/*

 * 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.collections4.iterators;

import java.util.ArrayList;

import java.util.Iterator;

import java.util.List;

import java.util.NoSuchElementException;

import org.apache.commons.collections4.FluentIterable;

/**

 * Provides an interleaved iteration over the elements contained in a

 * collection of Iterators.

 * <p>

 * Given two {@link Iterator} instances {@code A} and {@code B}, the

 * {@link #next} method on this iterator will switch between {@code A.next()}

 * and {@code B.next()} until both iterators are exhausted.

 * </p>

 *

 * @since 4.1

 */

public class ZippingIterator<E> implements Iterator<E> {

    /** The {@link Iterator}s to evaluate. */

    private final Iterator<Iterator<? extends E>> iterators;

    /** The next iterator to use for next(). */

    private Iterator<? extends E> nextIterator = null;

    /** The last iterator which was used for next(). */

    private Iterator<? extends E> lastReturned = null;

    // Constructors

    // ----------------------------------------------------------------------

    /**

     * Constructs a new <code>ZippingIterator</code> that will provide

     * interleaved iteration over the two given iterators.

     *

     * @param a  the first child iterator

     * @param b  the second child iterator

     * @throws NullPointerException if either iterator is null

     */

    @SuppressWarnings("unchecked")

    public ZippingIterator(final Iterator<? extends E> a, final Iterator<? extends E> b) {

        this(new Iterator[] {a, b});

    }

    /**

     * Constructs a new <code>ZippingIterator</code> that will provide

     * interleaved iteration over the three given iterators.

     *

     * @param a  the first child iterator

     * @param b  the second child iterator

     * @param c  the third child iterator

     * @throws NullPointerException if either iterator is null

     */

    @SuppressWarnings("unchecked")

    public ZippingIterator(final Iterator<? extends E> a,

                           final Iterator<? extends E> b,

                           final Iterator<? extends E> c) {

        this(new Iterator[] {a, b, c});

    }

    /**

     * Constructs a new <code>ZippingIterator</code> that will provide

     * interleaved iteration of the specified iterators.

     *

     * @param iterators  the array of iterators

     * @throws NullPointerException if any iterator is null

     */

    public ZippingIterator(final Iterator<? extends E>... iterators) {

        // create a mutable list to be able to remove exhausted iterators

        final List<Iterator<? extends E>> list = new ArrayList<>();

        for (final Iterator<? extends E> iterator : iterators) {

            if (iterator == null) {

                throw new NullPointerException("Iterator must not be null.");

            }

            list.add(iterator);

        }

        this.iterators = FluentIterable.of(list).loop().iterator();

    }

    // Iterator Methods

    // -------------------------------------------------------------------

    /**

     * Returns {@code true} if any child iterator has remaining elements.

     *

     * @return true if this iterator has remaining elements

     */

    @Override

    public boolean hasNext() {

        // the next iterator has already been determined

        // this might happen if hasNext() is called multiple

        if (nextIterator != null) {

            return true;

        }

        while(iterators.hasNext()) {

            final Iterator<? extends E> childIterator = iterators.next();

            if (childIterator.hasNext()) {

                nextIterator = childIterator;

                return true;

            }

            // iterator is exhausted, remove it

            iterators.remove();

        }

        return false;

    }

    /**

     * Returns the next element from a child iterator.

     *

     * @return the next interleaved element

     * @throws NoSuchElementException if no child iterator has any more elements

     */

    @Override

    public E next() throws NoSuchElementException {

        if (!hasNext()) {

            throw new NoSuchElementException();

        }

        final E val = nextIterator.next();

        lastReturned = nextIterator;

        nextIterator = null;

        return val;

    }

    /**

     * Removes the last returned element from the child iterator that produced it.

     *

     * @throws IllegalStateException if there is no last returned element, or if

     *   the last returned element has already been removed

     */

    @Override

    public void remove() {

        if (lastReturned == null) {

            throw new IllegalStateException("No value can be removed at present");

        }

        lastReturned.remove();

        lastReturned = null;

    }

}