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/*

 * 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/licenses/publicdomain

 */

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. Additionally, the

 * bulk operations <tt>addAll</tt>, <tt>removeAll</tt>,

 * <tt>retainAll</tt>, and <tt>containsAll</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 static final Unsafe unsafe = Unsafe.getUnsafe();

    private static final long mapOffset;

    static {

        try {

            mapOffset = unsafe.objectFieldOffset

                (ConcurrentSkipListSet.class.getDeclaredField("m"));

        } catch (Exception ex) { throw new Error(ex); }

    }

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

        unsafe.putObjectVolatile(this, mapOffset, map);

    }

}