utils/src/main/java/org/apache/clerezza/utils/IteratorMerger.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.clerezza.utils;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Iterator;

/**
 * Flattens an Iterator of Iterators to an Iterator
 * 
 * @author reto
 */
public class IteratorMerger<T> implements Iterator<T> {

    /**
     * This is the iterator of the merged iterators.
     */
    protected Iterator<Iterator<T>> baseIterators;
    
    /**
     * This is the current iterator of the merged iterators. Over this iterator
     * iterates the merged iterator currently.
     */
    protected Iterator<T> current;

    /**
     * constructs an iterator that will return the elements of the baseIterators
     * 
     * @param baseIterators
     */
    public IteratorMerger(Iterator<Iterator<T>> baseIterators) {
        init(baseIterators);
    }

    /**
     * constructs an iterator that will return the elements of the baseIterators
     *
     * @param baseIterators
     */
    public IteratorMerger(Iterator<T>... baseIterators) {
        init(Arrays.asList(baseIterators).iterator());
    }

    /**
     * Constructs an iterator that iterates over all elements of the collections
     * contained in the collection.
     * 
     * @param collectionOfCollections
     */
    public IteratorMerger(Collection<Collection<T>> collectionOfCollections) {
        final Iterator<Collection<T>> setIter = collectionOfCollections.iterator();
        init(new Iterator<Iterator<T>>() {

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

            @Override
            public Iterator<T> next() {
                return setIter.next().iterator();
            }

            @Override
            public void remove() {
                throw new UnsupportedOperationException("Not supported yet.");
            }
        });
    }

    private void init(Iterator<Iterator<T>> baseIterators) {
        this.baseIterators = baseIterators;
        if (baseIterators.hasNext()) {
            current = baseIterators.next();
        } else {
            current = new ArrayList<T>(0).iterator();
        }
    }

    private void updateCurrentIfNeeded() {
        while (!current.hasNext()) {
            if (baseIterators.hasNext()) {
                current = baseIterators.next();
            } else {
                return;
            }
        }
    }

    @Override
    public boolean hasNext() {
        updateCurrentIfNeeded();
        return current.hasNext();
    }

    @Override
    public T next() {
        updateCurrentIfNeeded();
        return current.next();
    }

    @Override
    public void remove() {
        current.remove();
    }

}