/*
 * Copyright (c) "Neo4j"
 * Neo4j Sweden AB [http://neo4j.com]
 *
 * This file is part of Neo4j.
 *
 * Neo4j is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
package org.neo4j.internal.helpers.collection;

import java.util.Iterator;
import java.util.NoSuchElementException;

/**
 * Abstract class for how you usually implement iterators when you don't know
 * how many objects there are (which is pretty much every time)
 *
 * Basically the {@link #hasNext()} method will look up the next object and
 * cache it. The cached object is then set to {@code null} in {@link #next()}.
 * So you only have to implement one method, {@code fetchNextOrNull} which
 * returns {@code null} when the iteration has reached the end, and you're done.
 */
public abstract class PrefetchingIterator<T> implements Iterator<T> {
    private boolean hasFetchedNext;
    private T nextObject;

    /**
     * @return {@code true} if there is a next item to be returned from the next
     * call to {@link #next()}.
     */
    @Override
    public boolean hasNext() {
        return peek() != null;
    }

    /**
     * @return the next element that will be returned from {@link #next()} without
     * actually advancing the iterator
     */
    public T peek() {
        if (hasFetchedNext) {
            return nextObject;
        }

        nextObject = fetchNextOrNull();
        hasFetchedNext = true;
        return nextObject;
    }

    /**
     * Uses {@link #hasNext()} to try to fetch the next item and returns it
     * if found, otherwise it throws a {@link NoSuchElementException}.
     *
     * @return the next item in the iteration, or throws
     * {@link NoSuchElementException} if there's no more items to return.
     */
    @Override
    public T next() {
        if (!hasNext()) {
            throw new NoSuchElementException();
        }
        T result = nextObject;
        nextObject = null;
        hasFetchedNext = false;
        return result;
    }

    public boolean hasFetchedNext() {
        return hasFetchedNext;
    }

    public T getNextObject() {
        return nextObject;
    }

    protected abstract T fetchNextOrNull();

    @Override
    public void remove() {
        throw new UnsupportedOperationException();
    }
}
