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Lunar Base is a simple game framework written in Java. It is written using an entity-component-system architecture.
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/*
 * This file is part of the lunar-base package.
 *
 * Copyright (c) 2014 Eric Fritz
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy of this software
 * and associated documentation files (the "Software"), to deal in the Software without
 * restriction, including without limitation the rights to use, copy, modify, merge, publish,
 * distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all copies or
 * substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING
 * BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
 * NONINFRINGEMENT.  IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
 * DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 */

package com.kauridev.lunarbase;

import java.util.HashMap;
import java.util.LinkedHashSet;
import java.util.Map;
import java.util.Set;

/**
 * A manager that handles associations between entities and components.
 *
 * @author Eric Fritz
 */
public class ComponentManager implements EntityRemovedEventListener
{
	/**
	 * The event manager.
	 */
	private EventManager eventManager;

	/**
	 * A map of components indexed by entity.
	 */
	private Map> components = new HashMap<>();

	/**
	 * Creates a new ComponentManager.
	 *
	 * @param eventManager The event manager.
	 */
	public ComponentManager(EventManager eventManager) {
		this.eventManager = eventManager;

		eventManager.addListener(EntityRemovedEvent.class, this);
	}

	/**
	 * Returns the component of a certain type associated with an entity.
	 *
	 * @param entity The entity.
	 * @param type   The component type.
	 *
	 * @return The component or null if it does not exist.
	 */
	public  C getComponent(Entity entity, Class type) {
		for (Component c : getComponents(entity)) {
			if (type.isAssignableFrom(c.getClass())) {
				return type.cast(c);
			}
		}

		return null;
	}

	/**
	 * Adds a component associated with an entity. This method fires the {@link EntityChangedEvent}
	 * event.
	 *
	 * @param entity    The entity.
	 * @param component The component.
	 */
	public void addComponent(Entity entity, Component component) {
		getComponents(entity).add(component);
		eventManager.dispatch(new EntityChangedEvent(entity));
	}

	/**
	 * Removes a component associated with an entity. This method fires the
	 * {@link EntityChangedEvent} event.
	 *
	 * @param entity    The entity.
	 * @param component The component.
	 */
	public void removeComponent(Entity entity, Component component) {
		getComponents(entity).remove(component);
		eventManager.dispatch(new EntityChangedEvent(entity));
	}

	@Override
	public EntityRemovedEvent onEntityRemoved(EntityRemovedEvent event) {
		getComponents(event.getEntity()).clear();
		return event;
	}

	/**
	 * Returns a list of components associated with the entity.
	 *
	 * @param entity The entity.
	 *
	 * @return A list of components.
	 */
	private Set getComponents(Entity entity) {
		if (components.get(entity.getId()) == null) {
			components.put(entity.getId(), new LinkedHashSet());
		}

		return components.get(entity.getId());
	}
}