package org.wamblee.system;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

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.
 *
 * @author Erik Brakkee
 */
public class SystemAssembler {

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

	private static final String ROOT_CONTEXT_NAME = "root";
	private String _context;
	private Component[] _systems;
	private ProvidedInterface[] _required; 

	public static RequiredInterface[] filterRequiredServices(
			ProvidedInterface aProvided,
			Collection<RequiredInterface> aDescriptors) {
		List<RequiredInterface> required = new ArrayList<RequiredInterface>();
		for (RequiredInterface descriptor : aDescriptors) {
			if (descriptor.implementedBy(aProvided)) {
				required.add(descriptor);
			}
		}
		return required.toArray(new RequiredInterface[0]);
	}

	public static ProvidedInterface[] filterProvidedServices(
			RequiredInterface aRequired,
			Collection<ProvidedInterface> aProvided) {
		List<ProvidedInterface> provided = new ArrayList<ProvidedInterface>();
		for (ProvidedInterface descriptor : aProvided) {
			if (aRequired.implementedBy(descriptor)) {
				provided.add(descriptor);
			}
		}
		return provided.toArray(new ProvidedInterface[0]);
	}

	/**
	 * 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(Component[] aSystems,
			ProvidedInterface[] 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, Component[] aSystems,
			ProvidedInterface[] aAvailableServices) {
		_context = aContext;
		_systems = aSystems;
		_required = aAvailableServices; 
		validate();
	}

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

		List<ProvidedInterface> allProvided = new ArrayList<ProvidedInterface>();
		for (ProvidedInterface descriptor : _required) {
			allProvided.add(descriptor);
		}
		for (Component system : _systems) {
			// Check if all required services are already provided by earlier
			// systems.
			RequiredInterface[] required = system.getRequiredServices();

			for (RequiredInterface descriptor : required) {
				ProvidedInterface[] filtered = filterProvidedServices(
						descriptor, allProvided);

				if (filtered.length == 0) {
					throw new SystemAssemblyException(
							"Service '"
									+ descriptor
									+ "' required by system '"
									+ system
									+ "' is not provided by systems that are started earlier");
				}
				if (filtered.length > 1) {
					throw new SystemAssemblyException(
							"Service '"
									+ descriptor
									+ "' required by system '"
									+ system
									+ "' matches multiple services provided by other systems: " + 
									Arrays.asList(filtered));
				}
			}

			// add all provided services
			ProvidedInterface[] provided = system.getProvidedServices();
			allProvided.addAll(Arrays.asList(provided));
		}
	}

	/**
	 * Starts the subsystems.
	 * 
	 * @param aRequiredServices
	 *            Services that are available from other systems that have been
	 *            started before.
	 */
	public void start() {
		LOG.info("Starting '" + _context + "'");
		Set<ProvidedInterface> allProvided = new HashSet<ProvidedInterface>();
		allProvided.addAll(Arrays.asList(_required));
		
		for (Component system : _systems) {
			
			// Compose a list of the required services required for the subsystem.
			
			RequiredInterface[] descriptors = system
					.getRequiredServices();
			List<ProvidedInterface> services = new ArrayList<ProvidedInterface>();
			for (RequiredInterface required : descriptors) {
				ProvidedInterface[] provided = filterProvidedServices(
						required, allProvided);
				assert provided.length == 1; 	
				services.add(provided[0]);
				required.setProvider(provided[0]);
			}
			
			// Start the service. 
			system.start(_context);
			
			// Add started services to the set of started services.
			for (ProvidedInterface service : system.getProvidedServices()) {
				allProvided.add(service);
			}

		}
	}

}