/*******************************************************************************
 * Copyright (c) 2004, 2016 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.springframework.ide.vscode.commons.util;

import java.util.Iterator;
import java.util.NoSuchElementException;

/**
 * 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.  For legacy code, 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>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 an enhanced 'for' loop to notify listeners. The recommended
 * code sequence for notifying all registered listeners of say,
 * <code>FooListener#eventHappened(Event)</code>, is:
 * </p>
 * <pre>
ListenerList&lt;FooListener&gt; fooListeners = new ListenerList<>();
//...
for (FooListener listener : fooListeners) {
	listener.eventHappened(event);
}
 * </pre>
 * <p>
 * Legacy code may still call {@link #getListeners()} and then use a 'for' loop
 * to iterate the {@code Object[]}. This might be insignificantly faster, but
 * it lacks type-safety and risks inadvertent modifications to the array.
 * </p>
 * <p>
 * This class can be used without OSGi running.
 * </p>
 * 
 * @param <E> the type of listeners in this list
 * @since org.eclipse.equinox.common 3.2
 */
public class ListenerList<E> implements Iterable<E> {

	/**
	 * The empty array singleton instance.
	 */
	private static final Object[] EmptyArray = new Object[0];

	/**
	 * Mode constant (value 0) indicating that listeners should be considered
	 * the <a href="ListenerList.html#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="ListenerList.html#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 Object[] listeners = EmptyArray;

	/**
	 * Creates a listener list in which listeners are compared using equality.
	 */
	public ListenerList() {
		this(EQUALITY);
	}

	/**
	 * Creates a listener list using the provided comparison mode.
	 * 
	 * @param mode The mode used to determine if listeners are the <a href="ListenerList.html#same">same</a>.
	 */
	public ListenerList(int mode) {
		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="ListenerList.html#same">same</a>
	 * listener is already registered.
	 * 
	 * @param listener the non-<code>null</code> listener to add
	 */
	public synchronized void add(E 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) {
			Object listener2 = listeners[i];
			if (identity ? listener == listener2 : listener.equals(listener2))
				return;
		}
		// Thread safety: create new array to avoid affecting concurrent readers
		Object[] newListeners = new Object[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.
	 * </p>
	 * <p>
	 * Note: The recommended and type-safe way to iterate this list is to use
	 * an enhanced 'for' statement, see {@link ListenerList}.
	 * This method is deprecated for new code.
	 * </p>
	 *
	 * @return the list of registered listeners
	 */
	public Object[] getListeners() {
		return listeners;
	}

	/**
	 * Returns an iterator over all the registered listeners.
	 * The resulting iterator is unaffected by subsequent adds or removes.
	 * 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.
	 * 
	 * @return an iterator
	 * @since org.eclipse.equinox.common 3.8
	 */
	@Override
	public Iterator<E> iterator() {
		return new ListenerListIterator<>(listeners);
	}

	private static class ListenerListIterator<E> implements Iterator<E> {
		private Object[] listeners;
		private int i;

		public ListenerListIterator(Object[] listeners) {
			this.listeners = listeners;
		}

		@Override
		public boolean hasNext() {
			return i < listeners.length;
		}

		@Override
		public E next() {
			if (i >= listeners.length) {
				throw new NoSuchElementException();
			}
			@SuppressWarnings("unchecked") // (E) is safe, because #add(E) only accepts Es
			E next = (E) listeners[i++];
			return next;
		}

		@Override
		public void remove() {
			throw new UnsupportedOperationException();
		}
	}

	/**
	 * 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="ListenerList.html#same">same</a> 
	 * listener was not already registered.
	 *
	 * @param listener the non-<code>null</code> listener to remove
	 */
	public synchronized void remove(Object 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) {
			Object listener2 = listeners[i];
			if (identity ? listener == listener2 : listener.equals(listener2)) {
				if (oldSize == 1) {
					listeners = EmptyArray;
				} else {
					// Thread safety: create new array to avoid affecting concurrent readers
					Object[] newListeners = new Object[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 = EmptyArray;
	}
}
