/*

 * 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.NoSuchElementException;

import org.apache.commons.collections4.ResettableListIterator;

/**

 * <code>SingletonIterator</code> is an {@link java.util.ListIterator} over a single

 * object instance.

 *

 * @since 2.1

 */

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

    private boolean beforeFirst = true;

    private boolean nextCalled = false;

    private boolean removed = false;

    private E object;

    /**

     * Constructs a new <code>SingletonListIterator</code>.

     *

     * @param object  the single object to return from the iterator

     */

    public SingletonListIterator(final E object) {

        super();

        this.object = object;

    }

    /**

     * Is another object available from the iterator?

     * <p>

     * This returns true if the single object hasn't been returned yet.

     *

     * @return true if the single object hasn't been returned yet

     */

    @Override

    public boolean hasNext() {

        return beforeFirst && !removed;

    }

    /**

     * Is a previous object available from the iterator?

     * <p>

     * This returns true if the single object has been returned.

     *

     * @return true if the single object has been returned

     */

    @Override

    public boolean hasPrevious() {

        return !beforeFirst && !removed;

    }

    /**

     * Returns the index of the element that would be returned by a subsequent

     * call to {@code next}.

     *

     * @return 0 or 1 depending on current state.

     */

    @Override

    public int nextIndex() {

        return beforeFirst ? 0 : 1;

    }

    /**

     * Returns the index of the element that would be returned by a subsequent

     * call to {@code previous}. A return value of -1 indicates that the iterator is currently at

     * the start.

     *

     * @return 0 or -1 depending on current state.

     */

    @Override

    public int previousIndex() {

        return beforeFirst ? -1 : 0;

    }

    /**

     * Get the next object from the iterator.

     * <p>

     * This returns the single object if it hasn't been returned yet.

     *

     * @return the single object

     * @throws NoSuchElementException if the single object has already

     *    been returned

     */

    @Override

    public E next() {

        if (!beforeFirst || removed) {

            throw new NoSuchElementException();

        }

        beforeFirst = false;

        nextCalled = true;

        return object;

    }

    /**

     * Get the previous object from the iterator.

     * <p>

     * This returns the single object if it has been returned.

     *

     * @return the single object

     * @throws NoSuchElementException if the single object has not already

     *    been returned

     */

    @Override

    public E previous() {

        if (beforeFirst || removed) {

            throw new NoSuchElementException();

        }

        beforeFirst = true;

        return object;

    }

    /**

     * Remove the object from this iterator.

     * @throws IllegalStateException if the {@code next} or {@code previous}

     *        method has not yet been called, or the {@code remove} method

     *        has already been called after the last call to {@code next}

     *        or {@code previous}.

     */

    @Override

    public void remove() {

        if(!nextCalled || removed) {

            throw new IllegalStateException();

        }

        object = null;

        removed = true;

    }

    /**

     * Add always throws {@link UnsupportedOperationException}.

     *

     * @param obj  the object to add

     * @throws UnsupportedOperationException always

     */

    @Override

    public void add(final E obj) {

        throw new UnsupportedOperationException("add() is not supported by this iterator");

    }

    /**

     * Set sets the value of the singleton.

     *

     * @param obj  the object to set

     * @throws IllegalStateException if {@code next} has not been called

     *          or the object has been removed

     */

    @Override

    public void set(final E obj) {

        if (!nextCalled || removed) {

            throw new IllegalStateException();

        }

        this.object = obj;

    }

    /**

     * Reset the iterator back to the start.

     */

    @Override

    public void reset() {

        beforeFirst = true;

        nextCalled = false;

    }

}