/*

  * 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.*;

 /**

  * A {@link java.util.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 (<tt>add</tt>, <tt>set</tt>, <tt>remove</tt>, etc.)

  *      are expensive since they usually entail copying the entire underlying

  *      array.

  *  <li>Iterators do not support the mutative <tt>remove</tt> 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<Handler>();

  *   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}/../technotes/guides/collections/index.html">

  * Java Collections Framework</a>.

  *

  * @see CopyOnWriteArrayList

  * @since 1.5

  * @author Doug Lea

  * @param <E> the type of elements held in this collection

  */

 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.

      */

     public CopyOnWriteArraySet() {

         al = new CopyOnWriteArrayList<E>();

     }

     /**

      * 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) {

         al = new CopyOnWriteArrayList<E>();

         al.addAllAbsent(c);

     }

     /**

      * Returns the number of elements in this set.

      *

      * @return the number of elements in this set

      */

     public int size() {

         return al.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 al.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==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.

      *

      * @param o element whose presence in this set is to be tested

      * @return <tt>true</tt> 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.

      *

      * <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.

      *

      * <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

      * <tt>null</tt>.  (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 <tt>x</tt> is a set known to contain only strings.

      * The following code can be used to dump the set into a newly allocated

      * array of <tt>String</tt>:

      *

      * <pre>

      *     String[] y = x.toArray(new String[0]);</pre>

      *

      * Note that <tt>toArray(new Object[0])</tt> is identical in function to

      * <tt>toArray()</tt>.

      *

      * @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.

      */

     public void clear() {

         al.clear();

     }

     /**

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

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

      * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;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

      */

     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 <tt>e</tt> to this set if

      * the set contains no element <tt>e2</tt> such that

      * <tt>(e==null&nbsp;?&nbsp;e2==null&nbsp;:&nbsp;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

      */

     public boolean add(E e) {

         return al.addIfAbsent(e);

     }

     /**

      * Returns <tt>true</tt> if this set contains all of the elements of the

      * specified collection.  If the specified collection is also a set, this

      * method returns <tt>true</tt> if it is a <i>subset</i> of this set.

      *

      * @param  c collection to be checked for containment in this set

      * @return <tt>true</tt> 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 al.containsAll(c);

     }

     /**

      * 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 <tt>addAll</tt> 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 <tt>true</tt> 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

      * <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 class of an element of this set

      *         is incompatible with the specified collection (optional)

      * @throws 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 #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 <i>intersection</i> of the

      * two sets.

      *

      * @param  c collection containing elements to be retained in this set

      * @return <tt>true</tt> 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 (optional)

      * @throws 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 #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.

      *

      * <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

      * <tt>remove</tt> 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 {@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 List#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 (e1==null ? e2==null : e1.equals(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) {

         if (o == this)

             return true;

         if (!(o instanceof Set))

             return false;

         Set<?> set = (Set<?>)(o);

         Iterator<?> it = set.iterator();

         // Uses O(n^2) algorithm that is only appropriate

         // for small sets, which CopyOnWriteArraySets should be.

         //  Use a single snapshot of underlying array

         Object[] elements = al.getArray();

         int len = elements.length;

         // Mark matched elements to avoid re-checking

         boolean[] matched = new boolean[len];

         int k = 0;

         outer: while (it.hasNext()) {

             if (++k > len)

                 return false;

             Object x = it.next();

             for (int i = 0; i < len; ++i) {

                 if (!matched[i] && eq(x, elements[i])) {

                     matched[i] = true;

                     continue outer;

                 }

             }

             return false;

         }

         return k == len;

     }

     /**

      * Test for equality, coping with nulls.

      */

     private static boolean eq(Object o1, Object o2) {

         return (o1 == null ? o2 == null : o1.equals(o2));

     }

 }