/*
 * 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.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.concurrent.atomic.AtomicReference;

import org.apache.commons.configuration2.ex.ConfigurationRuntimeException;
import org.apache.commons.lang3.mutable.Mutable;
import org.apache.commons.lang3.mutable.MutableObject;

/**
 * <p>
 * A specialized node model implementation which operates on {@link ImmutableNode} structures.
 * </p>
 * <p>
 * This {@code NodeModel} implementation keeps all its data as a tree of {@link ImmutableNode} objects in memory. The
 * managed structure can be manipulated in a thread-safe, non-blocking way. This is achieved by using atomic variables:
 * The root of the tree is stored in an atomic reference variable. Each update operation causes a new structure to be
 * constructed (which reuses as much from the original structure as possible). The old root node is then replaced by the
 * new one using an atomic compare-and-set operation. If this fails, the manipulation has to be done anew on the updated
 * structure.
 * </p>
 *
 * @since 2.0
 */
public class InMemoryNodeModel implements NodeModel<ImmutableNode> {
    /**
     * An interface used internally for handling concurrent updates. An implementation has to populate the passed in
     * {@code ModelTransaction}. The transaction is then executed, and an atomic update of the model's {@code TreeData} is
     * attempted. If this fails - because another update came across -, the whole operation has to be tried anew.
     */
    private interface TransactionInitializer {
        /**
         * Initializes the specified transaction for an update operation. The return value indicates whether the transaction
         * should be executed. A result of <b>false</b> means that the update is to be aborted (maybe another update method was
         * called).
         *
         * @param tx the transaction to be initialized
         * @return a flag whether the update should continue
         */
        boolean initTransaction(ModelTransaction tx);
    }

    /**
     * A dummy node handler instance used in operations which require only a limited functionality.
     */
    private static final NodeHandler<ImmutableNode> DUMMY_HANDLER = new TreeData(null, Collections.<ImmutableNode, ImmutableNode>emptyMap(),
        Collections.<ImmutableNode, ImmutableNode>emptyMap(), null, new ReferenceTracker());

    /**
     * Handles an add property operation if the property to be added is an attribute.
     *
     * @param tx the transaction
     * @param addData the {@code NodeAddData}
     * @param values the collection with node values
     */
    private static void addAttributeProperty(final ModelTransaction tx, final NodeAddData<ImmutableNode> addData, final Iterable<?> values) {
        if (addData.getPathNodes().isEmpty()) {
            tx.addAttributeOperation(addData.getParent(), addData.getNewNodeName(), values.iterator().next());
        } else {
            final int pathNodeCount = addData.getPathNodes().size();
            final ImmutableNode childWithAttribute = new ImmutableNode.Builder().name(addData.getPathNodes().get(pathNodeCount - 1))
                .addAttribute(addData.getNewNodeName(), values.iterator().next()).create();
            final ImmutableNode newChild = pathNodeCount > 1
                ? createNodeOnPath(addData.getPathNodes().subList(0, pathNodeCount - 1).iterator(), Collections.singleton(childWithAttribute))
                : childWithAttribute;
            tx.addAddNodeOperation(addData.getParent(), newChild);
        }
    }

    /**
     * Handles an add property operation if the property to be added is a node.
     *
     * @param tx the transaction
     * @param addData the {@code NodeAddData}
     * @param values the collection with node values
     */
    private static void addNodeProperty(final ModelTransaction tx, final NodeAddData<ImmutableNode> addData, final Iterable<?> values) {
        final Collection<ImmutableNode> newNodes = createNodesToAdd(addData.getNewNodeName(), values);
        addNodesByAddData(tx, addData, newNodes);
    }

    /**
     * Initializes a transaction to add a collection of nodes as described by a {@code NodeAddData} object. If necessary,
     * new path nodes are created. Eventually, the new nodes are added as children to the specified target node.
     *
     * @param tx the transaction
     * @param addData the {@code NodeAddData}
     * @param newNodes the collection of new child nodes
     */
    private static void addNodesByAddData(final ModelTransaction tx, final NodeAddData<ImmutableNode> addData, final Collection<ImmutableNode> newNodes) {
        if (addData.getPathNodes().isEmpty()) {
            tx.addAddNodesOperation(addData.getParent(), newNodes);
        } else {
            final ImmutableNode newChild = createNodeToAddWithPath(addData, newNodes);
            tx.addAddNodeOperation(addData.getParent(), newChild);
        }
    }

    /**
     * Creates an exception referring to an invalid key for adding properties. Such an exception is thrown when an operation
     * tries to add something to an attribute.
     *
     * @param key the invalid key causing this exception
     * @return the exception
     */
    private static IllegalArgumentException attributeKeyException(final String key) {
        return new IllegalArgumentException("New nodes cannot be added to an attribute key: " + key);
    }

    /**
     * Checks if the passed in node is defined. Result is <b>true</b> if the node contains any data.
     *
     * @param node the node in question
     * @return <b>true</b> if the node is defined, <b>false</b> otherwise
     */
    static boolean checkIfNodeDefined(final ImmutableNode node) {
        return node.getValue() != null || !node.getChildren().isEmpty() || !node.getAttributes().isEmpty();
    }

    /**
     * Creates a new data object with a tracked child node of the given parent node. If such a child node already exists, it
     * is used. Otherwise, a new one is created.
     *
     * @param current the current {@code TreeData} object
     * @param parent the parent node
     * @param childName the name of the child node
     * @param resolver the {@code NodeKeyResolver}
     * @param refSelector here the newly created {@code NodeSelector} is returned
     * @return the new {@code TreeData} instance
     */
    private static TreeData createDataWithTrackedChildNode(final TreeData current, final ImmutableNode parent, final String childName,
        final NodeKeyResolver<ImmutableNode> resolver, final MutableObject<NodeSelector> refSelector) {
        final TreeData newData;
        final List<ImmutableNode> namedChildren = current.getChildren(parent, childName);
        if (!namedChildren.isEmpty()) {
            newData = updateDataWithNewTrackedNode(current, namedChildren.get(0), resolver, refSelector);
        } else {
            final ImmutableNode child = new ImmutableNode.Builder().name(childName).create();
            final ModelTransaction tx = new ModelTransaction(current, null, resolver);
            tx.addAddNodeOperation(parent, child);
            newData = updateDataWithNewTrackedNode(tx.execute(), child, resolver, refSelector);
        }
        return newData;
    }

    /**
     * Recursive helper method for creating a path node for an add operation. All path nodes except for the last have a
     * single child. The last path node has the new nodes as children.
     *
     * @param it the iterator over the names of the path nodes
     * @param newNodes the collection of new child nodes
     * @return the newly created path node
     */
    private static ImmutableNode createNodeOnPath(final Iterator<String> it, final Collection<ImmutableNode> newNodes) {
        final String nodeName = it.next();
        final ImmutableNode.Builder builder;
        if (it.hasNext()) {
            builder = new ImmutableNode.Builder(1);
            builder.addChild(createNodeOnPath(it, newNodes));
        } else {
            builder = new ImmutableNode.Builder(newNodes.size());
            builder.addChildren(newNodes);
        }
        return builder.name(nodeName).create();
    }

    /**
     * Creates a collection with new nodes with a given name and a value from a given collection.
     *
     * @param newNodeName the name of the new nodes
     * @param values the collection with node values
     * @return the newly created collection
     */
    private static Collection<ImmutableNode> createNodesToAdd(final String newNodeName, final Iterable<?> values) {
        final Collection<ImmutableNode> nodes = new LinkedList<>();
        values.forEach(value -> nodes.add(new ImmutableNode.Builder().name(newNodeName).value(value).create()));
        return nodes;
    }

    /**
     * Creates a node structure consisting of the path nodes defined by the passed in {@code NodeAddData} instance and all
     * new child nodes.
     *
     * @param addData the {@code NodeAddData}
     * @param newNodes the collection of new child nodes
     * @return the parent node of the newly created hierarchy
     */
    private static ImmutableNode createNodeToAddWithPath(final NodeAddData<ImmutableNode> addData, final Collection<ImmutableNode> newNodes) {
        return createNodeOnPath(addData.getPathNodes().iterator(), newNodes);
    }

    /**
     * Creates tracked node entries for the specified nodes and creates the corresponding selectors.
     *
     * @param refSelectors the reference where to store the selectors
     * @param nodes the nodes to be tracked
     * @param current the current {@code TreeData} object
     * @param resolver the {@code NodeKeyResolver}
     * @return the updated {@code TreeData} object
     */
    private static TreeData createSelectorsForTrackedNodes(final Mutable<Collection<NodeSelector>> refSelectors, final List<ImmutableNode> nodes,
            final TreeData current, final NodeKeyResolver<ImmutableNode> resolver) {
        final List<NodeSelector> selectors = new ArrayList<>(nodes.size());
        final Map<ImmutableNode, String> cache = new HashMap<>();
        nodes.forEach(node -> selectors.add(new NodeSelector(resolver.nodeKey(node, cache, current))));
        refSelectors.setValue(selectors);
        final NodeTracker newTracker = current.getNodeTracker().trackNodes(selectors, nodes);
        return current.updateNodeTracker(newTracker);
    }

    /**
     * Determines the name of the root node for a merge operation. If a root name is provided, it is used. Otherwise, if the
     * current root node has no name, the name of the node to be merged is used. A result of <b>null</b> means that no node
     * name has to be set.
     *
     * @param rootNode the current root node
     * @param node the node to be merged with the root node
     * @param rootName the name of the resulting node
     * @return the new name of the root node
     */
    private static String determineRootName(final ImmutableNode rootNode, final ImmutableNode node, final String rootName) {
        if (rootName != null) {
            return rootName;
        }
        if (rootNode.getNodeName() == null) {
            return node.getNodeName();
        }
        return null;
    }

    /**
     * Initializes a transaction to clear the values of a property based on the passed in collection of affected results.
     *
     * @param tx the transaction to be initialized
     * @param results a collection with results pointing to the nodes to be cleared
     * @return a flag whether there are elements to be cleared
     */
    private static boolean initializeClearTransaction(final ModelTransaction tx, final Collection<QueryResult<ImmutableNode>> results) {
        results.forEach(result -> {
            if (result.isAttributeResult()) {
                tx.addRemoveAttributeOperation(result.getNode(), result.getAttributeName());
            } else {
                tx.addClearNodeValueOperation(result.getNode());
            }
        });

        return !results.isEmpty();
    }

    /**
     * Initializes a transaction to change the values of some query results based on the passed in map.
     *
     * @param tx the transaction to be initialized
     * @param changedValues the map defining the elements to be changed
     * @return a flag whether there are elements to be updated
     */
    private static boolean initializeUpdateTransaction(final ModelTransaction tx, final Map<QueryResult<ImmutableNode>, Object> changedValues) {
        changedValues.forEach((k, v) -> {
            final ImmutableNode node = k.getNode();
            if (k.isAttributeResult()) {
                tx.addAttributeOperation(node, k.getAttributeName(), v);
            } else {
                tx.addChangeNodeValueOperation(node, v);
            }
        });

        return !changedValues.isEmpty();
    }

    /**
     * Determines the initial root node of this model. If a root node has been provided, it is used. Otherwise, an empty
     * dummy root node is created.
     *
     * @param providedRoot the passed in root node
     * @return the root node to be used
     */
    private static ImmutableNode initialRootNode(final ImmutableNode providedRoot) {
        return providedRoot != null ? providedRoot : new ImmutableNode.Builder().create();
    }

    /**
     * Adds a tracked node that has already been resolved to the specified data object.
     *
     * @param current the current {@code TreeData} object
     * @param node the node in question
     * @param resolver the {@code NodeKeyResolver}
     * @param refSelector here the newly created {@code NodeSelector} is returned
     * @return the new {@code TreeData} instance
     */
    private static TreeData updateDataWithNewTrackedNode(final TreeData current, final ImmutableNode node, final NodeKeyResolver<ImmutableNode> resolver,
        final MutableObject<NodeSelector> refSelector) {
        final NodeSelector selector = new NodeSelector(resolver.nodeKey(node, new HashMap<>(), current));
        refSelector.setValue(selector);
        final NodeTracker newTracker = current.getNodeTracker().trackNodes(Collections.singleton(selector), Collections.singleton(node));
        return current.updateNodeTracker(newTracker);
    }

    /**
     * Updates the mapping from nodes to their parents for the passed in hierarchy of nodes. This method traverses all
     * children and grand-children of the passed in root node. For each node in the subtree the parent relation is added to
     * the map.
     *
     * @param parents the map with parent nodes
     * @param root the root node of the current tree
     */
    static void updateParentMapping(final Map<ImmutableNode, ImmutableNode> parents, final ImmutableNode root) {
        NodeTreeWalker.INSTANCE.walkBFS(root, new ConfigurationNodeVisitorAdapter<ImmutableNode>() {
            @Override
            public void visitBeforeChildren(final ImmutableNode node, final NodeHandler<ImmutableNode> handler) {
                node.forEach(c -> parents.put(c, node));
            }
        }, DUMMY_HANDLER);
    }

    /**
     * Checks whether the specified collection with values is not empty.
     *
     * @param values the collection with node values
     * @return <b>true</b> if values are provided, <b>false</b> otherwise
     */
    private static boolean valuesNotEmpty(final Iterable<?> values) {
        return values.iterator().hasNext();
    }

    /** Stores information about the current nodes structure. */
    private final AtomicReference<TreeData> structure;

    /**
     * Creates a new instance of {@code InMemoryNodeModel} which is initialized with an empty root node.
     */
    public InMemoryNodeModel() {
        this(null);
    }

    /**
     * Creates a new instance of {@code InMemoryNodeModel} and initializes it from the given root node. If the passed in
     * node is <b>null</b>, a new, empty root node is created.
     *
     * @param root the new root node for this model
     */
    public InMemoryNodeModel(final ImmutableNode root) {
        structure = new AtomicReference<>(createTreeData(initialRootNode(root), null));
    }

    @Override
    public void addNodes(final String key, final Collection<? extends ImmutableNode> nodes, final NodeKeyResolver<ImmutableNode> resolver) {
        addNodes(key, null, nodes, resolver);
    }

    /**
     * Adds new nodes using a tracked node as root node. This method works like the normal {@code addNodes()} method, but
     * the origin of the operation (also for the interpretation of the passed in key) is a tracked node identified by the
     * passed in {@code NodeSelector}. The selector can be <b>null</b>, then the root node is assumed.
     *
     * @param key the key
     * @param selector the {@code NodeSelector} defining the root node (or <b>null</b>)
     * @param nodes the collection of new nodes to be added
     * @param resolver the {@code NodeKeyResolver}
     * @throws ConfigurationRuntimeException if the selector cannot be resolved
     */
    public void addNodes(final String key, final NodeSelector selector, final Collection<? extends ImmutableNode> nodes,
        final NodeKeyResolver<ImmutableNode> resolver) {
        if (nodes != null && !nodes.isEmpty()) {
            updateModel(tx -> {
                final List<QueryResult<ImmutableNode>> results = resolver.resolveKey(tx.getQueryRoot(), key, tx.getCurrentData());
                if (results.size() == 1) {
                    if (results.get(0).isAttributeResult()) {
                        throw attributeKeyException(key);
                    }
                    tx.addAddNodesOperation(results.get(0).getNode(), nodes);
                } else {
                    final NodeAddData<ImmutableNode> addData = resolver.resolveAddKey(tx.getQueryRoot(), key, tx.getCurrentData());
                    if (addData.isAttribute()) {
                        throw attributeKeyException(key);
                    }
                    final ImmutableNode newNode = new ImmutableNode.Builder(nodes.size()).name(addData.getNewNodeName()).addChildren(nodes).create();
                    addNodesByAddData(tx, addData, Collections.singleton(newNode));
                }
                return true;
            }, selector, resolver);
        }
    }

    @Override
    public void addProperty(final String key, final Iterable<?> values, final NodeKeyResolver<ImmutableNode> resolver) {
        addProperty(key, null, values, resolver);
    }

    /**
     * Adds new property values using a tracked node as root node. This method works like the normal {@code addProperty()}
     * method, but the origin of the operation (also for the interpretation of the passed in key) is a tracked node
     * identified by the passed in {@code NodeSelector}. The selector can be <b>null</b>, then the root node is assumed.
     *
     * @param key the key
     * @param selector the {@code NodeSelector} defining the root node (or <b>null</b>)
     * @param values the values to be added
     * @param resolver the {@code NodeKeyResolver}
     * @throws ConfigurationRuntimeException if the selector cannot be resolved
     */
    public void addProperty(final String key, final NodeSelector selector, final Iterable<?> values, final NodeKeyResolver<ImmutableNode> resolver) {
        if (valuesNotEmpty(values)) {
            updateModel(tx -> {
                initializeAddTransaction(tx, key, values, resolver);
                return true;
            }, selector, resolver);
        }
    }

    /**
     * {@inheritDoc} A new empty root node is created with the same name as the current root node. Implementation note:
     * Because this is a hard reset the usual dance for dealing with concurrent updates is not required here.
     *
     * @param resolver the {@code NodeKeyResolver}
     */
    @Override
    public void clear(final NodeKeyResolver<ImmutableNode> resolver) {
        final ImmutableNode newRoot = new ImmutableNode.Builder().name(getRootNode().getNodeName()).create();
        setRootNode(newRoot);
    }

    /**
     * {@inheritDoc} If this operation leaves an affected node in an undefined state, it is removed from the model.
     */
    @Override
    public void clearProperty(final String key, final NodeKeyResolver<ImmutableNode> resolver) {
        clearProperty(key, null, resolver);
    }

    /**
     * Clears a property using a tracked node as root node. This method works like the normal {@code clearProperty()}
     * method, but the origin of the operation (also for the interpretation of the passed in key) is a tracked node
     * identified by the passed in {@code NodeSelector}. The selector can be <b>null</b>, then the root node is assumed.
     *
     * @param key the key
     * @param selector the {@code NodeSelector} defining the root node (or <b>null</b>)
     * @param resolver the {@code NodeKeyResolver}
     * @throws ConfigurationRuntimeException if the selector cannot be resolved
     */
    public void clearProperty(final String key, final NodeSelector selector, final NodeKeyResolver<ImmutableNode> resolver) {
        updateModel(tx -> {
            final List<QueryResult<ImmutableNode>> results = resolver.resolveKey(tx.getQueryRoot(), key, tx.getCurrentData());
            return initializeClearTransaction(tx, results);
        }, selector, resolver);
    }

    /**
     * {@inheritDoc} This implementation checks whether nodes become undefined after subtrees have been removed. If this is
     * the case, such nodes are removed, too. Return value is a collection with {@code QueryResult} objects for the elements
     * to be removed from the model.
     */
    @Override
    public List<QueryResult<ImmutableNode>> clearTree(final String key, final NodeKeyResolver<ImmutableNode> resolver) {
        return clearTree(key, null, resolver);
    }

    /**
     * Clears a whole sub tree using a tracked node as root node. This method works like the normal {@code clearTree()}
     * method, but the origin of the operation (also for the interpretation of the passed in key) is a tracked node
     * identified by the passed in {@code NodeSelector}. The selector can be <b>null</b>, then the root node is assumed.
     *
     * @param key the key
     * @param selector the {@code NodeSelector} defining the root node (or <b>null</b>)
     * @param resolver the {@code NodeKeyResolver}
     * @return a list with the results to be removed
     * @throws ConfigurationRuntimeException if the selector cannot be resolved
     */
    public List<QueryResult<ImmutableNode>> clearTree(final String key, final NodeSelector selector, final NodeKeyResolver<ImmutableNode> resolver) {
        final List<QueryResult<ImmutableNode>> removedElements = new LinkedList<>();
        updateModel(tx -> {
            boolean changes = false;
            final TreeData currentStructure = tx.getCurrentData();
            final List<QueryResult<ImmutableNode>> results = resolver.resolveKey(tx.getQueryRoot(), key, currentStructure);
            removedElements.clear();
            removedElements.addAll(results);
            for (final QueryResult<ImmutableNode> result : results) {
                if (result.isAttributeResult()) {
                    tx.addRemoveAttributeOperation(result.getNode(), result.getAttributeName());
                } else {
                    if (result.getNode() == currentStructure.getRootNode()) {
                        // the whole model is to be cleared
                        clear(resolver);
                        return false;
                    }
                    tx.addRemoveNodeOperation(currentStructure.getParent(result.getNode()), result.getNode());
                }
                changes = true;
            }
            return changes;
        }, selector, resolver);

        return removedElements;
    }

    /**
     * Creates the mapping to parent nodes for the nodes structured represented by the passed in root node. Each node is
     * assigned its parent node. Here an iterative algorithm is used rather than a recursive one to avoid stack overflow for
     * huge structures.
     *
     * @param root the root node of the structure
     * @return the parent node mapping
     */
    private Map<ImmutableNode, ImmutableNode> createParentMapping(final ImmutableNode root) {
        final Map<ImmutableNode, ImmutableNode> parents = new HashMap<>();
        updateParentMapping(parents, root);
        return parents;
    }

    /**
     * Creates a {@code TreeData} object for the specified root node.
     *
     * @param root the root node of the current tree
     * @param current the current {@code TreeData} object (may be <b>null</b>)
     * @return the {@code TreeData} describing the current tree
     */
    private TreeData createTreeData(final ImmutableNode root, final TreeData current) {
        final NodeTracker newTracker = current != null ? current.getNodeTracker().detachAllTrackedNodes() : new NodeTracker();
        return createTreeDataForRootAndTracker(root, newTracker);
    }

    /**
     * Creates a {@code TreeData} object for the specified root node and {@code NodeTracker}. Other parameters are set to
     * default values.
     *
     * @param root the new root node for this model
     * @param newTracker the new {@code NodeTracker}
     * @return the new {@code TreeData} object
     */
    private TreeData createTreeDataForRootAndTracker(final ImmutableNode root, final NodeTracker newTracker) {
        return new TreeData(root, createParentMapping(root), Collections.<ImmutableNode, ImmutableNode>emptyMap(), newTracker, new ReferenceTracker());
    }

    /**
     * Executes a transaction on the current data of this model. This method is called if an operation is to be executed on
     * the model's root node or a tracked node which is not yet detached.
     *
     * @param txInit the {@code TransactionInitializer}
     * @param selector an optional {@code NodeSelector} defining the target node
     * @param currentData the current data of the model
     * @param resolver the {@code NodeKeyResolver}
     * @return a flag whether the operation has been completed successfully
     */
    private boolean executeTransactionOnCurrentStructure(final TransactionInitializer txInit, final NodeSelector selector, final TreeData currentData,
        final NodeKeyResolver<ImmutableNode> resolver) {
        final boolean done;
        final ModelTransaction tx = new ModelTransaction(currentData, selector, resolver);
        if (!txInit.initTransaction(tx)) {
            done = true;
        } else {
            final TreeData newData = tx.execute();
            done = structure.compareAndSet(tx.getCurrentData(), newData);
        }
        return done;
    }

    /**
     * Tries to execute a transaction on the model of a detached tracked node. This method checks whether the target node of
     * the transaction is a tracked node and if this node is already detached. If this is the case, the update operation is
     * independent on this model and has to be executed on the specific model for the detached node.
     *
     * @param txInit the {@code TransactionInitializer}
     * @param selector an optional {@code NodeSelector} defining the target node
     * @param currentData the current data of the model
     * @param resolver the {@code NodeKeyResolver} @return a flag whether the transaction could be executed
     * @throws ConfigurationRuntimeException if the selector cannot be resolved
     */
    private boolean executeTransactionOnDetachedTrackedNode(final TransactionInitializer txInit, final NodeSelector selector, final TreeData currentData,
        final NodeKeyResolver<ImmutableNode> resolver) {
        if (selector != null) {
            final InMemoryNodeModel detachedNodeModel = currentData.getNodeTracker().getDetachedNodeModel(selector);
            if (detachedNodeModel != null) {
                detachedNodeModel.updateModel(txInit, null, resolver);
                return true;
            }
        }

        return false;
    }

    /**
     * {@inheritDoc} This implementation simply returns the current root node of this model.
     */
    @Override
    public ImmutableNode getInMemoryRepresentation() {
        return getTreeData().getRootNode();
    }

    /**
     * {@inheritDoc} {@code InMemoryNodeModel} implements the {@code NodeHandler} interface itself. So this implementation
     * just returns the <strong>this</strong> reference.
     */
    @Override
    public NodeHandler<ImmutableNode> getNodeHandler() {
        return getReferenceNodeHandler();
    }

    /**
     * Gets a {@code ReferenceNodeHandler} object for this model. This extended node handler can be used to query
     * references objects stored for this model.
     *
     * @return the {@code ReferenceNodeHandler}
     */
    public ReferenceNodeHandler getReferenceNodeHandler() {
        return getTreeData();
    }

    /**
     * Gets the root node of this mode. Note: This method should be used with care. The model may be updated concurrently
     * which causes the root node to be replaced. If the root node is to be processed further (e.g. by executing queries on
     * it), the model should be asked for its {@code NodeHandler}, and the root node should be obtained from there. The
     * connection between a node handler and its root node remain constant because an update of the model causes the whole
     * node handler to be replaced.
     *
     * @return the current root node
     */
    public ImmutableNode getRootNode() {
        return getTreeData().getRootNode();
    }

    /**
     * Gets the current {@code ImmutableNode} instance associated with the given {@code NodeSelector}. The node must be a
     * tracked node, i.e. {@link #trackNode(NodeSelector, NodeKeyResolver)} must have been called before with the given
     * selector.
     *
     * @param selector the {@code NodeSelector} defining the desired node
     * @return the current {@code ImmutableNode} associated with this selector
     * @throws ConfigurationRuntimeException if the selector is unknown
     */
    public ImmutableNode getTrackedNode(final NodeSelector selector) {
        return structure.get().getNodeTracker().getTrackedNode(selector);
    }

    /**
     * Gets a {@code NodeHandler} for a tracked node. Such a handler may be required for operations on a sub tree of the
     * model. The handler to be returned depends on the current state of the tracked node. If it is still active, a handler
     * is used which shares some data (especially the parent mapping) with this model. Detached track nodes in contrast have
     * their own separate model; in this case a handler associated with this model is returned.
     *
     * @param selector the {@code NodeSelector} defining the tracked node
     * @return a {@code NodeHandler} for this tracked node
     * @throws ConfigurationRuntimeException if the selector is unknown
     */
    public NodeHandler<ImmutableNode> getTrackedNodeHandler(final NodeSelector selector) {
        final TreeData currentData = structure.get();
        final InMemoryNodeModel detachedNodeModel = currentData.getNodeTracker().getDetachedNodeModel(selector);
        return detachedNodeModel != null ? detachedNodeModel.getNodeHandler()
            : new TrackedNodeHandler(currentData.getNodeTracker().getTrackedNode(selector), currentData);
    }

    /**
     * Gets the current {@code TreeData} object. This object contains all information about the current node structure.
     *
     * @return the current {@code TreeData} object
     */
    TreeData getTreeData() {
        return structure.get();
    }

    /**
     * Initializes a transaction for an add operation.
     *
     * @param tx the transaction to be initialized
     * @param key the key
     * @param values the collection with node values
     * @param resolver the {@code NodeKeyResolver}
     */
    private void initializeAddTransaction(final ModelTransaction tx, final String key, final Iterable<?> values,
        final NodeKeyResolver<ImmutableNode> resolver) {
        final NodeAddData<ImmutableNode> addData = resolver.resolveAddKey(tx.getQueryRoot(), key, tx.getCurrentData());
        if (addData.isAttribute()) {
            addAttributeProperty(tx, addData, values);
        } else {
            addNodeProperty(tx, addData, values);
        }
    }

    /**
     * Returns a flag whether the specified tracked node is detached. As long as the {@code NodeSelector} associated with
     * that node returns a single instance, the tracked node is said to be <em>life</em>. If now an update of the model
     * happens which invalidates the selector (maybe the target node was removed), the tracked node becomes detached. It is
     * still possible to query the node; here the latest valid instance is returned. But further changes on the node model
     * are no longer tracked for this node. So even if there are further changes which would make the {@code NodeSelector}
     * valid again, the tracked node stays in detached state.
     *
     * @param selector the {@code NodeSelector} defining the desired node
     * @return a flag whether this tracked node is in detached state
     * @throws ConfigurationRuntimeException if the selector is unknown
     */
    public boolean isTrackedNodeDetached(final NodeSelector selector) {
        return structure.get().getNodeTracker().isTrackedNodeDetached(selector);
    }

    /**
     * Merges the root node of this model with the specified node. This method is typically caused by configuration
     * implementations when a configuration source is loaded, and its data has to be added to the model. It is possible to
     * define the new name of the root node and to pass in a map with reference objects.
     *
     * @param node the node to be merged with the root node
     * @param rootName the new name of the root node; can be <b>null</b>, then the name of the root node is not changed
     *        unless it is <b>null</b>
     * @param references an optional map with reference objects
     * @param rootRef an optional reference object for the new root node
     * @param resolver the {@code NodeKeyResolver}
     */
    public void mergeRoot(final ImmutableNode node, final String rootName, final Map<ImmutableNode, ?> references, final Object rootRef,
        final NodeKeyResolver<ImmutableNode> resolver) {
        updateModel(tx -> {
            final TreeData current = tx.getCurrentData();
            final String newRootName = determineRootName(current.getRootNode(), node, rootName);
            if (newRootName != null) {
                tx.addChangeNodeNameOperation(current.getRootNode(), newRootName);
            }
            tx.addAddNodesOperation(current.getRootNode(), node.getChildren());
            tx.addAttributesOperation(current.getRootNode(), node.getAttributes());
            if (node.getValue() != null) {
                tx.addChangeNodeValueOperation(current.getRootNode(), node.getValue());
            }
            if (references != null) {
                tx.addNewReferences(references);
            }
            if (rootRef != null) {
                tx.addNewReference(current.getRootNode(), rootRef);
            }
            return true;
        }, null, resolver);
    }

    /**
     * Replaces an active tracked node. The node then becomes detached.
     *
     * @param currentData the current data of the model
     * @param selector the {@code NodeSelector} defining the tracked node
     * @param newNode the node replacing the tracked node
     * @return a flag whether the operation was successful
     */
    private boolean replaceActiveTrackedNode(final TreeData currentData, final NodeSelector selector, final ImmutableNode newNode) {
        final NodeTracker newTracker = currentData.getNodeTracker().replaceAndDetachTrackedNode(selector, newNode);
        return structure.compareAndSet(currentData, currentData.updateNodeTracker(newTracker));
    }

    /**
     * Replaces a tracked node if it is already detached.
     *
     * @param currentData the current data of the model
     * @param selector the {@code NodeSelector} defining the tracked node
     * @param newNode the node replacing the tracked node
     * @return a flag whether the operation was successful
     */
    private boolean replaceDetachedTrackedNode(final TreeData currentData, final NodeSelector selector, final ImmutableNode newNode) {
        final InMemoryNodeModel detachedNodeModel = currentData.getNodeTracker().getDetachedNodeModel(selector);
        if (detachedNodeModel != null) {
            detachedNodeModel.setRootNode(newNode);
            return true;
        }

        return false;
    }

    /**
     * Replaces the root node of this model. This method is similar to {@link #setRootNode(ImmutableNode)}; however, tracked
     * nodes will not get lost. The model applies the selectors of all tracked nodes on the new nodes hierarchy, so that
     * corresponding nodes are selected (this may cause nodes to become detached if a select operation fails). This
     * operation is useful if the new nodes hierarchy to be set is known to be similar to the old one. Note that reference
     * objects are lost; there is no way to automatically match nodes between the old and the new nodes hierarchy.
     *
     * @param newRoot the new root node to be set (must not be <b>null</b>)
     * @param resolver the {@code NodeKeyResolver}
     * @throws IllegalArgumentException if the new root node is <b>null</b>
     */
    public void replaceRoot(final ImmutableNode newRoot, final NodeKeyResolver<ImmutableNode> resolver) {
        if (newRoot == null) {
            throw new IllegalArgumentException("Replaced root node must not be null!");
        }

        final TreeData current = structure.get();
        // this step is needed to get a valid NodeHandler
        final TreeData temp = createTreeDataForRootAndTracker(newRoot, current.getNodeTracker());
        structure.set(temp.updateNodeTracker(temp.getNodeTracker().update(newRoot, null, resolver, temp)));
    }

    /**
     * Replaces a tracked node by another node. If the tracked node is not yet detached, it becomes now detached. The passed
     * in node (which must not be <b>null</b>) becomes the new root node of an independent model for this tracked node.
     * Further updates of this model do not affect the tracked node's model and vice versa.
     *
     * @param selector the {@code NodeSelector} defining the tracked node
     * @param newNode the node replacing the tracked node (must not be <b>null</b>)
     * @throws ConfigurationRuntimeException if the selector cannot be resolved
     * @throws IllegalArgumentException if the replacement node is <b>null</b>
     */
    public void replaceTrackedNode(final NodeSelector selector, final ImmutableNode newNode) {
        if (newNode == null) {
            throw new IllegalArgumentException("Replacement node must not be null!");
        }

        boolean done;
        do {
            final TreeData currentData = structure.get();
            done = replaceDetachedTrackedNode(currentData, selector, newNode) || replaceActiveTrackedNode(currentData, selector, newNode);
        } while (!done);
    }

    /**
     * Allows tracking all nodes selected by a key. This method evaluates the specified key on the current nodes structure.
     * For all selected nodes corresponding {@code NodeSelector} objects are created, and they are tracked. The returned
     * collection of {@code NodeSelector} objects can be used for interacting with the selected nodes.
     *
     * @param key the key for selecting the nodes to track
     * @param resolver the {@code NodeKeyResolver}
     * @return a collection with the {@code NodeSelector} objects for the new tracked nodes
     */
    public Collection<NodeSelector> selectAndTrackNodes(final String key, final NodeKeyResolver<ImmutableNode> resolver) {
        final Mutable<Collection<NodeSelector>> refSelectors = new MutableObject<>();
        boolean done;
        do {
            final TreeData current = structure.get();
            final List<ImmutableNode> nodes = resolver.resolveNodeKey(current.getRootNode(), key, current);
            if (nodes.isEmpty()) {
                return Collections.emptyList();
            }
            done = structure.compareAndSet(current, createSelectorsForTrackedNodes(refSelectors, nodes, current, resolver));
        } while (!done);
        return refSelectors.getValue();
    }

    /**
     * Sets the value of a property using a tracked node as root node. This method works like the normal
     * {@code setProperty()} method, but the origin of the operation (also for the interpretation of the passed in key) is a
     * tracked node identified by the passed in {@code NodeSelector}. The selector can be <b>null</b>, then the root node is
     * assumed.
     *
     * @param key the key
     * @param selector the {@code NodeSelector} defining the root node (or <b>null</b>)
     * @param value the new value for this property
     * @param resolver the {@code NodeKeyResolver}
     * @throws ConfigurationRuntimeException if the selector cannot be resolved
     */
    public void setProperty(final String key, final NodeSelector selector, final Object value, final NodeKeyResolver<ImmutableNode> resolver) {
        updateModel(tx -> {
            boolean added = false;
            final NodeUpdateData<ImmutableNode> updateData = resolver.resolveUpdateKey(tx.getQueryRoot(), key, value, tx.getCurrentData());
            if (!updateData.getNewValues().isEmpty()) {
                initializeAddTransaction(tx, key, updateData.getNewValues(), resolver);
                added = true;
            }
            final boolean cleared = initializeClearTransaction(tx, updateData.getRemovedNodes());
            final boolean updated = initializeUpdateTransaction(tx, updateData.getChangedValues());
            return added || cleared || updated;
        }, selector, resolver);
    }

    @Override
    public void setProperty(final String key, final Object value, final NodeKeyResolver<ImmutableNode> resolver) {
        setProperty(key, null, value, resolver);
    }

    /**
     * {@inheritDoc} All tracked nodes and reference objects managed by this model are cleared.Care has to be taken when
     * this method is used and the model is accessed by multiple threads. It is not deterministic which concurrent
     * operations see the old root and which see the new root node.
     *
     * @param newRoot the new root node to be set (can be <b>null</b>, then an empty root node is set)
     */
    @Override
    public void setRootNode(final ImmutableNode newRoot) {
        structure.set(createTreeData(initialRootNode(newRoot), structure.get()));
    }

    /**
     * Tracks all nodes which are children of the node selected by the passed in key. If the key selects exactly one node,
     * for all children of this node {@code NodeSelector} objects are created, and they become tracked nodes. The returned
     * collection of {@code NodeSelector} objects can be used for interacting with the selected nodes.
     *
     * @param key the key for selecting the parent node whose children are to be tracked
     * @param resolver the {@code NodeKeyResolver}
     * @return a collection with the {@code NodeSelector} objects for the new tracked nodes
     */
    public Collection<NodeSelector> trackChildNodes(final String key, final NodeKeyResolver<ImmutableNode> resolver) {
        final Mutable<Collection<NodeSelector>> refSelectors = new MutableObject<>();
        boolean done;
        do {
            refSelectors.setValue(Collections.<NodeSelector>emptyList());
            final TreeData current = structure.get();
            final List<ImmutableNode> nodes = resolver.resolveNodeKey(current.getRootNode(), key, current);
            if (nodes.size() == 1) {
                final ImmutableNode node = nodes.get(0);
                done = node.getChildren().isEmpty()
                    || structure.compareAndSet(current, createSelectorsForTrackedNodes(refSelectors, node.getChildren(), current, resolver));
            } else {
                done = true;
            }
        } while (!done);
        return refSelectors.getValue();
    }

    /**
     * Tracks a node which is a child of another node selected by the passed in key. If the selected node has a child node
     * with this name, it is tracked and its selector is returned. Otherwise, a new child node with this name is created
     * first.
     *
     * @param key the key for selecting the parent node
     * @param childName the name of the child node
     * @param resolver the {@code NodeKeyResolver}
     * @return the {@code NodeSelector} for the tracked child node
     * @throws ConfigurationRuntimeException if the passed in key does not select a single node
     */
    public NodeSelector trackChildNodeWithCreation(final String key, final String childName, final NodeKeyResolver<ImmutableNode> resolver) {
        final MutableObject<NodeSelector> refSelector = new MutableObject<>();
        boolean done;

        do {
            final TreeData current = structure.get();
            final List<ImmutableNode> nodes = resolver.resolveNodeKey(current.getRootNode(), key, current);
            if (nodes.size() != 1) {
                throw new ConfigurationRuntimeException("Key does not select a single node: " + key);
            }

            final ImmutableNode parent = nodes.get(0);
            final TreeData newData = createDataWithTrackedChildNode(current, parent, childName, resolver, refSelector);

            done = structure.compareAndSet(current, newData);
        } while (!done);

        return refSelector.getValue();
    }

    /**
     * Adds a node to be tracked. After this method has been called with a specific {@code NodeSelector}, the node
     * associated with this key can be always obtained using {@link #getTrackedNode(NodeSelector)} with the same selector.
     * This is useful because during updates of a model parts of the structure are replaced. Therefore, it is not a good
     * idea to simply hold a reference to a node; this might become outdated soon. Rather, the node should be tracked. This
     * mechanism ensures that always the correct node reference can be obtained.
     *
     * @param selector the {@code NodeSelector} defining the desired node
     * @param resolver the {@code NodeKeyResolver}
     * @throws ConfigurationRuntimeException if the selector does not select a single node
     */
    public void trackNode(final NodeSelector selector, final NodeKeyResolver<ImmutableNode> resolver) {
        boolean done;
        do {
            final TreeData current = structure.get();
            final NodeTracker newTracker = current.getNodeTracker().trackNode(current.getRootNode(), selector, resolver, current);
            done = structure.compareAndSet(current, current.updateNodeTracker(newTracker));
        } while (!done);
    }

    /**
     * Removes a tracked node. This method is the opposite of {@code trackNode()}. It has to be called if there is no longer
     * the need to track a specific node. Note that for each call of {@code trackNode()} there has to be a corresponding
     * {@code untrackNode()} call. This ensures that multiple observers can track the same node.
     *
     * @param selector the {@code NodeSelector} defining the desired node
     * @throws ConfigurationRuntimeException if the specified node is not tracked
     */
    public void untrackNode(final NodeSelector selector) {
        boolean done;
        do {
            final TreeData current = structure.get();
            final NodeTracker newTracker = current.getNodeTracker().untrackNode(selector);
            done = structure.compareAndSet(current, current.updateNodeTracker(newTracker));
        } while (!done);
    }

    /**
     * Performs a non-blocking, thread-safe update of this model based on a transaction initialized by the passed in
     * initializer. This method uses the atomic reference for the model's current data to ensure that an update was
     * successful even if the model is concurrently accessed.
     *
     * @param txInit the {@code TransactionInitializer}
     * @param selector an optional {@code NodeSelector} defining the target node of the transaction
     * @param resolver the {@code NodeKeyResolver}
     */
    private void updateModel(final TransactionInitializer txInit, final NodeSelector selector, final NodeKeyResolver<ImmutableNode> resolver) {
        boolean done;

        do {
            final TreeData currentData = getTreeData();
            done = executeTransactionOnDetachedTrackedNode(txInit, selector, currentData, resolver)
                || executeTransactionOnCurrentStructure(txInit, selector, currentData, resolver);
        } while (!done);
    }
}