/*
    * Licensed to the Apache Software Foundation (ASF) under one or more
    * contributor license agreements.  See the NOTICE file distributed with
    * this work for additional information regarding copyright ownership.
    * The ASF licenses this file to You under the Apache 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.apache.org/licenses/LICENSE-2.0
   *
   * 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.apache.commons.configuration2.tree;
  
  import java.util.ArrayList;
  import java.util.Collection;
  import java.util.Collections;
  import java.util.HashMap;
  import java.util.HashSet;
  import java.util.LinkedList;
  import java.util.List;
  import java.util.Map;
  import java.util.Set;
  import java.util.SortedMap;
  import java.util.TreeMap;
  
  /**
   * <p>
   * An internal helper class for a atomic updates of an {@link InMemoryNodeModel}.
   * </p>
   * <p>
   * This class performs updates on the node structure of a node model consisting of {@link ImmutableNode} objects.
   * Because the nodes themselves cannot be changed updates are achieved by replacing parts of the structure with new
   * nodes; the new nodes are copies of original nodes with the corresponding manipulations applied. Therefore, each
   * update of a node in the structure results in a new structure in which the affected node is replaced by a new one, and
   * this change bubbles up to the root node (because all parent nodes have to be replaced by instances with an updated
   * child reference).
   * </p>
   * <p>
   * A single update of a model may consist of multiple changes on nodes. For instance, a remove property operation can
   * include many nodes. There are some reasons why such updates should be handled in a single "transaction" rather than
   * executing them on altered node structures one by one:
   * <ul>
   * <li>An operation is typically executed on a set of source nodes from the original node hierarchy. While manipulating
   * nodes, nodes of this set may be replaced by new ones. The handling of these replacements complicates things a
   * lot.</li>
   * <li>Performing all updates one after the other may cause more updates of nodes than necessary. Nodes near to the root
   * node always have to be replaced when a child of them gets manipulated. If all these updates are deferred and handled
   * in a single transaction, the resulting operation is more efficient.</li>
   * </ul>
   * </p>
   */
  final class ModelTransaction {
  
      /**
       * A specialized operation class for adding an attribute to a target node.
       */
      private static final class AddAttributeOperation extends Operation {
          /** The attribute name. */
          private final String attributeName;
  
          /** The attribute value. */
          private final Object attributeValue;
  
          /**
           * Creates a new instance of {@code AddAttributeOperation}.
           *
           * @param name the name of the attribute
           * @param value the value of the attribute
           */
          public AddAttributeOperation(final String name, final Object value) {
              attributeName = name;
              attributeValue = value;
          }
  
          @Override
          protected ImmutableNode apply(final ImmutableNode target, final Operations operations) {
              return target.setAttribute(attributeName, attributeValue);
          }
      }
  
      /**
       * A specialized operation class for adding multiple attributes to a target node.
       */
      private static final class AddAttributesOperation extends Operation {
          /** The map with attributes. */
          private final Map<String, Object> attributes;
  
          /**
           * Creates a new instance of {@code AddAttributesOperation}.
           *
           * @param attrs the map with attributes
           */
          public AddAttributesOperation(final Map<String, Object> attrs) {
              attributes = attrs;
         }
 
         @Override
         protected ImmutableNode apply(final ImmutableNode target, final Operations operations) {
             return target.setAttributes(attributes);
         }
     }
 
     /**
      * A specialized operation class which changes the name of a node.
      */
     private static final class ChangeNodeNameOperation extends Operation {
         /** The new node name. */
         private final String newName;
 
         /**
          * Creates a new instance of {@code ChangeNodeNameOperation} and sets the new node name.
          *
          * @param name the new node name
          */
         public ChangeNodeNameOperation(final String name) {
             newName = name;
         }
 
         @Override
         protected ImmutableNode apply(final ImmutableNode target, final Operations operations) {
             return target.setName(newName);
         }
     }
 
     /**
      * A specialized operation class which changes the value of a node.
      */
     private static final class ChangeNodeValueOperation extends Operation {
         /** The new value for the affected node. */
         private final Object newValue;
 
         /**
          * Creates a new instance of {@code ChangeNodeValueOperation} and initializes it with the new value to set for the node.
          *
          * @param value the new node value
          */
         public ChangeNodeValueOperation(final Object value) {
             newValue = value;
         }
 
         @Override
         protected ImmutableNode apply(final ImmutableNode target, final Operations operations) {
             return target.setValue(newValue);
         }
     }
 
     /**
      * A specialized {@code Operation} implementation for replacing the children of a target node. All other properties are
      * not touched. With this operation single children of a node can be altered or removed; new children can be added. This
      * operation is frequently used because each update of a node causes updates of the children of all parent nodes.
      * Therefore, it is treated in a special way and allows adding further sub operations dynamically.
      */
     private final class ChildrenUpdateOperation extends Operation {
         /** A collection with new nodes to be added. */
         private Collection<ImmutableNode> newNodes;
 
         /** A collection with nodes to be removed. */
         private Set<ImmutableNode> nodesToRemove;
 
         /**
          * A map with nodes to be replaced by others. The keys are the nodes to be replaced, the values the replacements.
          */
         private Map<ImmutableNode, ImmutableNode> nodesToReplace;
 
         /**
          * Adds a node to be added to the target of the operation.
          *
          * @param node the new node to be added
          */
         public void addNewNode(final ImmutableNode node) {
             newNodes = append(newNodes, node);
         }
 
         /**
          * Adds a collection of nodes to be added to the target of the operation.
          *
          * @param nodes the collection with new nodes
          */
         public void addNewNodes(final Collection<? extends ImmutableNode> nodes) {
             newNodes = concatenate(newNodes, nodes);
         }
 
         /**
          * Adds a node for a remove operation. This child node is going to be removed from its parent.
          *
          * @param node the child node to be removed
          */
         public void addNodeToRemove(final ImmutableNode node) {
             nodesToRemove = append(nodesToRemove, node);
         }
 
         /**
          * Adds a node for a replacement operation. The original node is going to be replaced by its replacement.
          *
          * @param org the original node
          * @param replacement the replacement node
          */
         public void addNodeToReplace(final ImmutableNode org, final ImmutableNode replacement) {
             nodesToReplace = append(nodesToReplace, org, replacement);
         }
 
         /**
          * {@inheritDoc} This implementation applies changes on the children of the passed in target node according to its
          * configuration: new nodes are added, replacements are performed, and nodes no longer needed are removed.
          */
         @Override
         protected ImmutableNode apply(final ImmutableNode target, final Operations operations) {
             final Map<ImmutableNode, ImmutableNode> replacements = fetchReplacementMap();
             final Set<ImmutableNode> removals = fetchRemovalSet();
             final List<ImmutableNode> resultNodes = new LinkedList<>();
 
             for (final ImmutableNode nd : target) {
                 final ImmutableNode repl = replacements.get(nd);
                 if (repl != null) {
                     resultNodes.add(repl);
                     replacedNodes.put(nd, repl);
                 } else if (removals.contains(nd)) {
                     removedNodes.add(nd);
                 } else {
                     resultNodes.add(nd);
                 }
             }
 
             concatenate(resultNodes, newNodes);
             operations.newNodesAdded(newNodes);
             return target.replaceChildren(resultNodes);
         }
 
         /**
          * Adds all operations defined by the specified object to this instance.
          *
          * @param op the operation to be combined
          */
         public void combine(final ChildrenUpdateOperation op) {
             newNodes = concatenate(newNodes, op.newNodes);
             nodesToReplace = concatenate(nodesToReplace, op.nodesToReplace);
             nodesToRemove = concatenate(nodesToRemove, op.nodesToRemove);
         }
 
         /**
          * Returns a set with nodes to be removed. If no remove operations are pending, an empty set is returned.
          *
          * @return the set with nodes to be removed
          */
         private Set<ImmutableNode> fetchRemovalSet() {
             return nodesToRemove != null ? nodesToRemove : Collections.<ImmutableNode>emptySet();
         }
 
         /**
          * Obtains the map with replacement nodes. If no replacements are defined, an empty map is returned.
          *
          * @return the map with replacement nodes
          */
         private Map<ImmutableNode, ImmutableNode> fetchReplacementMap() {
             return nodesToReplace != null ? nodesToReplace : Collections.<ImmutableNode, ImmutableNode>emptyMap();
         }
     }
 
     /**
      * An abstract base class representing an operation to be performed on a node. Concrete subclasses implement specific
      * update operations.
      */
     private abstract static class Operation {
         /**
          * Executes this operation on the provided target node returning the result.
          *
          * @param target the target node for this operation
          * @param operations the current {@code Operations} instance
          * @return the manipulated node
          */
         protected abstract ImmutableNode apply(ImmutableNode target, Operations operations);
     }
 
     /**
      * A helper class which collects multiple update operations to be executed on a single node.
      */
     private final class Operations {
         /** An operation for manipulating child nodes. */
         private ChildrenUpdateOperation childrenOperation;
 
         /**
          * A collection for the other operations to be performed on the target node.
          */
         private Collection<Operation> operations;
 
         /** A collection with nodes added by an operation. */
         private Collection<ImmutableNode> addedNodesInOperation;
 
         /**
          * Adds an operation which manipulates children.
          *
          * @param co the operation
          */
         public void addChildrenOperation(final ChildrenUpdateOperation co) {
             if (childrenOperation == null) {
                 childrenOperation = co;
             } else {
                 childrenOperation.combine(co);
             }
         }
 
         /**
          * Adds an operation.
          *
          * @param op the operation
          */
         public void addOperation(final Operation op) {
             operations = append(operations, op);
         }
 
         /**
          * Executes all operations stored in this object on the given target node. The resulting node then has to be integrated
          * in the current node hierarchy. Unless the root node is already reached, this causes another updated operation to be
          * created which replaces the manipulated child in the parent node.
          *
          * @param target the target node for this operation
          * @param level the level of the target node
          */
         public void apply(final ImmutableNode target, final int level) {
             ImmutableNode node = target;
             if (childrenOperation != null) {
                 node = childrenOperation.apply(node, this);
             }
 
             if (operations != null) {
                 for (final Operation op : operations) {
                     node = op.apply(node, this);
                 }
             }
 
             handleAddedNodes(node);
             if (level == 0) {
                 // reached the root node
                 newRoot = node;
                 replacedNodes.put(target, node);
             } else {
                 // propagate change
                 propagateChange(target, node, level);
             }
         }
 
         /**
          * Checks whether new nodes have been added during operation execution. If so, the parent mapping has to be updated.
          *
          * @param node the resulting node after applying all operations
          */
         private void handleAddedNodes(final ImmutableNode node) {
             if (addedNodesInOperation != null) {
                 addedNodesInOperation.forEach(child -> {
                     parentMapping.put(child, node);
                     addedNodes.add(child);
                 });
             }
         }
 
         /**
          * Notifies this object that new nodes have been added by a sub operation. It has to be ensured that these nodes are
          * added to the parent mapping.
          *
          * @param newNodes the collection of newly added nodes
          */
         public void newNodesAdded(final Collection<ImmutableNode> newNodes) {
             addedNodesInOperation = concatenate(addedNodesInOperation, newNodes);
         }
 
         /**
          * Propagates the changes on the target node to the next level above of the hierarchy. If the updated node is no longer
          * defined, it can even be removed from its parent. Otherwise, it is just replaced.
          *
          * @param target the target node for this operation
          * @param node the resulting node after applying all operations
          * @param level the level of the target node
          */
         private void propagateChange(final ImmutableNode target, final ImmutableNode node, final int level) {
             final ImmutableNode parent = getParent(target);
             final ChildrenUpdateOperation co = new ChildrenUpdateOperation();
             if (InMemoryNodeModel.checkIfNodeDefined(node)) {
                 co.addNodeToReplace(target, node);
             } else {
                 co.addNodeToRemove(target);
             }
             fetchOperations(parent, level - 1).addChildrenOperation(co);
         }
     }
 
     /**
      * A specialized operation class for removing an attribute from a target node.
      */
     private static final class RemoveAttributeOperation extends Operation {
         /** The attribute name. */
         private final String attributeName;
 
         /**
          * Creates a new instance of {@code RemoveAttributeOperation}.
          *
          * @param name the name of the attribute
          */
         public RemoveAttributeOperation(final String name) {
             attributeName = name;
         }
 
         @Override
         protected ImmutableNode apply(final ImmutableNode target, final Operations operations) {
             return target.removeAttribute(attributeName);
         }
     }
 
     /**
      * Constant for the maximum number of entries in the replacement mapping. If this number is exceeded, the parent mapping
      * is reconstructed. The number is a bit arbitrary. If it is too low, updates - especially on large node structures -
      * are expensive because the parent mapping is often rebuild. If it is too big, read access to the model is slowed down
      * because looking up the parent of a node is more complicated.
      */
     private static final int MAX_REPLACEMENTS = 200;
 
     /** Constant for an unknown level. */
     private static final int LEVEL_UNKNOWN = -1;
 
     /**
      * Appends a single element to a collection. The collection may be null, then it is created.
      *
      * @param col the collection
      * @param node the element to be added
      * @param <E> the type of elements involved
      * @return the resulting collection
      */
     private static <E> Collection<E> append(final Collection<E> col, final E node) {
         final Collection<E> result = col != null ? col : new LinkedList<>();
         result.add(node);
         return result;
     }
 
     /**
      * Adds a single key-value pair to a map. The map may be null, then it is created.
      *
      * @param map the map
      * @param key the key
      * @param value the value
      * @param <K> the type of the key
      * @param <V> the type of the value
      * @return the resulting map
      */
     private static <K, V> Map<K, V> append(final Map<K, V> map, final K key, final V value) {
         final Map<K, V> result = map != null ? map : new HashMap<>();
         result.put(key, value);
         return result;
     }
 
     /**
      * Appends a single element to a set. The set may be null then it is created.
      *
      * @param col the set
      * @param elem the element to be added
      * @param <E> the type of the elements involved
      * @return the resulting set
      */
     private static <E> Set<E> append(final Set<E> col, final E elem) {
         final Set<E> result = col != null ? col : new HashSet<>();
         result.add(elem);
         return result;
     }
 
     /**
      * Constructs the concatenation of two collections. Both can be null.
      *
      * @param col1 the first collection
      * @param col2 the second collection
      * @param <E> the type of the elements involved
      * @return the resulting collection
      */
     private static <E> Collection<E> concatenate(final Collection<E> col1, final Collection<? extends E> col2) {
         if (col2 == null) {
             return col1;
         }
 
         final Collection<E> result = col1 != null ? col1 : new ArrayList<>(col2.size());
         result.addAll(col2);
         return result;
     }
 
     /**
      * Constructs the concatenation of two maps. Both can be null.
      *
      * @param map1 the first map
      * @param map2 the second map
      * @param <K> the type of the keys
      * @param <V> the type of the values
      * @return the resulting map
      */
     private static <K, V> Map<K, V> concatenate(final Map<K, V> map1, final Map<? extends K, ? extends V> map2) {
         if (map2 == null) {
             return map1;
         }
 
         final Map<K, V> result = map1 != null ? map1 : new HashMap<>();
         result.putAll(map2);
         return result;
     }
 
     /**
      * Constructs the concatenation of two sets. Both can be null.
      *
      * @param set1 the first set
      * @param set2 the second set
      * @param <E> the type of the elements involved
      * @return the resulting set
      */
     private static <E> Set<E> concatenate(final Set<E> set1, final Set<? extends E> set2) {
         if (set2 == null) {
             return set1;
         }
 
         final Set<E> result = set1 != null ? set1 : new HashSet<>();
         result.addAll(set2);
         return result;
     }
 
     /** Stores the current tree data of the calling node model. */
     private final TreeData currentData;
 
     /** The root node for query operations. */
     private final ImmutableNode queryRoot;
 
     /** The selector to the root node of this transaction. */
     private final NodeSelector rootNodeSelector;
 
     /** The {@code NodeKeyResolver} to be used for this transaction. */
     private final NodeKeyResolver<ImmutableNode> resolver;
 
     /** A new replacement mapping. */
     private final Map<ImmutableNode, ImmutableNode> replacementMapping;
 
     /** The nodes replaced in this transaction. */
     private final Map<ImmutableNode, ImmutableNode> replacedNodes;
 
     /** A new parent mapping. */
     private final Map<ImmutableNode, ImmutableNode> parentMapping;
 
     /** A collection with nodes which have been added. */
     private final Collection<ImmutableNode> addedNodes;
 
     /** A collection with nodes which have been removed. */
     private final Collection<ImmutableNode> removedNodes;
 
     /**
      * Stores all nodes which have been removed in this transaction (not only the root nodes of removed trees).
      */
     private final Collection<ImmutableNode> allRemovedNodes;
 
     /**
      * Stores the operations to be executed during this transaction. The map is sorted by the levels of the nodes to be
      * manipulated: Operations on nodes down in the hierarchy are executed first because they affect the nodes closer to the
      * root.
      */
     private final SortedMap<Integer, Map<ImmutableNode, Operations>> operations;
 
     /** A map with reference objects to be added during this transaction. */
     private Map<ImmutableNode, Object> newReferences;
 
     /** The new root node. */
     private ImmutableNode newRoot;
 
     /**
      * Creates a new instance of {@code ModelTransaction} for the current tree data.
      *
      * @param treeData the current {@code TreeData} structure to operate on
      * @param selector an optional {@code NodeSelector} defining the target root node for this transaction; this can be used
      *        to perform operations on tracked nodes
      * @param resolver the {@code NodeKeyResolver}
      */
     public ModelTransaction(final TreeData treeData, final NodeSelector selector, final NodeKeyResolver<ImmutableNode> resolver) {
         currentData = treeData;
         this.resolver = resolver;
         replacementMapping = getCurrentData().copyReplacementMapping();
         replacedNodes = new HashMap<>();
         parentMapping = getCurrentData().copyParentMapping();
         operations = new TreeMap<>();
         addedNodes = new LinkedList<>();
         removedNodes = new LinkedList<>();
         allRemovedNodes = new LinkedList<>();
         queryRoot = initQueryRoot(treeData, selector);
         rootNodeSelector = selector;
     }
 
     /**
      * Adds an operation for adding a new child to a given parent node.
      *
      * @param parent the parent node
      * @param newChild the new child to be added
      */
     public void addAddNodeOperation(final ImmutableNode parent, final ImmutableNode newChild) {
         final ChildrenUpdateOperation op = new ChildrenUpdateOperation();
         op.addNewNode(newChild);
         fetchOperations(parent, LEVEL_UNKNOWN).addChildrenOperation(op);
     }
 
     /**
      * Adds an operation for adding a number of new children to a given parent node.
      *
      * @param parent the parent node
      * @param newNodes the collection of new child nodes
      */
     public void addAddNodesOperation(final ImmutableNode parent, final Collection<? extends ImmutableNode> newNodes) {
         final ChildrenUpdateOperation op = new ChildrenUpdateOperation();
         op.addNewNodes(newNodes);
         fetchOperations(parent, LEVEL_UNKNOWN).addChildrenOperation(op);
     }
 
     /**
      * Adds an operation for adding an attribute to a target node.
      *
      * @param target the target node
      * @param name the name of the attribute
      * @param value the value of the attribute
      */
     public void addAttributeOperation(final ImmutableNode target, final String name, final Object value) {
         fetchOperations(target, LEVEL_UNKNOWN).addOperation(new AddAttributeOperation(name, value));
     }
 
     /**
      * Adds an operation for adding multiple attributes to a target node.
      *
      * @param target the target node
      * @param attributes the map with attributes to be set
      */
     public void addAttributesOperation(final ImmutableNode target, final Map<String, Object> attributes) {
         fetchOperations(target, LEVEL_UNKNOWN).addOperation(new AddAttributesOperation(attributes));
     }
 
     /**
      * Adds an operation for changing the name of a target node.
      *
      * @param target the target node
      * @param newName the new name for this node
      */
     public void addChangeNodeNameOperation(final ImmutableNode target, final String newName) {
         fetchOperations(target, LEVEL_UNKNOWN).addOperation(new ChangeNodeNameOperation(newName));
     }
 
     /**
      * Adds an operation for changing the value of a target node.
      *
      * @param target the target node
      * @param newValue the new value for this node
      */
     public void addChangeNodeValueOperation(final ImmutableNode target, final Object newValue) {
         fetchOperations(target, LEVEL_UNKNOWN).addOperation(new ChangeNodeValueOperation(newValue));
     }
 
     /**
      * Adds an operation for clearing the value of a target node.
      *
      * @param target the target node
      */
     public void addClearNodeValueOperation(final ImmutableNode target) {
         addChangeNodeValueOperation(target, null);
     }
 
     /**
      * Adds a new reference object for the given node.
      *
      * @param node the affected node
      * @param ref the reference object for this node
      */
     public void addNewReference(final ImmutableNode node, final Object ref) {
         fetchReferenceMap().put(node, ref);
     }
 
     /**
      * Adds a map with new reference objects. The entries in this map are passed to the {@code ReferenceTracker} during
      * execution of this transaction.
      *
      * @param refs the map with new reference objects
      */
     public void addNewReferences(final Map<ImmutableNode, ?> refs) {
         fetchReferenceMap().putAll(refs);
     }
 
     /**
      * Adds an operation for removing an attribute from a target node.
      *
      * @param target the target node
      * @param name the name of the attribute
      */
     public void addRemoveAttributeOperation(final ImmutableNode target, final String name) {
         fetchOperations(target, LEVEL_UNKNOWN).addOperation(new RemoveAttributeOperation(name));
     }
 
     /**
      * Adds an operation for removing a child node of a given node.
      *
      * @param parent the parent node
      * @param node the child node to be removed
      */
     public void addRemoveNodeOperation(final ImmutableNode parent, final ImmutableNode node) {
         final ChildrenUpdateOperation op = new ChildrenUpdateOperation();
         op.addNodeToRemove(node);
         fetchOperations(parent, LEVEL_UNKNOWN).addChildrenOperation(op);
     }
 
     /**
      * Executes this transaction resulting in a new {@code TreeData} object. The object returned by this method serves as
      * the definition of a new node structure for the calling model.
      *
      * @return the updated {@code TreeData}
      */
     public TreeData execute() {
         executeOperations();
         updateParentMapping();
         return new TreeData(newRoot, parentMapping, replacementMapping,
             currentData.getNodeTracker().update(newRoot, rootNodeSelector, getResolver(), getCurrentData()), updateReferenceTracker());
     }
 
     /**
      * Executes all operations in this transaction.
      */
     private void executeOperations() {
         while (!operations.isEmpty()) {
             final Integer level = operations.lastKey(); // start down in hierarchy
             operations.remove(level).forEach((k, v) -> v.apply(k, level));
         }
     }
 
     /**
      * Obtains the {@code Operations} object for manipulating the specified node. If no such object exists yet, it is
      * created. The level can be undefined, then it is determined based on the target node.
      *
      * @param target the target node
      * @param level the level of the target node (may be undefined)
      * @return the {@code Operations} object for this node
      */
     Operations fetchOperations(final ImmutableNode target, final int level) {
         final Integer nodeLevel = Integer.valueOf(level == LEVEL_UNKNOWN ? level(target) : level);
         final Map<ImmutableNode, Operations> levelOperations = operations.computeIfAbsent(nodeLevel, k -> new HashMap<>());
         return levelOperations.computeIfAbsent(target, k -> new Operations());
     }
 
     /**
      * Returns the map with new reference objects. It is created if necessary.
      *
      * @return the map with reference objects
      */
     private Map<ImmutableNode, Object> fetchReferenceMap() {
         if (newReferences == null) {
             newReferences = new HashMap<>();
         }
         return newReferences;
     }
 
     /**
      * Gets the current {@code TreeData} object this transaction operates on.
      *
      * @return the associated {@code TreeData} object
      */
     public TreeData getCurrentData() {
         return currentData;
     }
 
     /**
      * Gets the parent node of the given node.
      *
      * @param node the node in question
      * @return the parent of this node
      */
     ImmutableNode getParent(final ImmutableNode node) {
         return getCurrentData().getParent(node);
     }
 
     /**
      * Gets the root node to be used within queries. This is not necessarily the current root node of the model. If the
      * operation is executed on a tracked node, this node has to be passed as root nodes to the expression engine.
      *
      * @return the root node for queries and calls to the expression engine
      */
     public ImmutableNode getQueryRoot() {
         return queryRoot;
     }
 
     /**
      * Gets the {@code NodeKeyResolver} used by this transaction.
      *
      * @return the {@code NodeKeyResolver}
      */
     public NodeKeyResolver<ImmutableNode> getResolver() {
         return resolver;
     }
 
     /**
      * Initializes the root node to be used within queries. If a tracked node selector is provided, this node becomes the
      * root node. Otherwise, the actual root node is used.
      *
      * @param treeData the current data of the model
      * @param selector an optional {@code NodeSelector} defining the target root
      * @return the query root node for this transaction
      */
     private ImmutableNode initQueryRoot(final TreeData treeData, final NodeSelector selector) {
         return selector == null ? treeData.getRootNode() : treeData.getNodeTracker().getTrackedNode(selector);
     }
 
     /**
      * Determines the level of the specified node in the current hierarchy. The level of the root node is 0, the children of
      * the root have level 1 and so on.
      *
      * @param node the node in question
      * @return the level of this node
      */
     private int level(final ImmutableNode node) {
         ImmutableNode current = getCurrentData().getParent(node);
         int level = 0;
         while (current != null) {
             level++;
             current = getCurrentData().getParent(current);
         }
         return level;
     }
 
     /**
      * Rebuilds the parent mapping from scratch. This method is called if the replacement mapping exceeds its maximum size.
      * In this case, it is cleared, and a new parent mapping is constructed for the new root node.
      */
     private void rebuildParentMapping() {
         replacementMapping.clear();
         parentMapping.clear();
         InMemoryNodeModel.updateParentMapping(parentMapping, newRoot);
     }
 
     /**
      * Removes the specified node completely from the replacement mapping. This also includes the nodes that replace the
      * given one.
      *
      * @param node the node to be removed
      */
     private void removeNodeFromReplacementMapping(final ImmutableNode node) {
         ImmutableNode replacement = node;
         do {
             replacement = replacementMapping.remove(replacement);
         } while (replacement != null);
     }
 
     /**
      * Removes a node and its children (recursively) from the parent and the replacement mappings.
      *
      * @param root the root of the subtree to be removed
      */
     private void removeNodesFromParentAndReplacementMapping(final ImmutableNode root) {
         NodeTreeWalker.INSTANCE.walkBFS(root, new ConfigurationNodeVisitorAdapter<ImmutableNode>() {
             @Override
             public void visitBeforeChildren(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
                 allRemovedNodes.add(node);
                 parentMapping.remove(node);
                 removeNodeFromReplacementMapping(node);
             }
         }, getCurrentData());
     }
 
     /**
      * Updates the parent mapping for the resulting {@code TreeData} instance. This method is called after all update
      * operations have been executed. It ensures that the parent mapping is updated for the changes on the nodes structure.
      */
     private void updateParentMapping() {
         replacementMapping.putAll(replacedNodes);
         if (replacementMapping.size() > MAX_REPLACEMENTS) {
             rebuildParentMapping();
         } else {
             updateParentMappingForAddedNodes();
             updateParentMappingForRemovedNodes();
         }
     }
 
     /**
      * Adds newly added nodes and their children to the parent mapping.
      */
     private void updateParentMappingForAddedNodes() {
         addedNodes.forEach(node -> InMemoryNodeModel.updateParentMapping(parentMapping, node));
     }
 
     /**
      * Removes nodes that have been removed during this transaction from the parent and replacement mappings.
      */
     private void updateParentMappingForRemovedNodes() {
         removedNodes.forEach(this::removeNodesFromParentAndReplacementMapping);
     }
 
     /**
      * Returns an updated {@code ReferenceTracker} instance. The changes performed during this transaction are applied to
      * the tracker.
      *
      * @return the updated tracker instance
      */
     private ReferenceTracker updateReferenceTracker() {
         ReferenceTracker tracker = currentData.getReferenceTracker();
         if (newReferences != null) {
             tracker = tracker.addReferences(newReferences);
         }
         return tracker.updateReferences(replacedNodes, allRemovedNodes);
     }
 }