/*
 * Copyright (C) 2010 The Guava Authors
 *
 * Licensed 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 com.google.common.collect;

import java.util.NoSuchElementException;

import javax.annotation.Nullable;

import com.google.common.annotations.GwtCompatible;

/**
 * This class provides a skeletal implementation of the {@code Iterator}
 * interface for sequences whose next element can always be derived from the
 * previous element. Null elements are not supported, nor is the
 * {@link #remove()} method.
 *
 * <p>
 * Example:
 * 
 * <pre>
 * {
 * 	&#64;code
 *
 * 	Iterator<Integer> powersOfTwo = new AbstractSequentialIterator<Integer>(1) {
 * 		protected Integer computeNext(Integer previous) {
 * 			return (previous == 1 << 30) ? null : previous * 2;
 * 		}
 * 	};
 * }
 * </pre>
 *
 * @author Chris Povirk
 * @since 12.0 (in Guava as {@code AbstractLinkedIterator} since 8.0)
 */
@GwtCompatible
public abstract class AbstractSequentialIterator<T> extends UnmodifiableIterator<T> {
	private T nextOrNull;

	/**
	 * Creates a new iterator with the given first element, or, if {@code
	 * firstOrNull} is null, creates a new empty iterator.
	 */
	protected AbstractSequentialIterator(@Nullable T firstOrNull) {
		this.nextOrNull = firstOrNull;
	}

	/**
	 * Returns the element that follows {@code previous}, or returns {@code null} if
	 * no elements remain. This method is invoked during each call to
	 * {@link #next()} in order to compute the result of a <i>future</i> call to
	 * {@code next()}.
	 */
	protected abstract T computeNext(T previous);

	@Override
	public final boolean hasNext() {
		return nextOrNull != null;
	}

	@Override
	public final T next() {
		if (!hasNext()) {
			throw new NoSuchElementException();
		}
		try {
			return nextOrNull;
		} finally {
			nextOrNull = computeNext(nextOrNull);
		}
	}
}