/*
 * Copyright (c) 1997, 2011, Oracle and/or its affiliates. All rights reserved.
 * Copyright (C) 2014-2020 MicroEJ Corp. - EDC compliance and optimizations.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code 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
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package java.util;

import ej.annotation.Nullable;

/**
 * Doubly-linked list implementation of the {@code List} and {@code Deque} interfaces. Implements all optional list
 * operations, and permits all elements (including {@code null}).
 *
 * <p>
 * All of the operations perform as could be expected for a doubly-linked list. Operations that index into the list will
 * traverse the list from the beginning or the end, whichever is closer to the specified index.
 *
 * <p>
 * <strong>Note that this implementation is not synchronized.</strong> If multiple threads access a linked list
 * concurrently, and at least one of the threads modifies the list structurally, it <i>must</i> be synchronized
 * externally. (A structural modification is any operation that adds or deletes one or more elements; merely setting the
 * value of an element is not a structural modification.) This is typically accomplished by synchronizing on some object
 * that naturally encapsulates the list.
 *
 * If no such object exists, the list should be "wrapped" using the {@link Collections#synchronizedList
 * Collections.synchronizedList} method. This is best done at creation time, to prevent accidental unsynchronized access
 * to the list:
 *
 * <pre>
 *   List list = Collections.synchronizedList(new LinkedList(...));
 * </pre>
 *
 * <p>
 * The iterators returned by this class's {@code iterator} and {@code listIterator} methods are <i>fail-fast</i>: if the
 * list is structurally modified at any time after the iterator is created, in any way except through the Iterator's own
 * {@code remove} or {@code add} methods, the iterator will throw a {@link ConcurrentModificationException}. Thus, in
 * the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary,
 * non-deterministic behavior at an undetermined time in the future.
 *
 * <p>
 * Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make
 * any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw
 * {@code ConcurrentModificationException} on a best-effort basis. Therefore, it would be wrong to write a program that
 * depended on this exception for its correctness: <i>the fail-fast behavior of iterators should be used only to detect
 * bugs.</i>
 *
 * <p>
 * This class is a member of the <a href="{@docRoot}/../technotes/guides/collections/index.html"> Java Collections
 * Framework</a>.
 *
 * @author Josh Bloch
 * @see List
 * @see ArrayList
 * @since 1.2
 * @param <E>
 *            the type of elements held in this collection
 */

public class LinkedList<E> extends AbstractSequentialList<E>
		implements List<E>, Deque<E>, Cloneable, java.io.Serializable {
	transient int size = 0;

	/**
	 * Pointer to first node. Invariant: (first == null && last == null) || (first.prev == null && first.item != null)
	 */
	@Nullable
	transient Node<E> first;

	/**
	 * Pointer to last node. Invariant: (first == null && last == null) || (last.next == null && last.item != null)
	 */
	@Nullable
	transient Node<E> last;

	/**
	 * Constructs an empty list.
	 */
	public LinkedList() {
	}

	/**
	 * Constructs a list containing the elements of the specified collection, in the order they are returned by the
	 * collection's iterator.
	 *
	 * @param c
	 *            the collection whose elements are to be placed into this list
	 * @throws NullPointerException
	 *             if the specified collection is null
	 */
	public LinkedList(Collection<? extends E> c) {
		this();
		addAll(c);
	}

	/**
	 * Links e as first element.
	 */
	private void linkFirst(E e) {
		final Node<E> f = this.first;
		final Node<E> newNode = new Node<>(null, e, f);
		this.first = newNode;
		if (f == null) {
			this.last = newNode;
		} else {
			f.prev = newNode;
		}
		this.size++;
		this.modCount++;
	}

	/**
	 * Links e as last element.
	 */
	void linkLast(E e) {
		final Node<E> l = this.last;
		final Node<E> newNode = new Node<>(l, e, null);
		this.last = newNode;
		if (l == null) {
			this.first = newNode;
		} else {
			l.next = newNode;
		}
		this.size++;
		this.modCount++;
	}

	/**
	 * Inserts element e before non-null Node succ.
	 */
	void linkBefore(E e, Node<E> succ) {
		// assert succ != null;
		final Node<E> pred = succ.prev;
		final Node<E> newNode = new Node<>(pred, e, succ);
		succ.prev = newNode;
		if (pred == null) {
			this.first = newNode;
		} else {
			pred.next = newNode;
		}
		this.size++;
		this.modCount++;
	}

	/**
	 * Unlinks non-null first node f.
	 */
	private E unlinkFirst(Node<E> f) {
		// assert f == first && f != null;
		final E element = f.item;
		final Node<E> next = f.next;
		f.item = null;
		f.next = null; // help GC
		this.first = next;
		if (next == null) {
			this.last = null;
		} else {
			next.prev = null;
		}
		this.size--;
		this.modCount++;
		return element;
	}

	/**
	 * Unlinks non-null last node l.
	 */
	private E unlinkLast(Node<E> l) {
		// assert l == last && l != null;
		final E element = l.item;
		final Node<E> prev = l.prev;
		l.item = null;
		l.prev = null; // help GC
		this.last = prev;
		if (prev == null) {
			this.first = null;
		} else {
			prev.next = null;
		}
		this.size--;
		this.modCount++;
		return element;
	}

	/**
	 * Unlinks non-null node x.
	 */
	E unlink(Node<E> x) {
		final E element = x.item;
		final Node<E> next = x.next;
		final Node<E> prev = x.prev;

		if (prev == null) {
			this.first = next;
		} else {
			prev.next = next;
			x.prev = null;
		}

		if (next == null) {
			this.last = prev;
		} else {
			next.prev = prev;
			x.next = null;
		}

		x.item = null;
		this.size--;
		this.modCount++;
		return element;
	}

	/**
	 * Returns the first element in this list.
	 *
	 * @return the first element in this list
	 * @throws NoSuchElementException
	 *             if this list is empty
	 */
	@Override
	public E getFirst() {
		final Node<E> f = this.first;
		if (f == null) {
			throw new NoSuchElementException();
		}
		return f.item;
	}

	/**
	 * Returns the last element in this list.
	 *
	 * @return the last element in this list
	 * @throws NoSuchElementException
	 *             if this list is empty
	 */
	@Override
	public E getLast() {
		final Node<E> l = this.last;
		if (l == null) {
			throw new NoSuchElementException();
		}
		return l.item;
	}

	/**
	 * Removes and returns the first element from this list.
	 *
	 * @return the first element from this list
	 * @throws NoSuchElementException
	 *             if this list is empty
	 */
	@Override
	public E removeFirst() {
		final Node<E> f = this.first;
		if (f == null) {
			throw new NoSuchElementException();
		}
		return unlinkFirst(f);
	}

	/**
	 * Removes and returns the last element from this list.
	 *
	 * @return the last element from this list
	 * @throws NoSuchElementException
	 *             if this list is empty
	 */
	@Override
	public E removeLast() {
		final Node<E> l = this.last;
		if (l == null) {
			throw new NoSuchElementException();
		}
		return unlinkLast(l);
	}

	/**
	 * Inserts the specified element at the beginning of this list.
	 *
	 * @param e
	 *            the element to add
	 */
	@Override
	public void addFirst(E e) {
		linkFirst(e);
	}

	/**
	 * Appends the specified element to the end of this list.
	 *
	 * <p>
	 * This method is equivalent to {@link #add}.
	 *
	 * @param e
	 *            the element to add
	 */
	@Override
	public void addLast(E e) {
		linkLast(e);
	}

	/**
	 * Returns {@code true} if this list contains the specified element. More formally, returns {@code true} if and only
	 * if this list contains at least one element {@code e} such that
	 * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
	 *
	 * @param o
	 *            element whose presence in this list is to be tested
	 * @return {@code true} if this list contains the specified element
	 */
	@Override
	public boolean contains(Object o) {
		return indexOf(o) != -1;
	}

	/**
	 * Returns the number of elements in this list.
	 *
	 * @return the number of elements in this list
	 */
	@Override
	public int size() {
		return this.size;
	}

	/**
	 * Appends the specified element to the end of this list.
	 *
	 * <p>
	 * This method is equivalent to {@link #addLast}.
	 *
	 * @param e
	 *            element to be appended to this list
	 * @return {@code true} (as specified by {@link Collection#add})
	 */
	@Override
	public boolean add(E e) {
		linkLast(e);
		return true;
	}

	/**
	 * Removes the first occurrence of the specified element from this list, if it is present. If this list does not
	 * contain the element, it is unchanged. More formally, removes the element with the lowest index {@code i} such
	 * that <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt> (if such an element exists).
	 * Returns {@code true} if this list contained the specified element (or equivalently, if this list changed as a
	 * result of the call).
	 *
	 * @param o
	 *            element to be removed from this list, if present
	 * @return {@code true} if this list contained the specified element
	 */
	@Override
	public boolean remove(Object o) {
		if (o == null) {
			for (Node<E> x = this.first; x != null; x = x.next) {
				if (x.item == null) {
					unlink(x);
					return true;
				}
			}
		} else {
			for (Node<E> x = this.first; x != null; x = x.next) {
				if (o.equals(x.item)) {
					unlink(x);
					return true;
				}
			}
		}
		return false;
	}

	/**
	 * Appends all of the elements in the specified collection to the end of this list, in the order that they are
	 * returned by the specified collection's iterator. The behavior of this operation is undefined if the specified
	 * collection is modified while the operation is in progress. (Note that this will occur if the specified collection
	 * is this list, and it's nonempty.)
	 *
	 * @param c
	 *            collection containing elements to be added to this list
	 * @return {@code true} if this list changed as a result of the call
	 * @throws NullPointerException
	 *             if the specified collection is null
	 */
	@Override
	public boolean addAll(Collection<? extends E> c) {
		return addAll(this.size, c);
	}

	/**
	 * Inserts all of the elements in the specified collection into this list, starting at the specified position.
	 * Shifts the element currently at that position (if any) and any subsequent elements to the right (increases their
	 * indices). The new elements will appear in the list in the order that they are returned by the specified
	 * collection's iterator.
	 *
	 * @param index
	 *            index at which to insert the first element from the specified collection
	 * @param c
	 *            collection containing elements to be added to this list
	 * @return {@code true} if this list changed as a result of the call
	 * @throws IndexOutOfBoundsException
	 *             {@inheritDoc}
	 * @throws NullPointerException
	 *             if the specified collection is null
	 */
	@Override
	public boolean addAll(int index, Collection<? extends E> c) {
		checkPositionIndex(index);

		Object[] a = c.toArray();
		int numNew = a.length;
		if (numNew == 0) {
			return false;
		}

		Node<E> pred, succ;
		if (index == this.size) {
			succ = null;
			pred = this.last;
		} else {
			succ = node(index);
			pred = succ.prev;
		}

		for (Object o : a) {
			@SuppressWarnings("unchecked")
			E e = (E) o;
			Node<E> newNode = new Node<>(pred, e, null);
			if (pred == null) {
				this.first = newNode;
			} else {
				pred.next = newNode;
			}
			pred = newNode;
		}

		if (succ == null) {
			this.last = pred;
		} else {
			pred.next = succ;
			succ.prev = pred;
		}

		this.size += numNew;
		this.modCount++;
		return true;
	}

	/**
	 * Removes all of the elements from this list. The list will be empty after this call returns.
	 */
	@Override
	public void clear() {
		// Clearing all of the links between nodes is "unnecessary", but:
		// - helps a generational GC if the discarded nodes inhabit
		// more than one generation
		// - is sure to free memory even if there is a reachable Iterator
		for (Node<E> x = this.first; x != null;) {
			Node<E> next = x.next;
			x.item = null;
			x.next = null;
			x.prev = null;
			x = next;
		}
		this.first = this.last = null;
		this.size = 0;
		this.modCount++;
	}

	// Positional Access Operations

	/**
	 * Returns the element at the specified position in this list.
	 *
	 * @param index
	 *            index of the element to return
	 * @return the element at the specified position in this list
	 * @throws IndexOutOfBoundsException
	 *             {@inheritDoc}
	 */
	@Override
	public E get(int index) {
		checkElementIndex(index);
		return node(index).item;
	}

	/**
	 * Replaces the element at the specified position in this list with the specified element.
	 *
	 * @param index
	 *            index of the element to replace
	 * @param element
	 *            element to be stored at the specified position
	 * @return the element previously at the specified position
	 * @throws IndexOutOfBoundsException
	 *             {@inheritDoc}
	 */
	@Override
	public E set(int index, E element) {
		checkElementIndex(index);
		Node<E> x = node(index);
		E oldVal = x.item;
		x.item = element;
		return oldVal;
	}

	/**
	 * Inserts the specified element at the specified position in this list. Shifts the element currently at that
	 * position (if any) and any subsequent elements to the right (adds one to their indices).
	 *
	 * @param index
	 *            index at which the specified element is to be inserted
	 * @param element
	 *            element to be inserted
	 * @throws IndexOutOfBoundsException
	 *             {@inheritDoc}
	 */
	@Override
	public void add(int index, E element) {
		checkPositionIndex(index);

		if (index == this.size) {
			linkLast(element);
		} else {
			linkBefore(element, node(index));
		}
	}

	/**
	 * Removes the element at the specified position in this list. Shifts any subsequent elements to the left (subtracts
	 * one from their indices). Returns the element that was removed from the list.
	 *
	 * @param index
	 *            the index of the element to be removed
	 * @return the element previously at the specified position
	 * @throws IndexOutOfBoundsException
	 *             {@inheritDoc}
	 */
	@Override
	public E remove(int index) {
		checkElementIndex(index);
		return unlink(node(index));
	}

	/**
	 * Tells if the argument is the index of an existing element.
	 */
	private boolean isElementIndex(int index) {
		return index >= 0 && index < this.size;
	}

	/**
	 * Tells if the argument is the index of a valid position for an iterator or an add operation.
	 */
	private boolean isPositionIndex(int index) {
		return index >= 0 && index <= this.size;
	}

	/**
	 * Constructs an IndexOutOfBoundsException detail message. Of the many possible refactorings of the error handling
	 * code, this "outlining" performs best with both server and client VMs.
	 */
	private String outOfBoundsMsg(int index) {
		return "Index: " + index + ", Size: " + this.size;
	}

	private void checkElementIndex(int index) {
		if (!isElementIndex(index)) {
			throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
		}
	}

	private void checkPositionIndex(int index) {
		if (!isPositionIndex(index)) {
			throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
		}
	}

	/**
	 * Returns the (non-null) Node at the specified element index.
	 */
	Node<E> node(int index) {
		// assert isElementIndex(index);

		if (index < (this.size >> 1)) {
			Node<E> x = this.first;
			for (int i = 0; i < index; i++) {
				x = x.next;
			}
			return x;
		} else {
			Node<E> x = this.last;
			for (int i = this.size - 1; i > index; i--) {
				x = x.prev;
			}
			return x;
		}
	}

	// Search Operations

	/**
	 * Returns the index of the first occurrence of the specified element in this list, or -1 if this list does not
	 * contain the element. More formally, returns the lowest index {@code i} such that
	 * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>, or -1 if there is no such index.
	 *
	 * @param o
	 *            element to search for
	 * @return the index of the first occurrence of the specified element in this list, or -1 if this list does not
	 *         contain the element
	 */
	@Override
	public int indexOf(Object o) {
		int index = 0;
		if (o == null) {
			for (Node<E> x = this.first; x != null; x = x.next) {
				if (x.item == null) {
					return index;
				}
				index++;
			}
		} else {
			for (Node<E> x = this.first; x != null; x = x.next) {
				if (o.equals(x.item)) {
					return index;
				}
				index++;
			}
		}
		return -1;
	}

	/**
	 * Returns the index of the last occurrence of the specified element in this list, or -1 if this list does not
	 * contain the element. More formally, returns the highest index {@code i} such that
	 * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>, or -1 if there is no such index.
	 *
	 * @param o
	 *            element to search for
	 * @return the index of the last occurrence of the specified element in this list, or -1 if this list does not
	 *         contain the element
	 */
	@Override
	public int lastIndexOf(Object o) {
		int index = this.size;
		if (o == null) {
			for (Node<E> x = this.last; x != null; x = x.prev) {
				index--;
				if (x.item == null) {
					return index;
				}
			}
		} else {
			for (Node<E> x = this.last; x != null; x = x.prev) {
				index--;
				if (o.equals(x.item)) {
					return index;
				}
			}
		}
		return -1;
	}

	// Queue operations.

	/**
	 * Retrieves, but does not remove, the head (first element) of this list.
	 *
	 * @return the head of this list, or {@code null} if this list is empty
	 * @since 1.5
	 */
	@Override
	@Nullable
	public E peek() {
		final Node<E> f = this.first;
		return (f == null) ? null : f.item;
	}

	/**
	 * Retrieves, but does not remove, the head (first element) of this list.
	 *
	 * @return the head of this list
	 * @throws NoSuchElementException
	 *             if this list is empty
	 * @since 1.5
	 */
	@Override
	public E element() {
		return getFirst();
	}

	/**
	 * Retrieves and removes the head (first element) of this list.
	 *
	 * @return the head of this list, or {@code null} if this list is empty
	 * @since 1.5
	 */
	@Override
	@Nullable
	public E poll() {
		final Node<E> f = this.first;
		return (f == null) ? null : unlinkFirst(f);
	}

	/**
	 * Retrieves and removes the head (first element) of this list.
	 *
	 * @return the head of this list
	 * @throws NoSuchElementException
	 *             if this list is empty
	 * @since 1.5
	 */
	@Override
	public E remove() {
		return removeFirst();
	}

	/**
	 * Adds the specified element as the tail (last element) of this list.
	 *
	 * @param e
	 *            the element to add
	 * @return {@code true} (as specified by {@link Queue#offer})
	 * @since 1.5
	 */
	@Override
	public boolean offer(E e) {
		return add(e);
	}

	// Deque operations
	/**
	 * Inserts the specified element at the front of this list.
	 *
	 * @param e
	 *            the element to insert
	 * @return {@code true} (as specified by {@link Deque#offerFirst})
	 * @since 1.6
	 */
	@Override
	public boolean offerFirst(E e) {
		addFirst(e);
		return true;
	}

	/**
	 * Inserts the specified element at the end of this list.
	 *
	 * @param e
	 *            the element to insert
	 * @return {@code true} (as specified by {@link Deque#offerLast})
	 * @since 1.6
	 */
	@Override
	public boolean offerLast(E e) {
		addLast(e);
		return true;
	}

	/**
	 * Retrieves, but does not remove, the first element of this list, or returns {@code null} if this list is empty.
	 *
	 * @return the first element of this list, or {@code null} if this list is empty
	 * @since 1.6
	 */
	@Override
	@Nullable
	public E peekFirst() {
		final Node<E> f = this.first;
		return (f == null) ? null : f.item;
	}

	/**
	 * Retrieves, but does not remove, the last element of this list, or returns {@code null} if this list is empty.
	 *
	 * @return the last element of this list, or {@code null} if this list is empty
	 * @since 1.6
	 */
	@Override
	@Nullable
	public E peekLast() {
		final Node<E> l = this.last;
		return (l == null) ? null : l.item;
	}

	/**
	 * Retrieves and removes the first element of this list, or returns {@code null} if this list is empty.
	 *
	 * @return the first element of this list, or {@code null} if this list is empty
	 * @since 1.6
	 */
	@Override
	@Nullable
	public E pollFirst() {
		final Node<E> f = this.first;
		return (f == null) ? null : unlinkFirst(f);
	}

	/**
	 * Retrieves and removes the last element of this list, or returns {@code null} if this list is empty.
	 *
	 * @return the last element of this list, or {@code null} if this list is empty
	 * @since 1.6
	 */
	@Override
	@Nullable
	public E pollLast() {
		final Node<E> l = this.last;
		return (l == null) ? null : unlinkLast(l);
	}

	/**
	 * Pushes an element onto the stack represented by this list. In other words, inserts the element at the front of
	 * this list.
	 *
	 * <p>
	 * This method is equivalent to {@link #addFirst}.
	 *
	 * @param e
	 *            the element to push
	 * @since 1.6
	 */
	@Override
	public void push(E e) {
		addFirst(e);
	}

	/**
	 * Pops an element from the stack represented by this list. In other words, removes and returns the first element of
	 * this list.
	 *
	 * <p>
	 * This method is equivalent to {@link #removeFirst()}.
	 *
	 * @return the element at the front of this list (which is the top of the stack represented by this list)
	 * @throws NoSuchElementException
	 *             if this list is empty
	 * @since 1.6
	 */
	@Override
	public E pop() {
		return removeFirst();
	}

	/**
	 * Removes the first occurrence of the specified element in this list (when traversing the list from head to tail).
	 * If the list does not contain the element, it is unchanged.
	 *
	 * @param o
	 *            element to be removed from this list, if present
	 * @return {@code true} if the list contained the specified element
	 * @since 1.6
	 */
	@Override
	public boolean removeFirstOccurrence(Object o) {
		return remove(o);
	}

	/**
	 * Removes the last occurrence of the specified element in this list (when traversing the list from head to tail).
	 * If the list does not contain the element, it is unchanged.
	 *
	 * @param o
	 *            element to be removed from this list, if present
	 * @return {@code true} if the list contained the specified element
	 * @since 1.6
	 */
	@Override
	public boolean removeLastOccurrence(Object o) {
		if (o == null) {
			for (Node<E> x = this.last; x != null; x = x.prev) {
				if (x.item == null) {
					unlink(x);
					return true;
				}
			}
		} else {
			for (Node<E> x = this.last; x != null; x = x.prev) {
				if (o.equals(x.item)) {
					unlink(x);
					return true;
				}
			}
		}
		return false;
	}

	/**
	 * Returns a list-iterator of the elements in this list (in proper sequence), starting at the specified position in
	 * the list. Obeys the general contract of {@code List.listIterator(int)}.
	 * <p>
	 *
	 * The list-iterator is <i>fail-fast</i>: if the list is structurally modified at any time after the Iterator is
	 * created, in any way except through the list-iterator's own {@code remove} or {@code add} methods, the
	 * list-iterator will throw a {@code ConcurrentModificationException}. Thus, in the face of concurrent modification,
	 * the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an
	 * undetermined time in the future.
	 *
	 * @param index
	 *            index of the first element to be returned from the list-iterator (by a call to {@code next})
	 * @return a ListIterator of the elements in this list (in proper sequence), starting at the specified position in
	 *         the list
	 * @throws IndexOutOfBoundsException
	 *             {@inheritDoc}
	 * @see List#listIterator(int)
	 */
	@Override
	public ListIterator<E> listIterator(int index) {
		checkPositionIndex(index);
		return new ListItr(index);
	}

	private class ListItr implements ListIterator<E> {
		@Nullable
		private Node<E> lastReturned = null;
		@Nullable
		private Node<E> next;
		private int nextIndex;
		private int expectedModCount = LinkedList.this.modCount;

		ListItr(int index) {
			// assert isPositionIndex(index);
			this.next = (index == LinkedList.this.size) ? null : node(index);
			this.nextIndex = index;
		}

		@Override
		public boolean hasNext() {
			return this.nextIndex < LinkedList.this.size;
		}

		@Override
		public E next() {
			checkForComodification();
			if (!hasNext()) {
				throw new NoSuchElementException();
			}

			this.lastReturned = this.next;
			this.next = this.next.next;
			this.nextIndex++;
			return this.lastReturned.item;
		}

		@Override
		public boolean hasPrevious() {
			return this.nextIndex > 0;
		}

		@Override
		public E previous() {
			checkForComodification();
			if (!hasPrevious()) {
				throw new NoSuchElementException();
			}

			this.lastReturned = this.next = (this.next == null) ? LinkedList.this.last : this.next.prev;
			this.nextIndex--;
			return this.lastReturned.item;
		}

		@Override
		public int nextIndex() {
			return this.nextIndex;
		}

		@Override
		public int previousIndex() {
			return this.nextIndex - 1;
		}

		@Override
		public void remove() {
			checkForComodification();
			if (this.lastReturned == null) {
				throw new IllegalStateException();
			}

			Node<E> lastNext = this.lastReturned.next;
			unlink(this.lastReturned);
			if (this.next == this.lastReturned) {
				this.next = lastNext;
			} else {
				this.nextIndex--;
			}
			this.lastReturned = null;
			this.expectedModCount++;
		}

		@Override
		public void set(E e) {
			if (this.lastReturned == null) {
				throw new IllegalStateException();
			}
			checkForComodification();
			this.lastReturned.item = e;
		}

		@Override
		public void add(E e) {
			checkForComodification();
			this.lastReturned = null;
			if (this.next == null) {
				linkLast(e);
			} else {
				linkBefore(e, this.next);
			}
			this.nextIndex++;
			this.expectedModCount++;
		}

		final void checkForComodification() {
			if (LinkedList.this.modCount != this.expectedModCount) {
				throw new ConcurrentModificationException();
			}
		}
	}

	private static class Node<E> {
		@Nullable
		E item;
		@Nullable
		Node<E> next;
		@Nullable
		Node<E> prev;

		Node(@Nullable Node<E> prev, E element, @Nullable Node<E> next) {
			this.item = element;
			this.next = next;
			this.prev = prev;
		}
	}

	/**
	 * @since 1.6
	 */
	@Override
	public Iterator<E> descendingIterator() {
		return new DescendingIterator();
	}

	/**
	 * Adapter to provide descending iterators via ListItr.previous
	 */
	private class DescendingIterator implements Iterator<E> {
		private final ListItr itr = new ListItr(size());

		@Override
		public boolean hasNext() {
			return this.itr.hasPrevious();
		}

		@Override
		public E next() {
			return this.itr.previous();
		}

		@Override
		public void remove() {
			this.itr.remove();
		}
	}

	@SuppressWarnings("unchecked")
	private LinkedList<E> superClone() {
		try {
			return (LinkedList<E>) super.clone();
		} catch (CloneNotSupportedException e) {
			throw new InternalError();
		}
	}

	/**
	 * Returns a shallow copy of this {@code LinkedList}. (The elements themselves are not cloned.)
	 *
	 * @return a shallow copy of this {@code LinkedList} instance
	 */
	@Override
	public Object clone() {
		LinkedList<E> clone = superClone();

		// Put clone into "virgin" state
		clone.first = clone.last = null;
		clone.size = 0;
		clone.modCount = 0;

		// Initialize clone with our elements
		for (Node<E> x = this.first; x != null; x = x.next) {
			clone.add(x.item);
		}

		return clone;
	}

	/**
	 * Returns an array containing all of the elements in this list in proper sequence (from first to last element).
	 *
	 * <p>
	 * The returned array will be "safe" in that no references to it are maintained by this list. (In other words, this
	 * method must allocate a new array). The caller is thus free to modify the returned array.
	 *
	 * <p>
	 * This method acts as bridge between array-based and collection-based APIs.
	 *
	 * @return an array containing all of the elements in this list in proper sequence
	 */
	@Override
	public Object[] toArray() {
		Object[] result = new Object[this.size];
		int i = 0;
		for (Node<E> x = this.first; x != null; x = x.next) {
			result[i++] = x.item;
		}
		return result;
	}

	/**
	 * Returns an array containing all of the elements in this list in proper sequence (from first to last element); the
	 * runtime type of the returned array is that of the specified array. If the list fits in the specified array, it is
	 * returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size
	 * of this list.
	 *
	 * <p>
	 * If the list fits in the specified array with room to spare (i.e., the array has more elements than the list), the
	 * element in the array immediately following the end of the list is set to {@code null}. (This is useful in
	 * determining the length of the list <i>only</i> if the caller knows that the list does not contain any null
	 * elements.)
	 *
	 * <p>
	 * Like the {@link #toArray()} method, this method acts as bridge between array-based and collection-based APIs.
	 * Further, this method allows precise control over the runtime type of the output array, and may, under certain
	 * circumstances, be used to save allocation costs.
	 *
	 * <p>
	 * Suppose {@code x} is a list known to contain only strings. The following code can be used to dump the list into a
	 * newly allocated array of {@code String}:
	 *
	 * <pre>
	 * String[] y = x.toArray(new String[0]);
	 * </pre>
	 *
	 * Note that {@code toArray(new Object[0])} is identical in function to {@code toArray()}.
	 *
	 * @param a
	 *            the array into which the elements of the list are to be stored, if it is big enough; otherwise, a new
	 *            array of the same runtime type is allocated for this purpose.
	 * @return an array containing the elements of the list
	 * @throws ArrayStoreException
	 *             if the runtime type of the specified array is not a supertype of the runtime type of every element in
	 *             this list
	 * @throws NullPointerException
	 *             if the specified array is null
	 */
	@Override
	@SuppressWarnings("unchecked")
	public <T> T[] toArray(T[] a) {
		if (a.length < this.size) {
			return super.toArray(a);
		} else {
			int i = 0;
			Object[] result = a;
			for (Node<E> x = this.first; x != null; x = x.next) {
				result[i++] = x.item;
			}

			if (a.length > this.size) {
				a[this.size] = null;
			}

			return a;
		}
	}

	private static final long serialVersionUID = 876323262645176354L;

}