/*******************************************************************************
 * Copyright (c) 2004, 2007 IBM Corporation and others.
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 * 
 * Contributors:
 *     IBM Corporation - initial API and implementation
 *******************************************************************************/
package org.python.pydev.shared_core.callbacks;

import java.lang.reflect.Array;

/**
 * This class is a thread safe list that is designed for storing lists of listeners.
 * The implementation is optimized for minimal memory footprint, frequent reads 
 * and infrequent writes.  Modification of the list is synchronized and relatively
 * expensive, while accessing the listeners is very fast.  Readers are given access 
 * to the underlying array data structure for reading, with the trust that they will 
 * not modify the underlying array.
 * <p>
 * <a name="same">A listener list handles the <i>same</i> listener being added 
 * multiple times, and tolerates removal of listeners that are the same as other
 * listeners in the list.  For this purpose, listeners can be compared with each other 
 * using either equality or identity, as specified in the list constructor.
 * </p>
 * <p>
 * Use the <code>getListeners</code> method when notifying listeners. The recommended
 * code sequence for notifying all registered listeners of say,
 * <code>FooListener.eventHappened</code>, is:
 * 
 * <pre>
 * Object[] listeners = myListenerList.getListeners();
 * for (int i = 0; i &lt; listeners.length; ++i) {
 * 	((FooListener) listeners[i]).eventHappened(event);
 * }
 * </pre>
 * 
 * </p><p>
 * This class can be used without OSGi running.
 * </p>
 * @since org.eclipse.equinox.common 3.2
 * 
 * Change: Same as ListenerList, but with template for listener type.
 */
public class ListenerList<X> {

    /**
     * The empty array singleton instance.
     */
    private final X[] EmptyArray;

    /**
     * Mode constant (value 0) indicating that listeners should be considered
     * the <a href="#same">same</a> if they are equal.
     */
    public static final int EQUALITY = 0;

    /**
     * Mode constant (value 1) indicating that listeners should be considered
     * the <a href="#same">same</a> if they are identical.
     */
    public static final int IDENTITY = 1;

    /**
     * Indicates the comparison mode used to determine if two
     * listeners are equivalent
     */
    private final boolean identity;

    /**
     * The list of listeners.  Initially empty but initialized
     * to an array of size capacity the first time a listener is added.
     * Maintains invariant: listeners != null
     */
    private volatile X[] listeners;

    private Class<X> genericType;

    /**
     * Creates a listener list in which listeners are compared using equality.
     */
    public ListenerList(Class<X> genericType) {
        this(genericType, EQUALITY);
    }

    /**
     * Creates a listener list using the provided comparison mode.
     * 
     * @param mode The mode used to determine if listeners are the <a href="#same">same</a>.
     */
    public ListenerList(Class<X> genericType, int mode) {
        this.genericType = genericType;
        EmptyArray = (X[]) Array.newInstance(genericType, 0);
        listeners = EmptyArray;

        if (mode != EQUALITY && mode != IDENTITY)
            throw new IllegalArgumentException();
        this.identity = mode == IDENTITY;
    }

    /**
     * Adds a listener to this list. This method has no effect if the <a href="#same">same</a>
     * listener is already registered.
     * 
     * @param listener the non-<code>null</code> listener to add
     */
    public synchronized void add(X listener) {
        // This method is synchronized to protect against multiple threads adding 
        // or removing listeners concurrently. This does not block concurrent readers.
        if (listener == null)
            throw new IllegalArgumentException();
        // check for duplicates 
        final int oldSize = listeners.length;
        for (int i = 0; i < oldSize; ++i) {
            X listener2 = listeners[i];
            if (identity ? listener == listener2 : listener.equals(listener2))
                return;
        }
        // Thread safety: create new array to avoid affecting concurrent readers
        X[] newListeners = (X[]) Array.newInstance(genericType, oldSize + 1);
        System.arraycopy(listeners, 0, newListeners, 0, oldSize);
        newListeners[oldSize] = listener;
        //atomic assignment
        this.listeners = newListeners;
    }

    /**
     * Returns an array containing all the registered listeners.
     * The resulting array is unaffected by subsequent adds or removes.
     * If there are no listeners registered, the result is an empty array.
     * Use this method when notifying listeners, so that any modifications
     * to the listener list during the notification will have no effect on 
     * the notification itself.
     * <p>
     * Note: Callers of this method <b>must not</b> modify the returned array. 
     *
     * @return the list of registered listeners
     */
    public X[] getListeners() {
        return listeners;
    }

    /**
     * Returns whether this listener list is empty.
     *
     * @return <code>true</code> if there are no registered listeners, and
     *   <code>false</code> otherwise
     */
    public boolean isEmpty() {
        return listeners.length == 0;
    }

    /**
     * Removes a listener from this list. Has no effect if the <a href="#same">same</a> 
     * listener was not already registered.
     *
     * @param listener the non-<code>null</code> listener to remove
     */
    public synchronized void remove(X listener) {
        // This method is synchronized to protect against multiple threads adding 
        // or removing listeners concurrently. This does not block concurrent readers.
        if (listener == null)
            throw new IllegalArgumentException();
        int oldSize = listeners.length;
        for (int i = 0; i < oldSize; ++i) {
            X listener2 = listeners[i];
            if (identity ? listener == listener2 : listener.equals(listener2)) {
                if (oldSize == 1) {
                    listeners = (X[]) EmptyArray;
                } else {
                    // Thread safety: create new array to avoid affecting concurrent readers
                    X[] newListeners = (X[]) Array.newInstance(genericType, oldSize - 1);
                    System.arraycopy(listeners, 0, newListeners, 0, i);
                    System.arraycopy(listeners, i + 1, newListeners, i, oldSize - i - 1);
                    //atomic assignment to field
                    this.listeners = newListeners;
                }
                return;
            }
        }
    }

    /**
     * Returns the number of registered listeners.
     *
     * @return the number of registered listeners
     */
    public int size() {
        return listeners.length;
    }

    /**
     * Removes all listeners from this list.
     */
    public synchronized void clear() {
        listeners = (X[]) EmptyArray;
    }
}
