package org.wamblee.system;

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

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

/**
 * Assembler to control multiple subsystems. It makes sure that
 * all dependencies are met and controls the order in which systems
 * are initialized. 
 */
public class SystemAssembler {

	private static final Log LOG = LogFactory.getLog(SystemAssembler.class);

	private static final String ROOT_CONTEXT_NAME = "root";
	private String _context;
	private SubSystem[] _systems;

	/**
	 * Constructs the assembler. 
	 * @param aSystems Systems that must be assembled.
	 * @param aAvailableServices Available services from other systems
	 *  outside of the systems that this assembler manages.
	 */
	public SystemAssembler(SubSystem[] aSystems, ServiceDescriptor[] aAvailableServices) {
		this(ROOT_CONTEXT_NAME, aSystems, aAvailableServices);
	}

	/**
	 * Constructs the assembler. 
	 * @param aContext Context (unique name) of the assembler. 
	 * @param aSystems Systems that must be assembled. 
	 * @param aAvailableServices Available services from other systems
	 *  outside of the systems that this assembler manages.
	 */
	public SystemAssembler(String aContext, SubSystem[] aSystems, 
			ServiceDescriptor[] aAvailableServices) {
		_context = aContext;
		_systems = aSystems;
		validate(aAvailableServices);
	}

	/**
	 * Determines if the systems are ordered appropriately so that all
	 * dependencies are met.
	 */
	private void validate(ServiceDescriptor[] aDescriptors) throws SystemAssemblyException {

		List<ServiceDescriptor> allProvided = new ArrayList<ServiceDescriptor>();
		for (ServiceDescriptor descriptor: aDescriptors) { 
			allProvided.add(descriptor);
		}
		for (SubSystem system : _systems) {
			// Check if all required services are already provided by earlier
			// systems.
			ServiceDescriptor[] required = system.getRequiredServices();
			for (ServiceDescriptor descriptor : required) {
				if (!(allProvided.contains(descriptor))) {
					throw new SystemAssemblyException(
							"Service '"
									+ descriptor
									+ "' required by system '"
									+ system
									+ "' is not provided by systems that are started earlier");
				}
			}

			// add all provided services
			ServiceDescriptor[] provided = system.getProvidedServices();
			for (ServiceDescriptor descriptor : provided) {
				allProvided.add(descriptor);
			}
		}
	}

	/**
	 * Starts the subsystems.
	 * @param aRequiredServices Services that are available from
	 *  other systems that have been started before. 
	 */
	public void start(Service[] aRequiredServices) {
		LOG.info("Starting '" + _context + "'");
		Map<ServiceDescriptor, Service> allProvided = new HashMap<ServiceDescriptor, Service>();

		for (Service service : aRequiredServices) {
			allProvided.put(service.getDescriptor(), service);
		}
		for (SubSystem system : _systems) {
			ServiceDescriptor[] descriptors = system.getRequiredServices();
			List<Service> services = new ArrayList<Service>();
			for (ServiceDescriptor descriptor : descriptors) {
				Service required = allProvided.get(descriptor);
				if (required == null) {
					throw new SystemAssemblyException("Service '" + descriptor
							+ "' required by '" + system + "' is null.");
				}
				services.add(required);
			}
			Service[] provided = system.initialize(_context, services
					.toArray(new Service[0]));
			for (Service service : provided) {
				allProvided.put(service.getDescriptor(), service);
			}

		}
	}

}