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	/*
	* 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.
	*/

	/*
	* 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.AbstractSet;
	import java.util.Collection;
	import java.util.Collections;
	import java.util.Comparator;
	import java.util.Iterator;
	import java.util.Map;
	import java.util.NavigableMap;
	import java.util.NavigableSet;
	import java.util.Set;
	import java.util.SortedSet;
	import java.util.Spliterator;

	/**
	* 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 {@code contains}, {@code add}, and {@code remove}
	* operations and their variants. Insertion, removal, and access
	* operations safely execute concurrently by multiple threads.
	*
	* <p>Iterators and spliterators are
	* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
	*
	* <p>Ascending ordered views and their iterators are faster than
	* descending ones.
	*
	* <p>Beware that, unlike in most collections, the {@code size}
	* 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 {@code addAll},
	* {@code removeAll}, {@code retainAll}, {@code containsAll},
	* {@code equals}, and {@code toArray} are <em>not</em> guaranteed
	* to be performed atomically. For example, an iterator operating
	* concurrently with an {@code addAll} 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 {@code null} elements,
	* because {@code null} 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 {@code null}, 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 {@code c} 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 {@code ConcurrentSkipListSet}
	* instance. (The elements themselves are not cloned.)
	*
	* @return a shallow copy of this set
	*/
	public ConcurrentSkipListSet<E> clone() {
	try {
	@SuppressWarnings("unchecked")
	ConcurrentSkipListSet<E> clone =
	(ConcurrentSkipListSet<E>) super.clone();
	clone.setMap(new ConcurrentSkipListMap<E,Object>(m));
	return clone;
	} catch (CloneNotSupportedException e) {
	throw new InternalError();
	}
	}

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

	/**
	* Returns the number of elements in this set. If this set
	* contains more than {@code Integer.MAX_VALUE} elements, it
	* returns {@code Integer.MAX_VALUE}.
	*
	* <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 {@code true} if this set contains no elements.
	* @return {@code true} if this set contains no elements
	*/
	public boolean isEmpty() {
	return m.isEmpty();
	}

	/**
	* Returns {@code true} if this set contains the specified element.
	* More formally, returns {@code true} if and only if this set
	* contains an element {@code e} such that {@code o.equals(e)}.
	*
	* @param o object to be checked for containment in this set
	* @return {@code true} 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 {@code e} to this set if
	* the set contains no element {@code e2} such that {@code e.equals(e2)}.
	* If this set already contains the element, the call leaves the set
	* unchanged and returns {@code false}.
	*
	* @param e element to be added to this set
	* @return {@code true} if this set did not already contain the
	* specified element
	* @throws ClassCastException if {@code e} 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 {@code e} such that
	* {@code o.equals(e)}, if this set contains such an element.
	* Returns {@code true} 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 {@code true} if this set contained the specified element
	* @throws ClassCastException if {@code o} 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
	* {@code true} 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 {@code true} 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 {@code true} 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 (Object e : c)
	if (remove(e))
	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 java.util.NoSuchElementException {@inheritDoc}
	*/
	public E first() {
	return m.firstKey();
	}

	/**
	* @throws java.util.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
	* {@link Collections#reverseOrder(Comparator) Collections.reverseOrder}{@code (comparator())}.
	* 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<E>(m.descendingMap());
	}

	/**
	* Returns a {@link Spliterator} over the elements in this set.
	*
	* <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT},
	* {@link Spliterator#NONNULL}, {@link Spliterator#DISTINCT},
	* {@link Spliterator#SORTED} and {@link Spliterator#ORDERED}, with an
	* encounter order that is ascending order. Overriding implementations
	* should document the reporting of additional characteristic values.
	*
	* <p>The spliterator's comparator (see
	* {@link java.util.Spliterator#getComparator()}) is {@code null} if
	* the set's comparator (see {@link #comparator()}) is {@code null}.
	* Otherwise, the spliterator's comparator is the same as or imposes the
	* same total ordering as the set's comparator.
	*
	* @return a {@code Spliterator} over the elements in this set
	* @since 1.8
	*/
	@SuppressWarnings("unchecked")
	public Spliterator<E> spliterator() {
	if (m instanceof ConcurrentSkipListMap)
	return ((ConcurrentSkipListMap<E,?>)m).keySpliterator();
	else
	return (Spliterator<E>)((ConcurrentSkipListMap.SubMap<E,?>)m).keyIterator();
	}

	// 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);
	}
	}
	}

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