/**
    * Copyright 2005-2012 The Kuali Foundation
    *
    * Licensed under the Educational Community 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.opensource.org/licenses/ecl2.php
    *
   * 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 org.kuali.rice.krms.impl.provider.repository;
  
  import org.kuali.rice.krms.api.engine.ExecutionEnvironment;
  import org.kuali.rice.krms.api.repository.proposition.PropositionDefinition;
  import org.kuali.rice.krms.framework.engine.Proposition;
  import org.kuali.rice.krms.framework.engine.PropositionResult;
  import org.kuali.rice.krms.framework.type.PropositionTypeService;
  import org.kuali.rice.krms.impl.type.KrmsTypeResolver;
  
  import java.util.Collections;
  import java.util.List;
  
  /**
   * TODO... 
   * 
   * @author Kuali Rice Team (rice.collab@kuali.org)
   *
   */
  final class LazyProposition implements Proposition {
  
  	private final PropositionDefinition propositionDefinition;
  	private final KrmsTypeResolver resolver;
  	
  	private final Object mutex = new Object();
  	
  	// volatile for double-checked locking idiom
  	private volatile Proposition proposition;
  	
  	LazyProposition(PropositionDefinition propositionDefinition, KrmsTypeResolver resolver) {
  		this.propositionDefinition = propositionDefinition;
  		this.resolver = resolver;
  		this.proposition = null;
  	}
  	
  	@Override
  	public PropositionResult evaluate(ExecutionEnvironment environment) {
  		return getProposition().evaluate(environment);
  	}
  	
  	/**
  	 * Gets the proposition using a lazy double-checked locking mechanism as documented in Effective Java Item 71.
  	 */
  	private Proposition getProposition() {
  		Proposition localProposition = proposition;
  		if (localProposition == null) {
  			synchronized (mutex) {
  				localProposition = proposition;
  				if (localProposition == null) {
  					proposition = localProposition = constructProposition();
  				}
  			}
  		}
  		return localProposition;
  	}
  	
  	private Proposition constructProposition() {
          Proposition proposition = null;
          if (propositionDefinition != null) {
  		    PropositionTypeService propositionTypeService = resolver.getPropositionTypeService(propositionDefinition);
  		    proposition = propositionTypeService.loadProposition(propositionDefinition);
          }
  		if (proposition == null) {
  			proposition = new Proposition() {
  				@Override
  				public PropositionResult evaluate(ExecutionEnvironment environment) {
  					return new PropositionResult(true);
  				}
  
  			    @Override
  			    public List<Proposition> getChildren() {
  			        return Collections.emptyList();
  			    }
  			    
  			    @Override
  			    public boolean isCompound() {
  			        return false;
  			    }
  			};
  		}
  		return proposition;
  	}
  
  	@Override
  	public List<Proposition> getChildren() {
 	    return getProposition().getChildren();
 	}
 	
 	@Override
 	public boolean isCompound() {
 	    return getProposition().isCompound();
 	}
 }