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/*
    Copyright 2014, Strategic Gains, Inc.

	Licensed under the Apache License, Version 2.0 (the "License");
	you may not use this file except in compliance with the License.
	You may obtain a copy of the License at

		http://www.apache.org/licenses/LICENSE-2.0

	Unless required by applicable law or agreed to in writing, software
	distributed under the License is distributed on an "AS IS" BASIS,
	WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	See the License for the specific language governing permissions and
	limitations under the License.
*/
package com.strategicgains.hyperexpress.expand;

import java.util.HashMap;
import java.util.List;
import java.util.Map;

import com.strategicgains.hyperexpress.domain.Resource;
import com.strategicgains.hyperexpress.exception.ExpansionException;

/**
 * A Singleton object used to register {@link ExpansionCallback}s and call them.
 * 
 * Registering callbacks is achieved by calling registerCallback(). Calling those
 * callbacks is accomplished by calling the expand() methods, which can be called
 * after {@code HyperExpress.createResource()} or {@code HyperExpress.createCollectionResource()}
 * methods.
 * 
 * @author toddf
 * @since Aug 8, 2014
 */
public class Expander
{
	private static final Expander INSTANCE = new Expander();

	private Map callbacks = new HashMap();

	private Expander()
	{
		// prevents external instantiation.
	}

	/**
	 * Call the registered {@link ExpansionCallback}s for the given {@link Expansion} and
	 * {@link Resource).
	 * 
	 * @param expansion
	 * @param type
	 * @param resource
	 * @return
	 */
	public static Resource expand(Expansion expansion, Class type, Resource resource)
	{
		return INSTANCE._expand(expansion, type, resource);
	}

	public static List expand(Expansion expansion, Class type, List resources)
    {
		return INSTANCE._expand(expansion, type, resources);
    }

	public static Expander registerCallback(Class type, ExpansionCallback callback)
	{
		return INSTANCE._registerCallback(type, callback);
	}

	private Expander _registerCallback(Class type, ExpansionCallback callback)
	{
		if (callbacks.put(type.getName(), callback) != null)
		{
			throw new ExpansionException("Duplicate expansion callback registered for type: " + type.getName());
		}

		return this;
	}

	private Resource _expand(Expansion expansion, Class type, Resource resource)
	{
		if (expansion == null) return resource;

		ExpansionCallback callback = callbacks.get(type.getName());
		return (callback == null ? resource : callback.expand(expansion, resource));
	}

	private List _expand(Expansion expansion, Class type, List resources)
	{
		if (expansion == null) return resources;

		ExpansionCallback callback = callbacks.get(type.getName());

		if (callback == null) return resources;

		for (Resource resource : resources)
		{
			callback.expand(expansion, resource);
		}

		return resources;
	}
}