impl.utils/src/main/java/org/apache/commons/rdf/impl/utils/simple/SimpleGraph.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
package org.apache.commons.rdf.impl.utils.simple;

import org.apache.commons.rdf.impl.utils.AbstractGraph;
import java.lang.ref.SoftReference;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.ConcurrentModificationException;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.Set;

import org.apache.commons.rdf.BlankNodeOrIri;
import org.apache.commons.rdf.ImmutableGraph;
import org.apache.commons.rdf.RdfTerm;
import org.apache.commons.rdf.Triple;
import org.apache.commons.rdf.Iri;

/**
 * For now this is a minimalistic implementation, without any indexes or other
 * optimizations.
 *
 * @author reto
 */
public class SimpleGraph extends AbstractGraph {

    final Set<Triple> triples;

    private boolean checkConcurrency = false;

    class SimpleIterator implements Iterator<Triple> {

        private Iterator<Triple> listIter;
        private boolean isValid = true;

        public SimpleIterator(Iterator<Triple> listIter) {
            this.listIter = listIter;
        }
        private Triple currentNext;

        @Override
        public boolean hasNext() {
            checkValidity();
            return listIter.hasNext();
        }

        @Override
        public Triple next() {
            checkValidity();
            currentNext = listIter.next();
            return currentNext;
        }        

        @Override
        public void remove() {
            checkValidity();
            listIter.remove();
            triples.remove(currentNext);            
            invalidateIterators(this);            
        }

        private void checkValidity() throws ConcurrentModificationException {
            if (checkConcurrency && !isValid) {
                throw new ConcurrentModificationException();
            }
        }

        private void invalidate() {
            isValid = false;
        }
    }    
    
    private final Set<SoftReference<SimpleIterator>> iterators =
            Collections.synchronizedSet(new HashSet<SoftReference<SimpleIterator>>());
    
    /**
     * Creates an empty SimpleGraph
     */
    public SimpleGraph() {
        triples = Collections.synchronizedSet(new HashSet<Triple>());
    }

    /**
     * Creates a SimpleGraph using the passed iterator, the iterator 
     * is consumed before the constructor returns
     * 
     * @param iterator
     */
    public SimpleGraph(Iterator<Triple> iterator) {
        triples = new HashSet<Triple>();
        while (iterator.hasNext()) {
            Triple triple = iterator.next();
            triples.add(triple);
        }
    }

    /**
     * Creates a SimpleGraph for the specified set of triples, 
     * subsequent modification of baseSet do affect the created instance.
     * 
     * @param baseSet
     */
    public SimpleGraph(Set<Triple> baseSet) {
        this.triples = baseSet;
    }

    /**
     * Creates a SimpleGraph for the specified collection of triples,
     * subsequent modification of baseSet do not affect the created instance.
     *
     * @param baseSet
     */
    public SimpleGraph(Collection<Triple> baseCollection) {
        this.triples = new HashSet<Triple>(baseCollection);
    }

    @Override
    public int performSize() {
        return triples.size();
    }

    @Override
    public Iterator<Triple> performFilter(final BlankNodeOrIri subject, final Iri predicate, final RdfTerm object) {
        final List<Triple> tripleList = new ArrayList<Triple>();
        synchronized (triples) {
            Iterator<Triple> baseIter = triples.iterator();
            while (baseIter.hasNext()) {
                Triple triple = baseIter.next();
                if ((subject != null)
                        && (!triple.getSubject().equals(subject))) {
                    continue;
                }
                if ((predicate != null)
                        && (!triple.getPredicate().equals(predicate))) {
                    continue;
                }
                if ((object != null)
                        && (!triple.getObject().equals(object))) {
                    continue;
                }
                tripleList.add(triple);
            }

            final Iterator<Triple> listIter = tripleList.iterator();
            SimpleIterator resultIter = new SimpleIterator(listIter);
            if (checkConcurrency) {
                iterators.add(new SoftReference<SimpleIterator>(resultIter));
            }
            return resultIter;
        }
    }


    @Override
    public boolean performAdd(Triple e) {
        boolean modified = triples.add(e);
        if (modified) {
            invalidateIterators(null);
        }
        return modified;
    }
    
    private void invalidateIterators(SimpleIterator caller) {
        if (!checkConcurrency) {
            return;
        }
        Set<SoftReference> oldReferences = new HashSet<SoftReference>();
        synchronized(iterators) {
            for (SoftReference<SimpleGraph.SimpleIterator> softReference : iterators) {
                SimpleIterator simpleIterator = softReference.get();
                if (simpleIterator == null) {
                    oldReferences.add(softReference);
                    continue;
                }
                if (simpleIterator != caller) {
                    simpleIterator.invalidate();
                }
            }
        }
        iterators.removeAll(oldReferences);
    }

    /**
     * Specifies whether or not to throw <code>ConcurrentModificationException</code>s,
     * if this simple triple collection is modified concurrently. Concurrency
     * check is set to false by default.
     *
     * @param bool Specifies whether or not to check concurrent modifications.
     */
    public void setCheckConcurrency(boolean bool) {
        checkConcurrency = bool;
    }
    
    
    @Override
    public ImmutableGraph getImmutableGraph() {
        return new SimpleImmutableGraph(this);
    }
}