/*

 * 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.ArrayDeque;

import java.util.Deque;

import java.util.Iterator;

/**

 * Decorates an iterator to support pushback of elements.

 * <p>

 * The decorator stores the pushed back elements in a LIFO manner: the last element

 * that has been pushed back, will be returned as the next element in a call to {@link #next()}.

 * <p>

 * The decorator does not support the removal operation. Any call to {@link #remove()} will

 * result in an {@link UnsupportedOperationException}.

 *

 * @since 4.0

 */

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

    /** The iterator being decorated. */

    private final Iterator<? extends E> iterator;

    /** The LIFO queue containing the pushed back items. */

    private final Deque<E> items = new ArrayDeque<>();

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

    /**

     * Decorates the specified iterator to support one-element lookahead.

     * <p>

     * If the iterator is already a {@link PushbackIterator} it is returned directly.

     *

     * @param <E>  the element type

     * @param iterator  the iterator to decorate

     * @return a new peeking iterator

     * @throws NullPointerException if the iterator is null

     */

    public static <E> PushbackIterator<E> pushbackIterator(final Iterator<? extends E> iterator) {

        if (iterator == null) {

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

        }

        if (iterator instanceof PushbackIterator<?>) {

            @SuppressWarnings("unchecked") // safe cast

            final PushbackIterator<E> it = (PushbackIterator<E>) iterator;

            return it;

        }

        return new PushbackIterator<>(iterator);

    }

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

    /**

     * Constructor.

     *

     * @param iterator  the iterator to decorate

     */

    public PushbackIterator(final Iterator<? extends E> iterator) {

        super();

        this.iterator = iterator;

    }

    /**

     * Push back the given element to the iterator.

     * <p>

     * Calling {@link #next()} immediately afterwards will return exactly this element.

     *

     * @param item  the element to push back to the iterator

     */

    public void pushback(final E item) {

        items.push(item);

    }

    @Override

    public boolean hasNext() {

        return !items.isEmpty() || iterator.hasNext();

    }

    @Override

    public E next() {

        return !items.isEmpty() ? items.pop() : iterator.next();

    }

    /**

     * This iterator will always throw an {@link UnsupportedOperationException}.

     *

     * @throws UnsupportedOperationException always

     */

    @Override

    public void remove() {

        throw new UnsupportedOperationException();

    }

}