingSet> bindings) {
this.iter = iter;
this.bindings = bindings;
this.bindingsIterator = bindings.listIterator();
}
@Override
protected BindingSet getNextElement() {
if (iter.hasNext()) {
var bIn = iter.next();
int bIndex = ((Literal) bIn.getValue(BoundJoinVALUESConversionIteration.INDEX_BINDING_NAME)).intValue();
seenBindingIndexes.add(bIndex);
return convert(bIn, bIndex);
}
while (bindingsIterator.hasNext()) {
if (seenBindingIndexes.contains(bindingsIterator.nextIndex())) {
// the binding was already processed as part of the optional
bindingsIterator.next();
continue;
}
return bindingsIterator.next();
}
return null;
}
@Override
protected void handleClose() {
iter.close();
}
protected BindingSet convert(BindingSet bIn, int bIndex) throws QueryEvaluationException {
QueryBindingSet res = new QueryBindingSet();
for (Binding b : bIn) {
if (b.getName().equals(BoundJoinVALUESConversionIteration.INDEX_BINDING_NAME)) {
continue;
}
res.addBinding(b);
}
for (Binding bs : bindings.get(bIndex)) {
res.setBinding(bs);
}
return res;
}
}