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  package org.apache.cayenne.graph;
  
  import java.util.ArrayList;
  import java.util.HashMap;
  import java.util.HashSet;
  import java.util.Iterator;
  import java.util.List;
  import java.util.Map;
  import java.util.Set;
  
  /**
   * A utility class that removes redundant and mutually exclusive graph changes from the
   * graph diff.
   * 
   * @since 3.0
   */
  public class GraphDiffCompressor {
  
      public GraphDiff compress(GraphDiff diff) {
  
          if (diff.isNoop()) {
              return diff;
          }
  
          CompressAction action = new CompressAction();
          diff.apply(action);
          return action.getCompressedDiff();
      }
  
      final class CompressAction implements GraphChangeHandler {
  
          private List<GraphDiff> compressed = new ArrayList<>();
          private Map<Object, List<NodeDiff>> diffsByNode = new HashMap<>();
          private Set<Object> deletedNodes;
          private Set<Object> createdNodes;
  
          GraphDiff getCompressedDiff() {
  
              // remove deleted nodes...
              if (deletedNodes != null) {
  
                  for (Object nodeId : deletedNodes) {
  
                      Iterator<GraphDiff> it = compressed.iterator();
  
                      // if the node was inserted in the same transaction and later deleted,
                      // remove all its ops. Otherwise preserve arc ops (since delete rules
                      // depend on them), and delete operation itself.
  
                      // TODO: andrus 2008/02/04 - this doesn't take into account a
                      // possibility that a deleted node was re-inserted... Although I don't
                      // see how this could possibly happen with the present Cayenne API.
                      if (createdNodes != null && createdNodes.contains(nodeId)) {
                          while (it.hasNext()) {
                              NodeDiff diff = (NodeDiff) it.next();
                              if (nodeId.equals(diff.getNodeId())) {
                                  it.remove();
                              }
                          }
                      }
                      else {
                          while (it.hasNext()) {
                              NodeDiff diff = (NodeDiff) it.next();
                              if (nodeId.equals(diff.getNodeId())) {
                                  if (diff instanceof NodePropertyChangeOperation) {
                                      it.remove();
                                  }
                              }
                          }
                      }
  
                  }
              }
  
              return new CompoundDiff(compressed);
          }
  
          @Override
          public void arcCreated(Object nodeId, Object targetNodeId, ArcId arcId) {
  
              if (targetNodeId != null) {
                  List<NodeDiff> diffs = diffsByNode.get(nodeId);
                  if (diffs != null) {
                      for (int i = diffs.size() - 1; i >= 0; i--) {
                          NodeDiff diff = diffs.get(i);
                          if (diff instanceof ArcDeleteOperation) {
                              ArcDeleteOperation arcDiff = (ArcDeleteOperation) diff;
                              if (arcId.equals(arcDiff.getArcId())
                                      && targetNodeId.equals(arcDiff.targetNodeId)) {
                                  diffs.remove(i);
                                  compressed.remove(arcDiff);
                                  return;
                              }
                          }
                      }
                  }
              }
  
              registerDiff(new ArcCreateOperation(nodeId, targetNodeId, arcId));
          }
  
          @Override
          public void arcDeleted(Object nodeId, Object targetNodeId, ArcId arcId) {
  
              if (targetNodeId != null) {
                  List<NodeDiff> diffs = diffsByNode.get(nodeId);
                  if (diffs != null) {
                      for (int i = diffs.size() - 1; i >= 0; i--) {
                          NodeDiff diff = diffs.get(i);
                          if (diff instanceof ArcCreateOperation) {
                              ArcCreateOperation arcDiff = (ArcCreateOperation) diff;
                              if (arcId.equals(arcDiff.getArcId())
                                      && targetNodeId.equals(arcDiff.targetNodeId)) {
                                  diffs.remove(i);
                                  compressed.remove(arcDiff);
                                  return;
                              }
                          }
                      }
                  }
              }
  
              registerDiff(new ArcDeleteOperation(nodeId, targetNodeId, arcId));
          }
  
          @Override
          public void nodeCreated(Object nodeId) {
              registerDiff(new NodeCreateOperation(nodeId));
  
              if (createdNodes == null) {
                  createdNodes = new HashSet<>();
              }
  
              createdNodes.add(nodeId);
          }
  
          @Override
          public void nodeIdChanged(Object nodeId, Object newId) {
              registerDiff(new NodeIdChangeOperation(nodeId, newId));
          }
  
          @Override
          public void nodeRemoved(Object nodeId) {
  
              registerDiff(new NodeDeleteOperation(nodeId));
  
              if (deletedNodes == null) {
                  deletedNodes = new HashSet<>();
              }
  
              deletedNodes.add(nodeId);
          }
  
          @Override
          public void nodePropertyChanged(
                  Object nodeId,
                  String property,
                  Object oldValue,
                  Object newValue) {
  
              List<NodeDiff> diffs = diffsByNode.get(nodeId);
              if (diffs != null) {
  
                  for (int i = diffs.size() - 1; i >= 0; i--) {
                      NodeDiff diff = diffs.get(i);
                      if (diff instanceof NodePropertyChangeOperation) {
                          NodePropertyChangeOperation propertyDiff = (NodePropertyChangeOperation) diff;
                          if (property.equals(propertyDiff.getProperty())) {
                              propertyDiff.setNewValue(newValue);
                              return;
                          }
                      }
                  }
              }
  
              registerDiff(new NodePropertyChangeOperation(
                      nodeId,
                      property,
                      oldValue,
                      newValue));
          }
  
          private void registerDiff(NodeDiff diff) {
  
              compressed.add(diff);
              diffsByNode
                      .computeIfAbsent(diff.getNodeId(), k -> new ArrayList<>())
                      .add(diff);
          }
      }
  }
  