/*

 * 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

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

 *

 * 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.

 *

 */

package java.util;

/**

 * Linked list implementation of the {@code List} interface.  Implements all

 * optional list operations, and permits all elements (including

 * {@code null}).  In addition to implementing the {@code List} interface,

 * the {@code LinkedList} class provides uniformly named methods to

 * {@code get}, {@code remove} and {@code insert} 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 {@code Deque} interface, providing

 * first-in-first-out queue operations for {@code add},

 * {@code poll}, 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><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)

     */

    transient Node<E> first;

    /**

     * Pointer to last node.

     * Invariant: (first == null && last == null) ||

     *            (last.next == null && last.item != null)

     */

    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 = first;

        final Node<E> newNode = new Node<E>(null, e, f);

        first = newNode;

        if (f == null)

            last = newNode;

        else

            f.prev = newNode;

        size++;

        modCount++;

    }

    /**

     * Links e as last element.

     */

    void linkLast(E e) {

        final Node<E> l = last;

        final Node<E> newNode = new Node<E>(l, e, null);

        last = newNode;

        if (l == null)

            first = newNode;

        else

            l.next = newNode;

        size++;

        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<E>(pred, e, succ);

        succ.prev = newNode;

        if (pred == null)

            first = newNode;

        else

            pred.next = newNode;

        size++;

        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

        first = next;

        if (next == null)

            last = null;

        else

            next.prev = null;

        size--;

        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

        last = prev;

        if (prev == null)

            first = null;

        else

            prev.next = null;

        size--;

        modCount++;

        return element;

    }

    /**

     * Unlinks non-null node x.

     */

    E unlink(Node<E> x) {

        // assert x != null;

        final E element = x.item;

        final Node<E> next = x.next;

        final Node<E> prev = x.prev;

        if (prev == null) {

            first = next;

        } else {

            prev.next = next;

            x.prev = null;

        }

        if (next == null) {

            last = prev;

        } else {

            next.prev = prev;

            x.next = null;

        }

        x.item = null;

        size--;

        modCount++;

        return element;

    }

    /**

     * Returns the first element in this list.

     *

     * @return the first element in this list

     * @throws NoSuchElementException if this list is empty

     */

    public E getFirst() {

        final Node<E> f = 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

     */

    public E getLast()  {

        final Node<E> l = 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

     */

    public E removeFirst() {

        final Node<E> f = 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

     */

    public E removeLast() {

        final Node<E> l = 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

     */

    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

     */

    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

     */

    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 {@code true} (as specified by {@link Collection#add})

     */

    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

     */

    public boolean remove(Object o) {

        if (o == null) {

            for (Node<E> x = first; x != null; x = x.next) {

                if (x.item == null) {

                    unlink(x);

                    return true;

                }

            }

        } else {

            for (Node<E> x = 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

     */

    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 {@code true} 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) {

        checkPositionIndex(index);

        Object[] a = c.toArray();

        int numNew = a.length;

        if (numNew == 0)

            return false;

        Node<E> pred, succ;

        if (index == size) {

            succ = null;

            pred = last;

        } else {

            succ = node(index);

            pred = succ.prev;

        }

        for (Object o : a) {

            @SuppressWarnings("unchecked") E e = (E) o;

            Node<E> newNode = new Node<E>(pred, e, null);

            if (pred == null)

                first = newNode;

            else

                pred.next = newNode;

            pred = newNode;

        }

        if (succ == null) {

            last = pred;

        } else {

            pred.next = succ;

            succ.prev = pred;

        }

        size += numNew;

        modCount++;

        return true;

    }

    /**

     * Removes all of the elements from this list.

     * The list will be empty after this call returns.

     */

    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 = first; x != null; ) {

            Node<E> next = x.next;

            x.item = null;

            x.next = null;

            x.prev = null;

            x = next;

        }

        first = last = null;

        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) {

        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}

     */

    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).