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/*

 * Written by Doug Lea with assistance from members of JCP JSR-166

 * Expert Group and released to the public domain. Use, modify, and

 * redistribute this code in any way without acknowledgement.

 */

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>

 * 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));

    }

}