src/main/java/org/apache/sling/query/iterator/UniqueIterator.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.sling.query.iterator;

import java.util.Collections;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

import org.apache.sling.query.api.internal.Option;

/**
 * Provides an iteration of unique objects. During the iteration process this
 * iterator maintains a set of previously seen items that will be used as filter
 * to prevent duplicates from being iterated through
 * 
 * @param <T> 
 */
public class UniqueIterator<T> extends AbstractIterator<Option<T>> {

	private Iterator<Option<T>> iterator;

	private Set<T> seen;

	public UniqueIterator(Iterator<Option<T>> input) {
		this.iterator = input;
		seen = new HashSet<>();
	}

	@Override
	protected Option<T> getElement() {
		if (!iterator.hasNext()) {
			iterator = Collections.emptyIterator();
			seen = null;
			return null;
		}
		Option<T> candidate = iterator.next();
		if (!candidate.isEmpty()) {
			if (!seen.add(candidate.getElement())) {
				return Option.empty(candidate.getArgumentId());
			}
		}
		return candidate;
	}

}