/*

 * 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.lang.reflect.Field;

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.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.

 *

 * <p>Bulk operations that add, remove, or examine multiple elements,

 * such as {@link #addAll}, {@link #removeIf} or {@link #forEach},

 * are <em>not</em> guaranteed to be performed atomically.

 * For example, a {@code forEach} traversal concurrent with an {@code

 * addAll} operation might observe 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}/java.base/java/util/package-summary.html#CollectionsFramework">

 * 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 | 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 class of an element of this set

     *         is incompatible with the specified collection

     * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>)

     * @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 {@linkplain Spliterator#getComparator() spliterator's comparator}

     * is {@code null} if the {@linkplain #comparator() set's 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

     */

    public Spliterator<E> spliterator() {

        return (m instanceof ConcurrentSkipListMap)

            ? ((ConcurrentSkipListMap<E,?>)m).keySpliterator()

            : ((ConcurrentSkipListMap.SubMap<E,?>)m).new SubMapKeyIterator();

    }

    /** Initializes map field; for use in clone. */

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

        Field mapField = java.security.AccessController.doPrivileged(

            (java.security.PrivilegedAction<Field>) () -> {

                try {

                    Field f = ConcurrentSkipListSet.class

                        .getDeclaredField("m");

                    f.setAccessible(true);

                    return f;