/*
 * 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.Iterator;
import java.util.Objects;
import java.util.Set;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.function.Consumer;
import java.util.function.Predicate;

A Set that uses an internal CopyOnWriteArrayList for all of its operations. Thus, it shares the same basic properties: 

 
It is best suited for applications in which set sizes generally
      stay small, read-only operations
      vastly outnumber mutative operations, and you need
      to prevent interference among threads during traversal.
 
It is thread-safe.
 
Mutative operations (add, set, remove, etc.) are expensive since they usually entail copying the entire underlying array. 
Iterators do not support the mutative remove operation. 
Traversal via iterators is fast and cannot encounter
     interference from other threads. Iterators rely on
     unchanging snapshots of the array at the time the iterators were
     constructed.

Sample Usage. The following code sketch uses a
copy-on-write set to maintain a set of Handler objects that
perform some action upon state updates.
 
class Handler { void handle(); ... }
class X {
  private final CopyOnWriteArraySet<Handler> handlers
    = new CopyOnWriteArraySet<>();
  public void addHandler(Handler h) { handlers.add(h); }
  private long internalState;
  private synchronized void changeState() { internalState = ...; }
  public void update() {
    changeState();
    for (Handler handler : handlers)
      handler.handle();
  }
 }
This class is a member of the

Java Collections Framework.
Author:
Doug Lea
Type parameters:
<E> – the type of elements held in this set
See Also:
CopyOnWriteArrayList
Since:
1.5/**
 * A {@link Set} that uses an internal {@link CopyOnWriteArrayList}
 * for all of its operations.  Thus, it shares the same basic properties:
 * <ul>
 *  <li>It is best suited for applications in which set sizes generally
 *       stay small, read-only operations
 *       vastly outnumber mutative operations, and you need
 *       to prevent interference among threads during traversal.
 *  <li>It is thread-safe.
 *  <li>Mutative operations ({@code add}, {@code set}, {@code remove}, etc.)
 *      are expensive since they usually entail copying the entire underlying
 *      array.
 *  <li>Iterators do not support the mutative {@code remove} operation.
 *  <li>Traversal via iterators is fast and cannot encounter
 *      interference from other threads. Iterators rely on
 *      unchanging snapshots of the array at the time the iterators were
 *      constructed.
 * </ul>
 *
 * <p><b>Sample Usage.</b> The following code sketch uses a
 * copy-on-write set to maintain a set of Handler objects that
 * perform some action upon state updates.
 *
 * <pre> {@code
 * class Handler { void handle(); ... }
 *
 * class X {
 *   private final CopyOnWriteArraySet<Handler> handlers
 *     = new CopyOnWriteArraySet<>();
 *   public void addHandler(Handler h) { handlers.add(h); }
 *
 *   private long internalState;
 *   private synchronized void changeState() { internalState = ...; }
 *
 *   public void update() {
 *     changeState();
 *     for (Handler handler : handlers)
 *       handler.handle();
 *   }
 * }}</pre>
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
 * Java Collections Framework</a>.
 *
 * @see CopyOnWriteArrayList
 * @since 1.5
 * @author Doug Lea
 * @param <E> the type of elements held in this set
 */
public class CopyOnWriteArraySet<E> extends AbstractSet<E>
        implements java.io.Serializable {
    private static final long serialVersionUID = 5457747651344034263L;

    private final CopyOnWriteArrayList<E> al;

    
Creates an empty set.
/**
     * Creates an empty set.
     */
    public CopyOnWriteArraySet() {
        al = new CopyOnWriteArrayList<E>();
    }

    
Creates a set containing all of the elements of the specified
collection.
Params:
c – the collection of elements to initially contain
Throws:
NullPointerException – if the specified collection is null/**
     * Creates a set containing all of the elements of the specified
     * collection.
     *
     * @param c the collection of elements to initially contain
     * @throws NullPointerException if the specified collection is null
     */
    public CopyOnWriteArraySet(Collection<? extends E> c) {
        if (c.getClass() == CopyOnWriteArraySet.class) {
            @SuppressWarnings("unchecked") CopyOnWriteArraySet<E> cc =
                (CopyOnWriteArraySet<E>)c;
            al = new CopyOnWriteArrayList<E>(cc.al);
        }
        else {
            al = new CopyOnWriteArrayList<E>();
            al.addAllAbsent(c);
        }
    }

    
Returns the number of elements in this set.
Returns:
the number of elements in this set/**
     * Returns the number of elements in this set.
     *
     * @return the number of elements in this set
     */
    public int size() {
        return al.size();
    }

    
Returns true if this set contains no elements. 
Returns:
true if this set contains no elements/**
     * Returns {@code true} if this set contains no elements.
     *
     * @return {@code true} if this set contains no elements
     */
    public boolean isEmpty() {
        return al.isEmpty();
    }

    
Returns true if this set contains the specified element. More formally, returns true if and only if this set contains an element e such that Objects.equals(o, e). 
Params:
o – element whose presence in this set is to be tested
Returns:
true if this set contains the specified element/**
     * 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 Objects.equals(o, e)}.
     *
     * @param o element whose presence in this set is to be tested
     * @return {@code true} if this set contains the specified element
     */
    public boolean contains(Object o) {
        return al.contains(o);
    }

    
Returns an array containing all of the elements in this set.
If this set makes any guarantees as to what order its elements
are returned by its iterator, this method must return the
elements in the same order.
The returned array will be "safe" in that no references to it
are maintained by this set.  (In other words, this method must
allocate a new array even if this set is backed by an array).
The caller is thus free to modify the returned array.
This method acts as bridge between array-based and collection-based
APIs.
Returns:
an array containing all the elements in this set/**
     * Returns an array containing all of the elements in this set.
     * If this set makes any guarantees as to what order its elements
     * are returned by its iterator, this method must return the
     * elements in the same order.
     *
     * <p>The returned array will be "safe" in that no references to it
     * are maintained by this set.  (In other words, this method must
     * allocate a new array even if this set is backed by an array).
     * The caller is thus free to modify the returned array.
     *
     * <p>This method acts as bridge between array-based and collection-based
     * APIs.
     *
     * @return an array containing all the elements in this set
     */
    public Object[] toArray() {
        return al.toArray();
    }

    
Returns an array containing all of the elements in this set; the
runtime type of the returned array is that of the specified array.
If the set fits in the specified array, it is returned therein.
Otherwise, a new array is allocated with the runtime type of the
specified array and the size of this set.
If this set fits in the specified array with room to spare (i.e., the array has more elements than this set), the element in the array immediately following the end of the set is set to null. (This is useful in determining the length of this set only if the caller knows that this set does not contain
any null elements.)
If this set makes any guarantees as to what order its elements
are returned by its iterator, this method must return the elements
in the same order.
Like the toArray() method, this method acts as bridge between array-based and collection-based APIs. Further, this method allows precise control over the runtime type of the output array, and may, under certain circumstances, be used to save allocation costs. 
Suppose x is a set known to contain only strings. The following code can be used to dump the set into a newly allocated array of String: 
 String[] y = x.toArray(new String[0]);
 Note that toArray(new Object[0]) is identical in function to toArray(). 
Params:
a – the array into which the elements of this set are to be
       stored, if it is big enough; otherwise, a new array of the same
       runtime type is allocated for this purpose.
Throws:
ArrayStoreException – if the runtime type of the specified array
        is not a supertype of the runtime type of every element in this
        set
NullPointerException – if the specified array is null
Returns:
an array containing all the elements in this set/**
     * Returns an array containing all of the elements in this set; the
     * runtime type of the returned array is that of the specified array.
     * If the set fits in the specified array, it is returned therein.
     * Otherwise, a new array is allocated with the runtime type of the
     * specified array and the size of this set.
     *
     * <p>If this set fits in the specified array with room to spare
     * (i.e., the array has more elements than this set), the element in
     * the array immediately following the end of the set is set to
     * {@code null}.  (This is useful in determining the length of this
     * set <i>only</i> if the caller knows that this set does not contain
     * any null elements.)
     *
     * <p>If this set makes any guarantees as to what order its elements
     * are returned by its iterator, this method must return the elements
     * in the same order.
     *
     * <p>Like the {@link #toArray()} method, this method acts as bridge between
     * array-based and collection-based APIs.  Further, this method allows
     * precise control over the runtime type of the output array, and may,
     * under certain circumstances, be used to save allocation costs.
     *
     * <p>Suppose {@code x} is a set known to contain only strings.
     * The following code can be used to dump the set into a newly allocated
     * array of {@code String}:
     *
     * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
     *
     * Note that {@code toArray(new Object[0])} is identical in function to
     * {@code toArray()}.
     *
     * @param a the array into which the elements of this set are to be
     *        stored, if it is big enough; otherwise, a new array of the same
     *        runtime type is allocated for this purpose.
     * @return an array containing all the elements in this set
     * @throws ArrayStoreException if the runtime type of the specified array
     *         is not a supertype of the runtime type of every element in this
     *         set
     * @throws NullPointerException if the specified array is null
     */
    public <T> T[] toArray(T[] a) {
        return al.toArray(a);
    }

    
Removes all of the elements from this set.
The set will be empty after this call returns.
/**
     * Removes all of the elements from this set.
     * The set will be empty after this call returns.
     */
    public void clear() {
        al.clear();
    }

    
Removes the specified element from this set if it is present. More formally, removes an element e such that Objects.equals(o, e), if this set contains such an element. Returns 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.) 
Params:
o – object to be removed from this set, if present
Returns:
true if this set contained the specified element/**
     * Removes the specified element from this set if it is present.
     * More formally, removes an element {@code e} such that
     * {@code Objects.equals(o, 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
     */
    public boolean remove(Object o) {
        return al.remove(o);
    }

    
Adds the specified element to this set if it is not already present. More formally, adds the specified element e to this set if the set contains no element e2 such that Objects.equals(e, e2). If this set already contains the element, the call leaves the set unchanged and returns false. 
Params:
e – element to be added to this set
Returns:
true if this set did not already contain the specified element/**
     * 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 Objects.equals(e, 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
     */
    public boolean add(E e) {
        return al.addIfAbsent(e);
    }

    
Returns true if this set contains all of the elements of the specified collection. If the specified collection is also a set, this method returns true if it is a subset of this set.
Params:
c – collection to be checked for containment in this set
Throws:
NullPointerException – if the specified collection is null
See Also:
contains(Object)
Returns:
true if this set contains all of the elements of the specified collection/**
     * Returns {@code true} if this set contains all of the elements of the
     * specified collection.  If the specified collection is also a set, this
     * method returns {@code true} if it is a <i>subset</i> of this set.
     *
     * @param  c collection to be checked for containment in this set
     * @return {@code true} if this set contains all of the elements of the
     *         specified collection
     * @throws NullPointerException if the specified collection is null
     * @see #contains(Object)
     */
    public boolean containsAll(Collection<?> c) {
        return (c instanceof Set)
            ? compareSets(al.getArray(), (Set<?>) c) >= 0
            : al.containsAll(c);
    }

    
Tells whether the objects in snapshot (regarded as a set) are a
superset of the given set.
Returns:
-1 if snapshot is not a superset, 0 if the two sets
contain precisely the same elements, and 1 if snapshot is a
proper superset of the given set/**
     * Tells whether the objects in snapshot (regarded as a set) are a
     * superset of the given set.
     *
     * @return -1 if snapshot is not a superset, 0 if the two sets
     * contain precisely the same elements, and 1 if snapshot is a
     * proper superset of the given set
     */
    private static int compareSets(Object[] snapshot, Set<?> set) {
        // Uses O(n^2) algorithm, that is only appropriate for small
        // sets, which CopyOnWriteArraySets should be.
        //
        // Optimize up to O(n) if the two sets share a long common prefix,
        // as might happen if one set was created as a copy of the other set.

        final int len = snapshot.length;
        // Mark matched elements to avoid re-checking
        final boolean[] matched = new boolean[len];

        // j is the largest int with matched[i] true for { i | 0 <= i < j }
        int j = 0;
        outer: for (Object x : set) {
            for (int i = j; i < len; i++) {
                if (!matched[i] && Objects.equals(x, snapshot[i])) {
                    matched[i] = true;
                    if (i == j)
                        do { j++; } while (j < len && matched[j]);
                    continue outer;
                }
            }
            return -1;
        }
        return (j == len) ? 0 : 1;
    }

    
Adds all of the elements in the specified collection to this set if they're not already present. If the specified collection is also a set, the addAll operation effectively modifies this set so that its value is the union of the two sets.  The behavior of
this operation is undefined if the specified collection is modified
while the operation is in progress.
Params:
c – collection containing elements to be added to this set
Throws:
NullPointerException – if the specified collection is null
See Also:
add(Object)
Returns:
true if this set changed as a result of the call/**
     * Adds all of the elements in the specified collection to this set if
     * they're not already present.  If the specified collection is also a
     * set, the {@code addAll} operation effectively modifies this set so
     * that its value is the <i>union</i> of the two sets.  The behavior of
     * this operation is undefined if the specified collection is modified
     * while the operation is in progress.
     *
     * @param  c collection containing elements to be added to this set
     * @return {@code true} if this set changed as a result of the call
     * @throws NullPointerException if the specified collection is null
     * @see #add(Object)
     */
    public boolean addAll(Collection<? extends E> c) {
        return al.addAllAbsent(c) > 0;
    }

    
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
asymmetric set difference of the two sets.
Params:
c – collection containing elements to be removed from this set
Throws:
ClassCastException – if the class of an element of this set
        is incompatible with the specified collection
(optional)
NullPointerException – if this set contains a null element and the
        specified collection does not permit null elements
(optional),
        or if the specified collection is null
See Also:
remove(Object)
Returns:
true if this set changed as a result of the call/**
     * 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 this set contains a null element and the
     *         specified collection does not permit null elements
     * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>),
     *         or if the specified collection is null
     * @see #remove(Object)
     */
    public boolean removeAll(Collection<?> c) {
        return al.removeAll(c);
    }

    
Retains only the elements in this set that are contained in the
specified collection.  In other words, removes from this set all of
its elements that are not 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 intersection of the
two sets.
Params:
c – collection containing elements to be retained in this set
Throws:
ClassCastException – if the class of an element of this set
        is incompatible with the specified collection
(optional)
NullPointerException – if this set contains a null element and the
        specified collection does not permit null elements
(optional),
        or if the specified collection is null
See Also:
remove(Object)
Returns:
true if this set changed as a result of the call/**
     * Retains only the elements in this set that are contained in the
     * specified collection.  In other words, removes from this set all of
     * its elements that are not 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>intersection</i> of the
     * two sets.
     *
     * @param  c collection containing elements to be retained in 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 this set contains a null element and the
     *         specified collection does not permit null elements
     * (<a href="{@docRoot}/java.base/java/util/Collection.html#optional-restrictions">optional</a>),
     *         or if the specified collection is null
     * @see #remove(Object)
     */
    public boolean retainAll(Collection<?> c) {
        return al.retainAll(c);
    }

    
Returns an iterator over the elements contained in this set
in the order in which these elements were added.
The returned iterator provides a snapshot of the state of the set
when the iterator was constructed. No synchronization is needed while
traversing the iterator. The iterator does NOT support the remove method. 
Returns:
an iterator over the elements in this set/**
     * Returns an iterator over the elements contained in this set
     * in the order in which these elements were added.
     *
     * <p>The returned iterator provides a snapshot of the state of the set
     * when the iterator was constructed. No synchronization is needed while
     * traversing the iterator. The iterator does <em>NOT</em> support the
     * {@code remove} method.
     *
     * @return an iterator over the elements in this set
     */
    public Iterator<E> iterator() {
        return al.iterator();
    }

    
Compares the specified object with this set for equality. Returns true if the specified object is the same object as this object, or if it is also a Set and the elements returned by an iterator over the specified set are the same as the elements returned by an iterator over this set. More formally, the two iterators are considered to return the same elements if they return the same number of elements and for every element e1 returned by the iterator over the specified set, there is an element e2 returned by the iterator over this set such that Objects.equals(e1, e2). 
Params:
o – object to be compared for equality with this set
Returns:
true if the specified object is equal to this set/**
     * Compares the specified object with this set for equality.
     * Returns {@code true} if the specified object is the same object
     * as this object, or if it is also a {@link Set} and the elements
     * returned by an {@linkplain Set#iterator() iterator} over the
     * specified set are the same as the elements returned by an
     * iterator over this set.  More formally, the two iterators are
     * considered to return the same elements if they return the same
     * number of elements and for every element {@code e1} returned by
     * the iterator over the specified set, there is an element
     * {@code e2} returned by the iterator over this set such that
     * {@code Objects.equals(e1, e2)}.
     *
     * @param o 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) {
        return (o == this)
            || ((o instanceof Set)
                && compareSets(al.getArray(), (Set<?>) o) == 0);
    }

    
Throws:
NullPointerException – {@inheritDoc}/**
     * @throws NullPointerException {@inheritDoc}
     */
    public boolean removeIf(Predicate<? super E> filter) {
        return al.removeIf(filter);
    }

    
Throws:
NullPointerException – {@inheritDoc}/**
     * @throws NullPointerException {@inheritDoc}
     */
    public void forEach(Consumer<? super E> action) {
        al.forEach(action);
    }

    
Returns a Spliterator over the elements in this set in the order in which these elements were added. 
The Spliterator reports Spliterator.IMMUTABLE, Spliterator.DISTINCT, Spliterator.SIZED, and Spliterator.SUBSIZED. 
The spliterator provides a snapshot of the state of the set
when the spliterator was constructed. No synchronization is needed while
operating on the spliterator.
Returns:
a Spliterator over the elements in this set
Since:
1.8/**
     * Returns a {@link Spliterator} over the elements in this set in the order
     * in which these elements were added.
     *
     * <p>The {@code Spliterator} reports {@link Spliterator#IMMUTABLE},
     * {@link Spliterator#DISTINCT}, {@link Spliterator#SIZED}, and
     * {@link Spliterator#SUBSIZED}.
     *
     * <p>The spliterator provides a snapshot of the state of the set
     * when the spliterator was constructed. No synchronization is needed while
     * operating on the spliterator.
     *
     * @return a {@code Spliterator} over the elements in this set
     * @since 1.8
     */
    public Spliterator<E> spliterator() {
        return Spliterators.spliterator
            (al.getArray(), Spliterator.IMMUTABLE | Spliterator.DISTINCT);
    }
}