/*

  * 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

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  */

 /*

  * This file is available under and governed by the GNU General Public

  * License version 2 only, as published by the Free Software Foundation.

  * However, the following notice accompanied the original version of this

  * file:

  *

  * Written by Doug Lea with assistance from members of JCP JSR-166

  * Expert Group and released to the public domain, as explained at

  * http://creativecommons.org/publicdomain/zero/1.0/

  */

 package java.util.concurrent;

 import java.util.*;

 import sun.misc.Unsafe;

 /**

  * A scalable concurrent {@link NavigableSet} implementation based on

  * a {@link ConcurrentSkipListMap}.  The elements of the set are kept

  * sorted according to their {@linkplain Comparable natural ordering},

  * or by a {@link Comparator} provided at set creation time, depending

  * on which constructor is used.

  *

  * <p>This implementation provides expected average <i>log(n)</i> time

  * cost for the <tt>contains</tt>, <tt>add</tt>, and <tt>remove</tt>

  * operations and their variants.  Insertion, removal, and access

  * operations safely execute concurrently by multiple threads.

  * Iterators are <i>weakly consistent</i>, returning elements

  * reflecting the state of the set at some point at or since the

  * creation of the iterator.  They do <em>not</em> throw {@link

  * ConcurrentModificationException}, and may proceed concurrently with

  * other operations.  Ascending ordered views and their iterators are

  * faster than descending ones.

  *

  * <p>Beware that, unlike in most collections, the <tt>size</tt>

  * method is <em>not</em> a constant-time operation. Because of the

  * asynchronous nature of these sets, determining the current number

  * of elements requires a traversal of the elements, and so may report

  * inaccurate results if this collection is modified during traversal.

  * Additionally, the bulk operations <tt>addAll</tt>,

  * <tt>removeAll</tt>, <tt>retainAll</tt>, <tt>containsAll</tt>,

  * <tt>equals</tt>, and <tt>toArray</tt> are <em>not</em> guaranteed

  * to be performed atomically. For example, an iterator operating

  * concurrently with an <tt>addAll</tt> operation might view only some

  * of the added elements.

  *

  * <p>This class and its iterators implement all of the

  * <em>optional</em> methods of the {@link Set} and {@link Iterator}

  * interfaces. Like most other concurrent collection implementations,

  * this class does not permit the use of <tt>null</tt> elements,

  * because <tt>null</tt> arguments and return values cannot be reliably

  * distinguished from the absence of elements.

  *

  * <p>This class is a member of the

  * <a href="{@docRoot}/../technotes/guides/collections/index.html">

  * Java Collections Framework</a>.

  *

  * @author Doug Lea

  * @param <E> the type of elements maintained by this set

  * @since 1.6

  */

 public class ConcurrentSkipListSet<E>

     extends AbstractSet<E>

     implements NavigableSet<E>, Cloneable, java.io.Serializable {

     private static final long serialVersionUID = -2479143111061671589L;

     /**

      * The underlying map. Uses Boolean.TRUE as value for each

      * element.  This field is declared final for the sake of thread

      * safety, which entails some ugliness in clone()

      */

     private final ConcurrentNavigableMap<E,Object> m;

     /**

      * Constructs a new, empty set that orders its elements according to

      * their {@linkplain Comparable natural ordering}.

      */

     public ConcurrentSkipListSet() {

         m = new ConcurrentSkipListMap<E,Object>();

     }

     /**

      * Constructs a new, empty set that orders its elements according to

      * the specified comparator.

      *

      * @param comparator the comparator that will be used to order this set.

      *        If <tt>null</tt>, the {@linkplain Comparable natural

      *        ordering} of the elements will be used.

      */

     public ConcurrentSkipListSet(Comparator<? super E> comparator) {

         m = new ConcurrentSkipListMap<E,Object>(comparator);

     }

     /**

      * Constructs a new set containing the elements in the specified

      * collection, that orders its elements according to their

      * {@linkplain Comparable natural ordering}.

      *

      * @param c The elements that will comprise the new set

      * @throws ClassCastException if the elements in <tt>c</tt> are

      *         not {@link Comparable}, or are not mutually comparable

      * @throws NullPointerException if the specified collection or any

      *         of its elements are null

      */

     public ConcurrentSkipListSet(Collection<? extends E> c) {

         m = new ConcurrentSkipListMap<E,Object>();

         addAll(c);

     }

     /**

      * Constructs a new set containing the same elements and using the

      * same ordering as the specified sorted set.

      *

      * @param s sorted set whose elements will comprise the new set

      * @throws NullPointerException if the specified sorted set or any

      *         of its elements are null

      */

     public ConcurrentSkipListSet(SortedSet<E> s) {

         m = new ConcurrentSkipListMap<E,Object>(s.comparator());

         addAll(s);

     }

     /**

      * For use by submaps

      */

     ConcurrentSkipListSet(ConcurrentNavigableMap<E,Object> m) {

         this.m = m;

     }

     /**

      * Returns a shallow copy of this <tt>ConcurrentSkipListSet</tt>

      * instance. (The elements themselves are not cloned.)

      *

      * @return a shallow copy of this set

      */

     public ConcurrentSkipListSet<E> clone() {

         ConcurrentSkipListSet<E> clone = null;

         try {

             clone = (ConcurrentSkipListSet<E>) super.clone();

             clone.setMap(new ConcurrentSkipListMap(m));

         } catch (CloneNotSupportedException e) {

             throw new InternalError();

         }

         return clone;

     }

     /* ---------------- Set operations -------------- */

     /**

      * Returns the number of elements in this set.  If this set

      * contains more than <tt>Integer.MAX_VALUE</tt> elements, it

      * returns <tt>Integer.MAX_VALUE</tt>.

      *

      * <p>Beware that, unlike in most collections, this method is

      * <em>NOT</em> a constant-time operation. Because of the

      * asynchronous nature of these sets, determining the current

      * number of elements requires traversing them all to count them.

      * Additionally, it is possible for the size to change during

      * execution of this method, in which case the returned result

      * will be inaccurate. Thus, this method is typically not very

      * useful in concurrent applications.

      *

      * @return the number of elements in this set

      */

     public int size() {

         return m.size();

     }

     /**

      * Returns <tt>true</tt> if this set contains no elements.

      * @return <tt>true</tt> if this set contains no elements

      */

     public boolean isEmpty() {

         return m.isEmpty();

     }

     /**

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

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

      * contains an element <tt>e</tt> such that <tt>o.equals(e)</tt>.

      *

      * @param o object to be checked for containment in this set

      * @return <tt>true</tt> if this set contains the specified element

      * @throws ClassCastException if the specified element cannot be

      *         compared with the elements currently in this set

      * @throws NullPointerException if the specified element is null

      */

     public boolean contains(Object o) {

         return m.containsKey(o);

     }

     /**

      * Adds the specified element to this set if it is not already present.

      * More formally, adds the specified element <tt>e</tt> to this set if

      * the set contains no element <tt>e2</tt> such that <tt>e.equals(e2)</tt>.

      * If this set already contains the element, the call leaves the set

      * unchanged and returns <tt>false</tt>.

      *

      * @param e element to be added to this set

      * @return <tt>true</tt> if this set did not already contain the

      *         specified element

      * @throws ClassCastException if <tt>e</tt> cannot be compared

      *         with the elements currently in this set

      * @throws NullPointerException if the specified element is null

      */

     public boolean add(E e) {

         return m.putIfAbsent(e, Boolean.TRUE) == null;

     }

     /**

      * Removes the specified element from this set if it is present.

      * More formally, removes an element <tt>e</tt> such that

      * <tt>o.equals(e)</tt>, if this set contains such an element.

      * Returns <tt>true</tt> if this set contained the element (or

      * equivalently, if this set changed as a result of the call).

      * (This set will not contain the element once the call returns.)

      *

      * @param o object to be removed from this set, if present

      * @return <tt>true</tt> if this set contained the specified element

      * @throws ClassCastException if <tt>o</tt> cannot be compared

      *         with the elements currently in this set

      * @throws NullPointerException if the specified element is null

      */

     public boolean remove(Object o) {

         return m.remove(o, Boolean.TRUE);

     }

     /**

      * Removes all of the elements from this set.

      */

     public void clear() {

         m.clear();

     }

     /**

      * Returns an iterator over the elements in this set in ascending order.

      *

      * @return an iterator over the elements in this set in ascending order

      */

     public Iterator<E> iterator() {

         return m.navigableKeySet().iterator();

     }

     /**

      * Returns an iterator over the elements in this set in descending order.

      *

      * @return an iterator over the elements in this set in descending order

      */

     public Iterator<E> descendingIterator() {

         return m.descendingKeySet().iterator();

     }

     /* ---------------- AbstractSet Overrides -------------- */

     /**

      * Compares the specified object with this set for equality.  Returns

      * <tt>true</tt> if the specified object is also a set, the two sets

      * have the same size, and every member of the specified set is

      * contained in this set (or equivalently, every member of this set is

      * contained in the specified set).  This definition ensures that the

      * equals method works properly across different implementations of the

      * set interface.

      *

      * @param o the object to be compared for equality with this set

      * @return <tt>true</tt> if the specified object is equal to this set

      */

     public boolean equals(Object o) {

         // Override AbstractSet version to avoid calling size()

         if (o == this)

             return true;

         if (!(o instanceof Set))

             return false;

         Collection<?> c = (Collection<?>) o;

         try {

             return containsAll(c) && c.containsAll(this);

         } catch (ClassCastException unused)   {

             return false;

         } catch (NullPointerException unused) {

             return false;

         }

     }

     /**

      * Removes from this set all of its elements that are contained in

      * the specified collection.  If the specified collection is also

      * a set, this operation effectively modifies this set so that its

      * value is the <i>asymmetric set difference</i> of the two sets.

      *

      * @param  c collection containing elements to be removed from this set

      * @return <tt>true</tt> if this set changed as a result of the call

      * @throws ClassCastException if the types of one or more elements in this

      *         set are incompatible with the specified collection

      * @throws NullPointerException if the specified collection or any

      *         of its elements are null

      */

     public boolean removeAll(Collection<?> c) {

         // Override AbstractSet version to avoid unnecessary call to size()

         boolean modified = false;

         for (Iterator<?> i = c.iterator(); i.hasNext(); )

             if (remove(i.next()))

                 modified = true;

         return modified;

     }

     /* ---------------- Relational operations -------------- */

     /**

      * @throws ClassCastException {@inheritDoc}

      * @throws NullPointerException if the specified element is null

      */

     public E lower(E e) {

         return m.lowerKey(e);

     }

     /**

      * @throws ClassCastException {@inheritDoc}

      * @throws NullPointerException if the specified element is null

      */

     public E floor(E e) {

         return m.floorKey(e);

     }

     /**

      * @throws ClassCastException {@inheritDoc}

      * @throws NullPointerException if the specified element is null

      */

     public E ceiling(E e) {

         return m.ceilingKey(e);

     }

     /**

      * @throws ClassCastException {@inheritDoc}

      * @throws NullPointerException if the specified element is null

      */

     public E higher(E e) {

         return m.higherKey(e);

     }

     public E pollFirst() {

         Map.Entry<E,Object> e = m.pollFirstEntry();

         return (e == null) ? null : e.getKey();

     }

     public E pollLast() {

         Map.Entry<E,Object> e = m.pollLastEntry();

         return (e == null) ? null : e.getKey();

     }

     /* ---------------- SortedSet operations -------------- */

     public Comparator<? super E> comparator() {

         return m.comparator();

     }

     /**

      * @throws NoSuchElementException {@inheritDoc}

      */

     public E first() {

         return m.firstKey();

     }

     /**

      * @throws NoSuchElementException {@inheritDoc}

      */

     public E last() {

         return m.lastKey();

     }

     /**

      * @throws ClassCastException {@inheritDoc}

      * @throws NullPointerException if {@code fromElement} or

      *         {@code toElement} is null

      * @throws IllegalArgumentException {@inheritDoc}

      */

     public NavigableSet<E> subSet(E fromElement,

                                   boolean fromInclusive,

                                   E toElement,

                                   boolean toInclusive) {

         return new ConcurrentSkipListSet<E>

             (m.subMap(fromElement, fromInclusive,

                       toElement,   toInclusive));

     }

     /**

      * @throws ClassCastException {@inheritDoc}

      * @throws NullPointerException if {@code toElement} is null

      * @throws IllegalArgumentException {@inheritDoc}

      */

     public NavigableSet<E> headSet(E toElement, boolean inclusive) {

         return new ConcurrentSkipListSet<E>(m.headMap(toElement, inclusive));

     }

     /**

      * @throws ClassCastException {@inheritDoc}

      * @throws NullPointerException if {@code fromElement} is null

      * @throws IllegalArgumentException {@inheritDoc}

      */

     public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {

         return new ConcurrentSkipListSet<E>(m.tailMap(fromElement, inclusive));

     }

     /**

      * @throws ClassCastException {@inheritDoc}

      * @throws NullPointerException if {@code fromElement} or

      *         {@code toElement} is null

      * @throws IllegalArgumentException {@inheritDoc}

      */

     public NavigableSet<E> subSet(E fromElement, E toElement) {

         return subSet(fromElement, true, toElement, false);

     }

     /**

      * @throws ClassCastException {@inheritDoc}

      * @throws NullPointerException if {@code toElement} is null

      * @throws IllegalArgumentException {@inheritDoc}

      */

     public NavigableSet<E> headSet(E toElement) {

         return headSet(toElement, false);

     }

     /**

      * @throws ClassCastException {@inheritDoc}

      * @throws NullPointerException if {@code fromElement} is null

      * @throws IllegalArgumentException {@inheritDoc}

      */

     public NavigableSet<E> tailSet(E fromElement) {

         return tailSet(fromElement, true);

     }

     /**

      * Returns a reverse order view of the elements contained in this set.

      * The descending set is backed by this set, so changes to the set are

      * reflected in the descending set, and vice-versa.

      *

      * <p>The returned set has an ordering equivalent to

      * <tt>{@link Collections#reverseOrder(Comparator) Collections.reverseOrder}(comparator())</tt>.

      * The expression {@code s.descendingSet().descendingSet()} returns a

      * view of {@code s} essentially equivalent to {@code s}.

      *

      * @return a reverse order view of this set

      */

     public NavigableSet<E> descendingSet() {

         return new ConcurrentSkipListSet(m.descendingMap());

     }

     // Support for resetting map in clone

     private void setMap(ConcurrentNavigableMap<E,Object> map) {

         UNSAFE.putObjectVolatile(this, mapOffset, map);

     }

     private static final sun.misc.Unsafe UNSAFE;

     private static final long mapOffset;

     static {

         try {

             UNSAFE = sun.misc.Unsafe.getUnsafe();

             Class k = ConcurrentSkipListSet.class;

             mapOffset = UNSAFE.objectFieldOffset

                 (k.getDeclaredField("m"));

         } catch (Exception e) {

             throw new Error(e);

         }

     }

 }