/*******************************************************************************
 * Copyright (c) 2002, 2013 Object Factory Inc.
 *
 * This program and the accompanying materials 
 * are made available under the terms of the Eclipse Public License 2.0
 * which accompanies this distribution, and is available at
 * https://www.eclipse.org/legal/epl-2.0/
 *
 * SPDX-License-Identifier: EPL-2.0
 * 
 * Contributors:
 *		Object Factory Inc. - Initial implementation
 *******************************************************************************/
package org.eclipse.ant.internal.ui.dtd.util;

import java.lang.ref.SoftReference;

/**
 * Factory maintains a free list and, with FactoryObject, serves as a basis for factories of all types. Factory should only be subclassed in singleton
 * classes; for static factories, it may be instantiated as a static object.
 * 
 * @author Bob Foster
 */
public class Factory {

	/**
	 * Return the first object on the free list or null if none.
	 */
	public FactoryObject getFree() {
		Head head = getHead();
		FactoryObject obj = head.next;
		if (obj != null) {
			head.next = obj.next();
			obj.next(null);
		}
		return obj;
	}

	/**
	 * Add an object to the free list.
	 */
	public void setFree(FactoryObject obj) {
		Head head = getHead();
		obj.next(head.next);
		head.next = obj;
	}

	private Head getHead() {
		Head head = free.get();
		if (head == null) {
			// head is needed because you can't change
			// the referent of a SoftReference.
			// Without head, we would need to create
			// a new SoftReference each time we remove
			// a map from the list. With head, getting
			// a free object only causes memory allocation
			// when the list has been previously collected.
			head = new Head();
			free = new SoftReference<>(head);
		}
		return head;
	}

	private static class Head {
		public FactoryObject next;
	}

	private SoftReference<Head> free = new SoftReference<>(new Head());
}