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   *  distributed with this work for additional information
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   *  to you under the Apache License, Version 2.0 (the
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   *    https://www.apache.org/licenses/LICENSE-2.0
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  package org.apache.cayenne.commitlog.model;
  
  import java.util.Collection;
  import java.util.Collections;
  import java.util.HashMap;
  import java.util.HashSet;
  import java.util.Map;
  
  import org.apache.cayenne.ObjectId;
  
  /**
   * A mutable implementation of {@link ChangeMap}.
   * 
   * @since 4.0
   */
  public class MutableChangeMap implements ChangeMap {
  
  	private Map<ObjectId, MutableObjectChange> changes;
  
  	public MutableObjectChange getOrCreate(ObjectId id, ObjectChangeType type) {
  		MutableObjectChange changeSet = getOrCreate(id);
  		changeSet.setType(type);
  		return changeSet;
  	}
  
  	private MutableObjectChange getOrCreate(ObjectId id) {
  
  		MutableObjectChange objectChange = changes != null ? changes.get(id) : null;
  
  		if (objectChange == null) {
  
  			if (changes == null) {
  				changes = new HashMap<>();
  			}
  
  			objectChange = new MutableObjectChange(id);
  			changes.put(id, objectChange);
  		}
  
  		return objectChange;
  	}
  
  	public MutableObjectChange aliasId(ObjectId preCommitId, ObjectId postCommitId) {
  		MutableObjectChange changeSet = getOrCreate(preCommitId);
  		changeSet.setPostCommitId(postCommitId);
  		changes.put(postCommitId, changeSet);
  		return changeSet;
  	}
  
  	@Override
  	public Collection<? extends ObjectChange> getUniqueChanges() {
  		// ensure distinct change set
  		return changes == null ? Collections.<ObjectChange> emptySet() : new HashSet<>(changes.values());
  	}
  
  	@Override
  	public Map<ObjectId, ? extends ObjectChange> getChanges() {
  		return changes == null ? Collections.<ObjectId, ObjectChange> emptyMap() : changes;
  	}
  }
  