org.eclipse.rdf4j.common.iteration.MinusIteration Maven / Gradle / Ivy
/*******************************************************************************
* Copyright (c) 2015 Eclipse RDF4J contributors, Aduna, and others.
* 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.
*******************************************************************************/
package org.eclipse.rdf4j.common.iteration;
import java.util.HashSet;
import java.util.Set;
/**
* An Iteration that returns the results of an Iteration (the left argument) minus the results of another
* Iteration (the right argument). Optionally, the Iteration can be configured to filter duplicates from the
* returned elements.
*
* Note that duplicates can also be filtered by wrapping this Iteration in a {@link DistinctIteration}, but
* that has a bit more overhead as it adds a second hash table lookup.
*/
public class MinusIteration extends FilterIteration {
/*-----------*
* Variables *
*-----------*/
private final Iteration rightArg;
private final boolean distinct;
private boolean initialized;
private Set excludeSet;
/*--------------*
* Constructors *
*--------------*/
/**
* Creates a new MinusIteration that returns the results of the left argument minus the results of the
* right argument. By default, duplicates are not filtered from the results.
*
* @param leftArg
* An Iteration containing the main set of elements.
* @param rightArg
* An Iteration containing the set of elements that should be filtered from the main set.
*/
public MinusIteration(Iteration leftArg, Iteration rightArg) {
this(leftArg, rightArg, false);
}
/**
* Creates a new MinusIteration that returns the results of the left argument minus the results of the
* right argument.
*
* @param leftArg
* An Iteration containing the main set of elements.
* @param rightArg
* An Iteration containing the set of elements that should be filtered from the main set.
* @param distinct
* Flag indicating whether duplicate elements should be filtered from the result.
*/
public MinusIteration(Iteration leftArg, Iteration rightArg,
boolean distinct)
{
super(leftArg);
assert rightArg != null;
this.rightArg = rightArg;
this.distinct = distinct;
this.initialized = false;
}
/*--------------*
* Constructors *
*--------------*/
// implements LookAheadIteration.getNextElement()
protected boolean accept(E object)
throws X
{
if (!initialized) {
// Build set of elements-to-exclude from right argument
excludeSet = Iterations.addAll(rightArg, new HashSet());
initialized = true;
}
if (!excludeSet.contains(object)) {
// Object is part of the result
if (distinct) {
// Prevent duplicates from being returned by
// adding the object to the exclude set
excludeSet.add(object);
}
return true;
}
return false;
}
@Override
protected void handleClose()
throws X
{
super.handleClose();
Iterations.closeCloseable(rightArg);
}
}