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The S-Space Package is a collection of algorithms for building Semantic Spaces as well as a highly-scalable library for designing new distributional semantics algorithms. Distributional algorithms process text corpora and represent the semantic for words as high dimensional feature vectors. This package also includes matrices, vectors, and numerous clustering algorithms. These approaches are known by many names, such as word spaces, semantic spaces, or distributed semantics and rest upon the Distributional Hypothesis: words that appear in similar contexts have similar meanings.

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/*
 * Copyright 2009 David Jurgens
 *
 * This file is part of the S-Space package and is covered under the terms and
 * conditions therein.
 *
 * The S-Space package is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as published
 * by the Free Software Foundation and distributed hereunder to you.
 *
 * THIS SOFTWARE IS PROVIDED "AS IS" AND NO REPRESENTATIONS OR WARRANTIES,
 * EXPRESS OR IMPLIED ARE MADE.  BY WAY OF EXAMPLE, BUT NOT LIMITATION, WE MAKE
 * NO REPRESENTATIONS OR WARRANTIES OF MERCHANT- ABILITY OR FITNESS FOR ANY
 * PARTICULAR PURPOSE OR THAT THE USE OF THE LICENSED SOFTWARE OR DOCUMENTATION
 * WILL NOT INFRINGE ANY THIRD PARTY PATENTS, COPYRIGHTS, TRADEMARKS OR OTHER
 * RIGHTS.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program. If not, see .
 */

package edu.ucla.sspace.util;

import java.util.ArrayDeque;
import java.util.Arrays;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Queue;


/**
 * An {@code Iterator} implemntation that combines multiple iterators into a
 * single instance.
 *
 * This class is thread-safe.
 *
 * @author David Jurgens
 */ 
public class CombinedIterator implements Iterator {

    /**
     * The iterators to use
     */
    private final Queue> iters;

    /**
     * The curruent iterator from which elements are being drawn.
     */
    private Iterator current;

    /**
     * The iterator that was last used to return an element.  This field is
     * needed to support {@code remove}
     */
    private Iterator toRemoveFrom;

    /**
     * Constructs a {@code CombinedIterator} from all of the provided iterators.
     */
    public CombinedIterator(Iterator... iterators) {
        this(Arrays.asList(iterators));
    }

    /**
     * Constructs a {@code CombinedIterator} from all of the iterators in the
     * provided collection.
     */
    public CombinedIterator(Collection> iterators) {
        this((Queue>)(new ArrayDeque>(iterators)));
    }

    /**
     * Constructs a {@code CombinedIterator} from all of the iterators in the
     * provided collection.
     */
    private CombinedIterator(Queue> iterators) {
        this.iters = iterators;
        advance();
    }

    /**
     * Joins the iterators of all the provided iterables as one unified
     * iterator.
     */
    public static  Iterator join(Collection> iterables) {
        Queue> iters = 
            new ArrayDeque>(iterables.size());
        for (Iterable i : iterables)
            iters.add(i.iterator());
        return new CombinedIterator(iters);
    }

    /**
     * Moves to the next iterator in the queue if the current iterator is out of
     * elements.
     */
    private void advance() {
        if (current == null || !current.hasNext()) {
            do {
                current = iters.poll();
            } while (current != null && !current.hasNext());
        }
    }

    /**
     * Returns true if there are still elements in at least one of the backing
     * iterators.
     */
    public synchronized boolean hasNext() {
        return current != null && current.hasNext();
    }

    /**
     * Returns the next element from some iterator.
     */
    public synchronized T next() {
        if (current == null) {
            throw new NoSuchElementException();
        }
        T t = current.next();
        // Once an element has been drawn from the iterator, the current
        // iterator should be used for any subsequent remove call.
        if (toRemoveFrom != current)
            toRemoveFrom = current;
        advance();
        return t;
    }

    /**
     * Removes the previously returned element using the backing iterator's
     * {@code remove} method.
     */
    public synchronized void remove() {
        if (toRemoveFrom == null) {
            throw new NoSuchElementException();
        }        
        toRemoveFrom.remove();
    }
}