/*
 * 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
 *
 *      https://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;
import java.util.NoSuchElementException;

import org.apache.commons.collections4.Transformer;

/**
 * An Iterator that can traverse multiple iterators down an object graph.
 * <p>
 * This iterator can extract multiple objects from a complex tree-like object graph.
 * The iteration starts from a single root object.
 * It uses a {@code Transformer} to extract the iterators and elements.
 * Its main benefit is that no intermediate {@code List} is created.
 * </p>
 * <p>
 * For example, consider an object graph:
 * </p>
 * <pre>
 *                 |- Branch -- Leaf
 *                 |         \- Leaf
 *         |- Tree |         /- Leaf
 *         |       |- Branch -- Leaf
 *  Forest |                 \- Leaf
 *         |       |- Branch -- Leaf
 *         |       |         \- Leaf
 *         |- Tree |         /- Leaf
 *                 |- Branch -- Leaf
 *                 |- Branch -- Leaf</pre>
 * <p>
 * The following {@code Transformer}, used in this class, will extract all
 * the Leaf objects without creating a combined intermediate list:
 * </p>
 * <pre>
 * public Object transform(Object input) {
 *   if (input instanceof Forest) {
 *     return ((Forest) input).treeIterator();
 *   }
 *   if (input instanceof Tree) {
 *     return ((Tree) input).branchIterator();
 *   }
 *   if (input instanceof Branch) {
 *     return ((Branch) input).leafIterator();
 *   }
 *   if (input instanceof Leaf) {
 *     return input;
 *   }
 *   throw new ClassCastException();
 * }</pre>
 * <p>
 * Internally, iteration starts from the root object. When next is called,
 * the transformer is called to examine the object. The transformer will return
 * either an iterator or an object. If the object is an Iterator, the next element
 * from that iterator is obtained and the process repeats. If the element is an object
 * it is returned.
 * </p>
 * <p>
 * Under many circumstances, linking Iterators together in this manner is
 * more efficient (and convenient) than using nested for loops to extract a list.
 * </p>
 *
 * @param <E> The type of elements returned by this iterator.
 * @since 3.1
 */
public class ObjectGraphIterator<E> implements Iterator<E> {

    /** The stack of iterators */
    private final Deque<Iterator<? extends E>> stack = new ArrayDeque<>(8);

    /** The root object in the tree */
    private E root;

    /** The transformer to use */
    private final Transformer<? super E, ? extends E> transformer;

    /** Whether there is another element in the iteration */
    private boolean hasNext;

    /** The current iterator */
    private Iterator<? extends E> currentIterator;

    /** The current value */
    private E currentValue;

    /** The last used iterator, needed for remove() */
    private Iterator<? extends E> lastUsedIterator;

    /**
     * Constructs an ObjectGraphIterator using a root object and transformer.
     * <p>
     * The root object can be an iterator, in which case it will be immediately
     * looped around.
     *
     * @param root  The root object, null will result in an empty iterator
     * @param transformer  The transformer to use, null will use a no effect transformer
     */
    @SuppressWarnings("unchecked")
    public ObjectGraphIterator(final E root, final Transformer<? super E, ? extends E> transformer) {
        if (root instanceof Iterator) {
            this.currentIterator = (Iterator<? extends E>) root;
        } else {
            this.root = root;
        }
        this.transformer = transformer;
    }

    /**
     * Constructs a ObjectGraphIterator that will handle an iterator of iterators.
     * <p>
     * This constructor exists for convenience to emphasise that this class can
     * be used to iterate over nested iterators. That is to say that the iterator
     * passed in here contains other iterators, which may in turn contain further
     * iterators.
     * </p>
     *
     * @param rootIterator  The root iterator, null will result in an empty iterator
     */
    public ObjectGraphIterator(final Iterator<? extends E> rootIterator) {
        this.currentIterator = rootIterator;
        this.transformer = null;
    }

    /**
     * Finds the next object in the iteration given any start object.
     *
     * @param value  The value to start from
     */
    @SuppressWarnings("unchecked")
    protected void findNext(final E value) {
        if (value instanceof Iterator) {
            // need to examine this iterator
            findNextByIterator((Iterator<? extends E>) value);
        } else {
            // next value found
            currentValue = value;
            hasNext = true;
        }
    }

    /**
     * Finds the next object in the iteration given an iterator.
     *
     * @param iterator  The iterator to start from
     */
    protected void findNextByIterator(final Iterator<? extends E> iterator) {
        if (iterator != currentIterator) {
            // recurse a level
            if (currentIterator != null) {
                stack.push(currentIterator);
            }
            currentIterator = iterator;
        }

        while (currentIterator.hasNext() && !hasNext) {
            E next = currentIterator.next();
            if (transformer != null) {
                next = transformer.apply(next);
            }
            findNext(next);
        }
        // if we haven't found the next value and iterators are not yet exhausted
        if (!hasNext && !stack.isEmpty()) {
            // current iterator exhausted, go up a level
            currentIterator = stack.pop();
            findNextByIterator(currentIterator);
        }
    }

    /**
     * Checks whether there are any more elements in the iteration to obtain.
     *
     * @return true if elements remain in the iteration
     */
    @Override
    public boolean hasNext() {
        updateCurrentIterator();
        return hasNext;
    }

    /**
     * Gets the next element of the iteration.
     *
     * @return The next element from the iteration
     * @throws NoSuchElementException if all the Iterators are exhausted
     */
    @Override
    public E next() {
        updateCurrentIterator();
        if (!hasNext) {
            throw new NoSuchElementException("No more elements in the iteration");
        }
        lastUsedIterator = currentIterator;
        final E result = currentValue;
        currentValue = null;
        hasNext = false;
        return result;
    }

    /**
     * Removes from the underlying collection the last element returned.
     * <p>
     * This method calls remove() on the underlying Iterator, and it may
     * throw an UnsupportedOperationException if the underlying Iterator
     * does not support this method.
     * </p>
     *
     * @throws UnsupportedOperationException
     *   if the remove operator is not supported by the underlying Iterator
     * @throws IllegalStateException
     *   if the next method has not yet been called, or the remove method has
     *   already been called after the last call to the next method.
     */
    @Override
    public void remove() {
        if (lastUsedIterator == null) {
            throw new IllegalStateException("Iterator remove() cannot be called at this time");
        }
        lastUsedIterator.remove();
        lastUsedIterator = null;
    }

    /**
     * Loops around the iterators to find the next value to return.
     */
    protected void updateCurrentIterator() {
        if (hasNext) {
            return;
        }
        if (currentIterator == null) {
            if (root == null) { // NOPMD
                // do nothing, hasNext will be false
            } else {
                if (transformer == null) {
                    findNext(root);
                } else {
                    findNext(transformer.apply(root));
                }
                root = null;
            }
        } else {
            findNextByIterator(currentIterator);
        }
    }

}