package java.util.concurrent;

import java.util.*;

/**
 * CopyOnWriteArraySets implement a java.util.Set that uses
 * 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> Mutative operations(add, set, remove, etc) are expensive
 *      since they usually entail copying the entire underlying array.
 *  <li> Loops involving repeated element-by-element mutative operations
 *      are so expensive that they should generally be avoided.
 *  <li> Iterators do not support the mutative remove operation
 *  <li> Traversal via iterators is very fast and cannot ever 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> Probably the main application
 * of copy-on-write sets are classes that maintain
 * sets of Handler objects
 * that must be multicasted to upon an update command. This
 * is a classic case where you do not want to be holding a synch
 * lock while sending a message, and where traversals normally
 * vastly overwhelm additions.
 * <pre>
 * class  Handler { void handle(); ... }
 *
 * class X {
 *    private final CopyOnWriteArraySet handlers = new CopyOnWriteArraySet();
 *    public void addHandler(Handler h) { handlers.add(h); }
 *
 *    private long internalState;
 *    private synchronized void changeState() { internalState = ...; }
 *
 *    public void update() {
 *       changeState();
 *       Iterator it = handlers.iterator();
 *       while (it.hasNext())
 *          ((Handler)(it.next()).handle();
 *    }
 * }
 * </pre>
 * <p>[<a href="http://gee.cs.oswego.edu/dl/classes/EDU/oswego/cs/dl/util/concurrent/intro.html"> Introduction to this package. </a>]
 * @see CopyOnWriteArrayList
 **/
public class CopyOnWriteArraySet<E> extends AbstractSet<E>
        implements Cloneable, java.io.Serializable {

    private final CopyOnWriteArrayList<E> al;

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

    /**
     * Constructs a set containing all of the elements of the specified
     * Collection.
     */
    public <T extends E> CopyOnWriteArraySet(Collection<T> c) {
        al = new CopyOnWriteArrayList<E>();
        al.addAllAbsent(c);
    }


    public int      size()                    { return al.size(); }
    public boolean  isEmpty()                 { return al.isEmpty(); }
    public boolean  contains(Object o)        { return al.contains(o); }
    public Object[] toArray()                 { return al.toArray(); }
    public <T> T[]  toArray(T[] a)            { return al.toArray(a); }
    public void     clear()                   {        al.clear(); }
    public Iterator<E>  iterator()            { return al.iterator(); }
    public boolean  remove(Object o)          { return al.remove(o); }
    public boolean  add(E o)                  { return al.addIfAbsent(o); }
    public <T> boolean  containsAll(Collection<T> c)      { return al.containsAll(c); }
    public <T extends E> boolean  addAll(Collection<T> c) { return al.addAllAbsent(c) > 0; }
    public <T> boolean  removeAll(Collection<T> c)        { return al.removeAll(c); }
    public <T> boolean  retainAll(Collection<T> c)        { return al.retainAll(c); }

}