/*

 * Copyright 1997-2006 Sun Microsystems, Inc.  All Rights Reserved.

 * 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.  Sun designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,

 * CA 95054 USA or visit www.sun.com if you need additional information or

 * have any questions.

 */

package java.util;

/**

 * Linked list implementation of the <tt>List</tt> interface.  Implements all

 * optional list operations, and permits all elements (including

 * <tt>null</tt>).  In addition to implementing the <tt>List</tt> interface,

 * the <tt>LinkedList</tt> class provides uniformly named methods to

 * <tt>get</tt>, <tt>remove</tt> and <tt>insert</tt> an element at the

 * beginning and end of the list.  These operations allow linked lists to be

 * used as a stack, {@linkplain Queue queue}, or {@linkplain Deque

 * double-ended queue}. <p>

 *

 * The class implements the <tt>Deque</tt> interface, providing

 * first-in-first-out queue operations for <tt>add</tt>,

 * <tt>poll</tt>, along with other stack and deque operations.<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>

 *

 * <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 <tt>iterator</tt> and

 * <tt>listIterator</tt> 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 <tt>remove</tt> or

 * <tt>add</tt> 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 <tt>ConcurrentModificationException</tt> 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

 * @see     Vector

 * @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

{

    private transient Entry<E> header = new Entry<E>(null, null, null);

    private transient int size = 0;

    /**

     * Constructs an empty list.

     */

    public LinkedList() {

        header.next = header.previous = header;

    }

    /**

     * 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);

    }

    /**

     * Returns the first element in this list.

     *

     * @return the first element in this list

     * @throws NoSuchElementException if this list is empty

     */

    public E getFirst() {

        if (size==0)

            throw new NoSuchElementException();

        return header.next.element;

    }

    /**

     * Returns the last element in this list.

     *

     * @return the last element in this list

     * @throws NoSuchElementException if this list is empty

     */

    public E getLast()  {

        if (size==0)

            throw new NoSuchElementException();

        return header.previous.element;

    }

    /**

     * Removes and returns the first element from this list.

     *

     * @return the first element from this list

     * @throws NoSuchElementException if this list is empty

     */

    public E removeFirst() {

        return remove(header.next);

    }

    /**

     * Removes and returns the last element from this list.

     *

     * @return the last element from this list

     * @throws NoSuchElementException if this list is empty

     */

    public E removeLast() {

        return remove(header.previous);

    }

    /**

     * Inserts the specified element at the beginning of this list.

     *

     * @param e the element to add

     */

    public void addFirst(E e) {

        addBefore(e, header.next);

    }

    /**

     * Appends the specified element to the end of this list.

     *

     * <p>This method is equivalent to {@link #add}.

     *

     * @param e the element to add

     */

    public void addLast(E e) {

        addBefore(e, header);

    }

    /**

     * Returns <tt>true</tt> if this list contains the specified element.

     * More formally, returns <tt>true</tt> if and only if this list contains

     * at least one element <tt>e</tt> 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 <tt>true</tt> if this list contains the specified element

     */

    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

     */

    public int size() {

        return 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 <tt>true</tt> (as specified by {@link Collection#add})

     */

    public boolean add(E e) {

        addBefore(e, header);

        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

     * <tt>i</tt> such that

     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>

     * (if such an element exists).  Returns <tt>true</tt> 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 <tt>true</tt> if this list contained the specified element

     */

    public boolean remove(Object o) {

        if (o==null) {

            for (Entry<E> e = header.next; e != header; e = e.next) {

                if (e.element==null) {

                    remove(e);

                    return true;

                }

            }

        } else {

            for (Entry<E> e = header.next; e != header; e = e.next) {

                if (o.equals(e.element)) {

                    remove(e);

                    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 <tt>true</tt> if this list changed as a result of the call

     * @throws NullPointerException if the specified collection is null

     */

    public boolean addAll(Collection<? extends E> c) {

        return addAll(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 <tt>true</tt> if this list changed as a result of the call

     * @throws IndexOutOfBoundsException {@inheritDoc}

     * @throws NullPointerException if the specified collection is null

     */

    public boolean addAll(int index, Collection<? extends E> c) {

        if (index < 0 || index > size)

            throw new IndexOutOfBoundsException("Index: "+index+

                                                ", Size: "+size);

        Object[] a = c.toArray();

        int numNew = a.length;

        if (numNew==0)

            return false;

        modCount++;

        Entry<E> successor = (index==size ? header : entry(index));

        Entry<E> predecessor = successor.previous;

        for (int i=0; i<numNew; i++) {

            Entry<E> e = new Entry<E>((E)a[i], successor, predecessor);

            predecessor.next = e;

            predecessor = e;

        }

        successor.previous = predecessor;

        size += numNew;

        return true;

    }

    /**

     * Removes all of the elements from this list.

     */

    public void clear() {

        Entry<E> e = header.next;

        while (e != header) {

            Entry<E> next = e.next;

            e.next = e.previous = null;

            e.element = null;

            e = next;

        }

        header.next = header.previous = header;

        size = 0;

        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}

     */

    public E get(int index) {

        return entry(index).element;

    }

    /**

     * 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}

     */

    public E set(int index, E element) {

        Entry<E> e = entry(index);

        E oldVal = e.element;

        e.element = 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}

     */

    public void add(int index, E element) {

        addBefore(element, (index==size ? header : entry(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}

     */

    public E remove(int index) {

        return remove(entry(index));

    }

    /**

     * Returns the indexed entry.

     */

    private Entry<E> entry(int index) {

        if (index < 0 || index >= size)

            throw new IndexOutOfBoundsException("Index: "+index+

                                                ", Size: "+size);

        Entry<E> e = header;

        if (index < (size >> 1)) {

            for (int i = 0; i <= index; i++)

                e = e.next;

        } else {

            for (int i = size; i > index; i--)

                e = e.previous;

        }

        return e;

    }

    // 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 <tt>i</tt> 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

     */

    public int indexOf(Object o) {

        int index = 0;

        if (o==null) {

            for (Entry e = header.next; e != header; e = e.next) {

                if (e.element==null)

                    return index;

                index++;

            }

        } else {

            for (Entry e = header.next; e != header; e = e.next) {

                if (o.equals(e.element))

                    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 <tt>i</tt> 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

     */

    public int lastIndexOf(Object o) {

        int index = size;

        if (o==null) {

            for (Entry e = header.previous; e != header; e = e.previous) {

                index--;

                if (e.element==null)

                    return index;

            }

        } else {

            for (Entry e = header.previous; e != header; e = e.previous) {

                index--;

                if (o.equals(e.element))

                    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 <tt>null</tt> if this list is empty

     * @since 1.5

     */

    public E peek() {

        if (size==0)

            return null;

        return getFirst();

    }

    /**

     * 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

     */

    public E element() {

        return getFirst();

    }

    /**

     * Retrieves and removes the head (first element) of this list

     * @return the head of this list, or <tt>null</tt> if this list is empty

     * @since 1.5

     */

    public E poll() {

        if (size==0)

            return null;

        return removeFirst();

    }

    /**

     * 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

     */

    public E remove() {

        return removeFirst();

    }

    /**

     * Adds the specified element as the tail (last element) of this list.

     *

     * @param e the element to add

     * @return <tt>true</tt> (as specified by {@link Queue#offer})

     * @since 1.5

     */

    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 <tt>true</tt> (as specified by {@link Deque#offerFirst})

     * @since 1.6

     */

    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