/*

 * 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.List;

import java.util.ListIterator;

import org.apache.commons.collections4.ResettableListIterator;

/**

 * Iterates backwards through a List, starting with the last element

 * and continuing to the first. This is useful for looping around

 * a list in reverse order without needing to actually reverse the list.

 * <p>

 * The first call to <code>next()</code> will return the last element

 * from the list, and so on. The <code>hasNext()</code> method works

 * in concert with the <code>next()</code> method as expected.

 * However, the <code>nextIndex()</code> method returns the correct

 * index in the list, thus it starts high and reduces as the iteration

 * continues. The previous methods work similarly.

 *

 * @since 3.2

 */

public class ReverseListIterator<E> implements ResettableListIterator<E> {

    /** The list being wrapped. */

    private final List<E> list;

    /** The list iterator being wrapped. */

    private ListIterator<E> iterator;

    /** Flag to indicate if updating is possible at the moment. */

    private boolean validForUpdate = true;

    /**

     * Constructor that wraps a list.

     *

     * @param list  the list to create a reversed iterator for

     * @throws NullPointerException if the list is null

     */

    public ReverseListIterator(final List<E> list) {

        super();

        if (list == null) {

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

        }

        this.list = list;

        iterator = list.listIterator(list.size());

    }

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

    /**

     * Checks whether there is another element.

     *

     * @return true if there is another element

     */

    @Override

    public boolean hasNext() {

        return iterator.hasPrevious();

    }

    /**

     * Gets the next element.

     * The next element is the previous in the list.

     *

     * @return the next element in the iterator

     */

    @Override

    public E next() {

        final E obj = iterator.previous();

        validForUpdate = true;

        return obj;

    }

    /**

     * Gets the index of the next element.

     *

     * @return the index of the next element in the iterator

     */

    @Override

    public int nextIndex() {

        return iterator.previousIndex();

    }

    /**

     * Checks whether there is a previous element.

     *

     * @return true if there is a previous element

     */

    @Override

    public boolean hasPrevious() {

        return iterator.hasNext();

    }

    /**

     * Gets the previous element.

     * The next element is the previous in the list.

     *

     * @return the previous element in the iterator

     */

    @Override

    public E previous() {

        final E obj = iterator.next();

        validForUpdate = true;

        return obj;

    }

    /**

     * Gets the index of the previous element.

     *

     * @return the index of the previous element in the iterator

     */

    @Override

    public int previousIndex() {

        return iterator.nextIndex();

    }

    /**

     * Removes the last returned element.

     *

     * @throws UnsupportedOperationException if the list is unmodifiable

     * @throws IllegalStateException if there is no element to remove

     */

    @Override

    public void remove() {

        if (validForUpdate == false) {

            throw new IllegalStateException("Cannot remove from list until next() or previous() called");

        }

        iterator.remove();

    }

    /**

     * Replaces the last returned element.

     *

     * @param obj  the object to set

     * @throws UnsupportedOperationException if the list is unmodifiable

     * @throws IllegalStateException if the iterator is not in a valid state for set

     */

    @Override

    public void set(final E obj) {

        if (validForUpdate == false) {

            throw new IllegalStateException("Cannot set to list until next() or previous() called");

        }

        iterator.set(obj);

    }

    /**

     * Adds a new element to the list between the next and previous elements.

     *

     * @param obj  the object to add

     * @throws UnsupportedOperationException if the list is unmodifiable

     * @throws IllegalStateException if the iterator is not in a valid state for set

     */

    @Override

    public void add(final E obj) {

        // the validForUpdate flag is needed as the necessary previous()

        // method call re-enables remove and add

        if (validForUpdate == false) {

            throw new IllegalStateException("Cannot add to list until next() or previous() called");

        }

        validForUpdate = false;

        iterator.add(obj);

        iterator.previous();

    }

    /**

     * Resets the iterator back to the start (which is the

     * end of the list as this is a reversed iterator)

     */

    @Override

    public void reset() {

        iterator = list.listIterator(list.size());

    }

}