/*
 * 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;

import java.io.ByteArrayOutputStream;
import java.io.PrintStream;
import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.stream.Collectors;

import org.apache.commons.configuration2.event.ConfigurationEvent;
import org.apache.commons.configuration2.event.EventListener;
import org.apache.commons.configuration2.event.EventSource;
import org.apache.commons.configuration2.event.EventType;
import org.apache.commons.configuration2.ex.ConfigurationRuntimeException;
import org.apache.commons.configuration2.sync.LockMode;
import org.apache.commons.configuration2.tree.DefaultConfigurationKey;
import org.apache.commons.configuration2.tree.DefaultExpressionEngine;
import org.apache.commons.configuration2.tree.ExpressionEngine;
import org.apache.commons.configuration2.tree.ImmutableNode;
import org.apache.commons.configuration2.tree.NodeCombiner;
import org.apache.commons.configuration2.tree.NodeTreeWalker;
import org.apache.commons.configuration2.tree.QueryResult;
import org.apache.commons.configuration2.tree.TreeUtils;
import org.apache.commons.configuration2.tree.UnionCombiner;
import org.apache.commons.lang3.StringUtils;

/**
 * <p>
 * A hierarchical composite configuration class.
 * </p>
 * <p>
 * This class maintains a list of configuration objects, which can be added using the diverse {@code addConfiguration()}
 * methods. After that the configurations can be accessed either by name (if one was provided when the configuration was
 * added) or by index. For the whole set of managed configurations a logical node structure is constructed. For this
 * purpose a {@link org.apache.commons.configuration2.tree.NodeCombiner NodeCombiner} object can be set. This makes it
 * possible to specify different algorithms for the combination process.
 * </p>
 * <p>
 * The big advantage of this class is that it creates a truly hierarchical structure of all the properties stored in the
 * contained configurations - even if some of them are no hierarchical configurations per se. So all enhanced features
 * provided by a hierarchical configuration (e.g. choosing an expression engine) are applicable.
 * </p>
 * <p>
 * The class works by registering itself as an event listener at all added configurations. So it gets notified whenever
 * one of these configurations is changed and can invalidate its internal node structure. The next time a property is
 * accessed the node structure will be re-constructed using the current state of the managed configurations. Note that,
 * depending on the used {@code NodeCombiner}, this may be a complex operation.
 * </p>
 * <p>
 * Because of the way a {@code CombinedConfiguration} is working it has more or less view character: it provides a logic
 * view on the configurations it contains. In this constellation not all methods defined for hierarchical configurations
 * - especially methods that update the stored properties - can be implemented in a consistent manner. Using such
 * methods (like {@code addProperty()}, or {@code clearProperty()} on a {@code CombinedConfiguration} is not strictly
 * forbidden, however, depending on the current {@link NodeCombiner} and the involved properties, the results may be
 * different than expected. Some examples may illustrate this:
 * </p>
 * <ul>
 * <li>Imagine a {@code CombinedConfiguration} <em>cc</em> containing two child configurations with the following
 * content:
 * <dl>
 * <dt>user.properties</dt>
 * <dd>
 *
 * <pre>
 * gui.background = blue
 * gui.position = (10, 10, 400, 200)
 * </pre>
 *
 * </dd>
 * <dt>default.properties</dt>
 * <dd>
 *
 * <pre>
 * gui.background = black
 * gui.foreground = white
 * home.dir = /data
 * </pre>
 *
 * </dd>
 * </dl>
 * As a {@code NodeCombiner} a {@link org.apache.commons.configuration2.tree.OverrideCombiner OverrideCombiner} is used.
 * This combiner will ensure that defined user settings take precedence over the default values. If the resulting
 * {@code CombinedConfiguration} is queried for the background color, {@code blue} will be returned because this value
 * is defined in {@code user.properties}. Now consider what happens if the key {@code gui.background} is removed from
 * the {@code CombinedConfiguration}:
 *
 * <pre>
 * cc.clearProperty(&quot;gui.background&quot;);
 * </pre>
 *
 * Will a {@code cc.containsKey("gui.background")} now return <b>false</b>? No, it won't! The {@code clearProperty()}
 * operation is executed on the node set of the combined configuration, which was constructed from the nodes of the two
 * child configurations. It causes the value of the <em>background</em> node to be cleared, which is also part of the
 * first child configuration. This modification of one of its child configurations causes the
 * {@code CombinedConfiguration} to be re-constructed. This time the {@code OverrideCombiner} cannot find a
 * {@code gui.background} property in the first child configuration, but it finds one in the second, and adds it to the
 * resulting combined configuration. So the property is still present (with a different value now).</li>
 * <li>{@code addProperty()} can also be problematic: Most node combiners use special view nodes for linking parts of
 * the original configurations' data together. If new properties are added to such a special node, they do not belong to
 * any of the managed configurations and thus hang in the air. Using the same configurations as in the last example, the
 * statement
 *
 * <pre>
 * addProperty(&quot;database.user&quot;, &quot;scott&quot;);
 * </pre>
 *
 * would cause such a hanging property. If now one of the child configurations is changed and the
 * {@code CombinedConfiguration} is re-constructed, this property will disappear! (Add operations are not problematic if
 * they result in a child configuration being updated. For instance an {@code addProperty("home.url", "localhost");}
 * will alter the second child configuration - because the prefix <em>home</em> is here already present; when the
 * {@code CombinedConfiguration} is re-constructed, this change is taken into account.)</li>
 * </ul>
 * <p>
 * Because of such problems it is recommended to perform updates only on the managed child configurations.
 * </p>
 * <p>
 * Whenever the node structure of a {@code CombinedConfiguration} becomes invalid (either because one of the contained
 * configurations was modified or because the {@code invalidate()} method was directly called) an event is generated. So
 * this can be detected by interested event listeners. This also makes it possible to add a combined configuration into
 * another one.
 * </p>
 * <p>
 * Notes about thread-safety: This configuration implementation uses a {@code Synchronizer} object to protect instances
 * against concurrent access. The concrete {@code Synchronizer} implementation used determines whether an instance of
 * this class is thread-safe or not. In contrast to other implementations derived from
 * {@link BaseHierarchicalConfiguration}, thread-safety is an issue here because the nodes structure used by this
 * configuration has to be constructed dynamically when a child configuration is changed. Therefore, when multiple
 * threads are involved which also manipulate one of the child configurations, a proper {@code Synchronizer} object
 * should be set. Note that the {@code Synchronizer} objects used by the child configurations do not really matter.
 * Because immutable in-memory nodes structures are used for them there is no danger that updates on child
 * configurations could interfere with read operations on the combined configuration.
 * </p>
 *
 * @since 1.3
 */
public class CombinedConfiguration extends BaseHierarchicalConfiguration implements EventListener<ConfigurationEvent> {
    /**
     * Constant for the event type fired when the internal node structure of a combined configuration becomes invalid.
     *
     * @since 2.0
     */
    public static final EventType<ConfigurationEvent> COMBINED_INVALIDATE = new EventType<>(ConfigurationEvent.ANY, "COMBINED_INVALIDATE");

    /** Constant for the expression engine for parsing the at path. */
    private static final DefaultExpressionEngine AT_ENGINE = DefaultExpressionEngine.INSTANCE;

    /** Constant for the default node combiner. */
    private static final NodeCombiner DEFAULT_COMBINER = new UnionCombiner();

    /** Constant for a root node for an empty configuration. */
    private static final ImmutableNode EMPTY_ROOT = new ImmutableNode.Builder().create();

    /** Stores the combiner. */
    private NodeCombiner nodeCombiner;

    /** Stores a list with the contained configurations. */
    private List<ConfigData> configurations;

    /** Stores a map with the named configurations. */
    private Map<String, Configuration> namedConfigurations;

    /**
     * An expression engine used for converting child configurations to hierarchical ones.
     */
    private ExpressionEngine conversionExpressionEngine;

    /** A flag whether this configuration is up-to-date. */
    private boolean upToDate;

    /**
     * Creates a new instance of {@code CombinedConfiguration} and initializes the combiner to be used.
     *
     * @param comb the node combiner (can be <b>null</b>, then a union combiner is used as default)
     */
    public CombinedConfiguration(final NodeCombiner comb) {
        nodeCombiner = comb != null ? comb : DEFAULT_COMBINER;
        initChildCollections();
    }

    /**
     * Creates a new instance of {@code CombinedConfiguration} that uses a union combiner.
     *
     * @see org.apache.commons.configuration2.tree.UnionCombiner
     */
    public CombinedConfiguration() {
        this(null);
    }

    /**
     * Gets the node combiner that is used for creating the combined node structure.
     *
     * @return the node combiner
     */
    public NodeCombiner getNodeCombiner() {
        beginRead(true);
        try {
            return nodeCombiner;
        } finally {
            endRead();
        }
    }

    /**
     * Sets the node combiner. This object will be used when the combined node structure is to be constructed. It must not
     * be <b>null</b>, otherwise an {@code IllegalArgumentException} exception is thrown. Changing the node combiner causes
     * an invalidation of this combined configuration, so that the new combiner immediately takes effect.
     *
     * @param nodeCombiner the node combiner
     */
    public void setNodeCombiner(final NodeCombiner nodeCombiner) {
        if (nodeCombiner == null) {
            throw new IllegalArgumentException("Node combiner must not be null!");
        }

        beginWrite(true);
        try {
            this.nodeCombiner = nodeCombiner;
            invalidateInternal();
        } finally {
            endWrite();
        }
    }

    /**
     * Gets the {@code ExpressionEngine} for converting flat child configurations to hierarchical ones.
     *
     * @return the conversion expression engine
     * @since 1.6
     */
    public ExpressionEngine getConversionExpressionEngine() {
        beginRead(true);
        try {
            return conversionExpressionEngine;
        } finally {
            endRead();
        }
    }

    /**
     * Sets the {@code ExpressionEngine} for converting flat child configurations to hierarchical ones. When constructing
     * the root node for this combined configuration the properties of all child configurations must be combined to a single
     * hierarchical node structure. In this process, non hierarchical configurations are converted to hierarchical ones
     * first. This can be problematic if a child configuration contains keys that are no compatible with the default
     * expression engine used by hierarchical configurations. Therefore it is possible to specify a specific expression
     * engine to be used for this purpose.
     *
     * @param conversionExpressionEngine the conversion expression engine
     * @see ConfigurationUtils#convertToHierarchical(Configuration, ExpressionEngine)
     * @since 1.6
     */
    public void setConversionExpressionEngine(final ExpressionEngine conversionExpressionEngine) {
        beginWrite(true);
        try {
            this.conversionExpressionEngine = conversionExpressionEngine;
        } finally {
            endWrite();
        }
    }

    /**
     * Adds a new configuration to this combined configuration. It is possible (but not mandatory) to give the new
     * configuration a name. This name must be unique, otherwise a {@code ConfigurationRuntimeException} will be thrown.
     * With the optional {@code at} argument you can specify where in the resulting node structure the content of the added
     * configuration should appear. This is a string that uses dots as property delimiters (independent on the current
     * expression engine). For instance if you pass in the string {@code "database.tables"}, all properties of the added
     * configuration will occur in this branch.
     *
     * @param config the configuration to add (must not be <b>null</b>)
     * @param name the name of this configuration (can be <b>null</b>)
     * @param at the position of this configuration in the combined tree (can be <b>null</b>)
     */
    public void addConfiguration(final Configuration config, final String name, final String at) {
        if (config == null) {
            throw new IllegalArgumentException("Added configuration must not be null!");
        }

        beginWrite(true);
        try {
            if (name != null && namedConfigurations.containsKey(name)) {
                throw new ConfigurationRuntimeException("A configuration with the name '" + name + "' already exists in this combined configuration!");
            }

            final ConfigData cd = new ConfigData(config, name, at);
            if (getLogger().isDebugEnabled()) {
                getLogger().debug("Adding configuration " + config + " with name " + name);
            }
            configurations.add(cd);
            if (name != null) {
                namedConfigurations.put(name, config);
            }

            invalidateInternal();
        } finally {
            endWrite();
        }
        registerListenerAt(config);
    }

    /**
     * Adds a new configuration to this combined configuration with an optional name. The new configuration's properties
     * will be added under the root of the combined node structure.
     *
     * @param config the configuration to add (must not be <b>null</b>)
     * @param name the name of this configuration (can be <b>null</b>)
     */
    public void addConfiguration(final Configuration config, final String name) {
        addConfiguration(config, name, null);
    }

    /**
     * Adds a new configuration to this combined configuration. The new configuration is not given a name. Its properties
     * will be added under the root of the combined node structure.
     *
     * @param config the configuration to add (must not be <b>null</b>)
     */
    public void addConfiguration(final Configuration config) {
        addConfiguration(config, null, null);
    }

    /**
     * Gets the number of configurations that are contained in this combined configuration.
     *
     * @return the number of contained configurations
     */
    public int getNumberOfConfigurations() {
        beginRead(true);
        try {
            return getNumberOfConfigurationsInternal();
        } finally {
            endRead();
        }
    }

    /**
     * Gets the configuration at the specified index. The contained configurations are numbered in the order they were
     * added to this combined configuration. The index of the first configuration is 0.
     *
     * @param index the index
     * @return the configuration at this index
     */
    public Configuration getConfiguration(final int index) {
        beginRead(true);
        try {
            final ConfigData cd = configurations.get(index);
            return cd.getConfiguration();
        } finally {
            endRead();
        }
    }

    /**
     * Gets the configuration with the given name. This can be <b>null</b> if no such configuration exists.
     *
     * @param name the name of the configuration
     * @return the configuration with this name
     */
    public Configuration getConfiguration(final String name) {
        beginRead(true);
        try {
            return namedConfigurations.get(name);
        } finally {
            endRead();
        }
    }

    /**
     * Gets a List of all the configurations that have been added.
     *
     * @return A List of all the configurations.
     * @since 1.7
     */
    public List<Configuration> getConfigurations() {
        beginRead(true);
        try {
            return configurations.stream().map(ConfigData::getConfiguration).collect(Collectors.toList());
        } finally {
            endRead();
        }
    }

    /**
     * Gets a List of the names of all the configurations that have been added in the order they were added. A NULL value
     * will be present in the list for each configuration that was added without a name.
     *
     * @return A List of all the configuration names.
     * @since 1.7
     */
    public List<String> getConfigurationNameList() {
        beginRead(true);
        try {
            return configurations.stream().map(ConfigData::getName).collect(Collectors.toList());
        } finally {
            endRead();
        }
    }

    /**
     * Removes the specified configuration from this combined configuration.
     *
     * @param config the configuration to be removed
     * @return a flag whether this configuration was found and could be removed
     */
    public boolean removeConfiguration(final Configuration config) {
        for (int index = 0; index < getNumberOfConfigurations(); index++) {
            if (configurations.get(index).getConfiguration() == config) {
                removeConfigurationAt(index);
                return true;
            }
        }

        return false;
    }

    /**
     * Removes the configuration at the specified index.
     *
     * @param index the index
     * @return the removed configuration
     */
    public Configuration removeConfigurationAt(final int index) {
        final ConfigData cd = configurations.remove(index);
        if (cd.getName() != null) {
            namedConfigurations.remove(cd.getName());
        }
        unregisterListenerAt(cd.getConfiguration());
        invalidateInternal();
        return cd.getConfiguration();
    }

    /**
     * Removes the configuration with the specified name.
     *
     * @param name the name of the configuration to be removed
     * @return the removed configuration (<b>null</b> if this configuration was not found)
     */
    public Configuration removeConfiguration(final String name) {
        final Configuration conf = getConfiguration(name);
        if (conf != null) {
            removeConfiguration(conf);
        }
        return conf;
    }

    /**
     * Gets a set with the names of all configurations contained in this combined configuration. Of course here are only
     * these configurations listed, for which a name was specified when they were added.
     *
     * @return a set with the names of the contained configurations (never <b>null</b>)
     */
    public Set<String> getConfigurationNames() {
        beginRead(true);
        try {
            return namedConfigurations.keySet();
        } finally {
            endRead();
        }
    }

    /**
     * Invalidates this combined configuration. This means that the next time a property is accessed the combined node
     * structure must be re-constructed. Invalidation of a combined configuration also means that an event of type
     * {@code EVENT_COMBINED_INVALIDATE} is fired. Note that while other events most times appear twice (once before and
     * once after an update), this event is only fired once (after update).
     */
    public void invalidate() {
        beginWrite(true);
        try {
            invalidateInternal();
        } finally {
            endWrite();
        }
    }

    /**
     * Event listener call back for configuration update events. This method is called whenever one of the contained
     * configurations was modified. It invalidates this combined configuration.
     *
     * @param event the update event
     */
    @Override
    public void onEvent(final ConfigurationEvent event) {
        if (event.isBeforeUpdate()) {
            invalidate();
        }
    }

    /**
     * Clears this configuration. All contained configurations will be removed.
     */
    @Override
    protected void clearInternal() {
        unregisterListenerAtChildren();
        initChildCollections();
        invalidateInternal();
    }

    /**
     * Returns a copy of this object. This implementation performs a deep clone, i.e. all contained configurations will be
     * cloned, too. For this to work, all contained configurations must be cloneable. Registered event listeners won't be
     * cloned. The clone will use the same node combiner than the original.
     *
     * @return the copied object
     */
    @Override
    public Object clone() {
        beginRead(false);
        try {
            final CombinedConfiguration copy = (CombinedConfiguration) super.clone();
            copy.initChildCollections();
            configurations.forEach(cd -> copy.addConfiguration(ConfigurationUtils.cloneConfiguration(cd.getConfiguration()), cd.getName(), cd.getAt()));

            return copy;
        } finally {
            endRead();
        }
    }

    /**
     * Gets the configuration source, in which the specified key is defined. This method will determine the configuration
     * node that is identified by the given key. The following constellations are possible:
     * <ul>
     * <li>If no node object is found for this key, <b>null</b> is returned.</li>
     * <li>If the key maps to multiple nodes belonging to different configuration sources, a
     * {@code IllegalArgumentException} is thrown (in this case no unique source can be determined).</li>
     * <li>If exactly one node is found for the key, the (child) configuration object, to which the node belongs is
     * determined and returned.</li>
     * <li>For keys that have been added directly to this combined configuration and that do not belong to the namespaces
     * defined by existing child configurations this configuration will be returned.</li>
     * </ul>
     *
     * @param key the key of a configuration property
     * @return the configuration, to which this property belongs or <b>null</b> if the key cannot be resolved
     * @throws IllegalArgumentException if the key maps to multiple properties and the source cannot be determined, or if
     *         the key is <b>null</b>
     * @since 1.5
     */
    public Configuration getSource(final String key) {
        if (key == null) {
            throw new IllegalArgumentException("Key must not be null!");
        }

        final Set<Configuration> sources = getSources(key);
        if (sources.isEmpty()) {
            return null;
        }
        final Iterator<Configuration> iterator = sources.iterator();
        final Configuration source = iterator.next();
        if (iterator.hasNext()) {
            throw new IllegalArgumentException("The key " + key + " is defined by multiple sources!");
        }
        return source;
    }

    /**
     * Gets a set with the configuration sources, in which the specified key is defined. This method determines the
     * configuration nodes that are identified by the given key. It then determines the configuration sources to which these
     * nodes belong and adds them to the result set. Note the following points:
     * <ul>
     * <li>If no node object is found for this key, an empty set is returned.</li>
     * <li>For keys that have been added directly to this combined configuration and that do not belong to the namespaces
     * defined by existing child configurations this combined configuration is contained in the result set.</li>
     * </ul>
     *
     * @param key the key of a configuration property
     * @return a set with the configuration sources, which contain this property
     * @since 2.0
     */
    public Set<Configuration> getSources(final String key) {
        beginRead(false);
        try {
            final List<QueryResult<ImmutableNode>> results = fetchNodeList(key);
            final Set<Configuration> sources = new HashSet<>();

            results.forEach(result -> {
                final Set<Configuration> resultSources = findSourceConfigurations(result.getNode());
                if (resultSources.isEmpty()) {
                    // key must be defined in combined configuration
                    sources.add(this);
                } else {
                    sources.addAll(resultSources);
                }
            });

            return sources;
        } finally {
            endRead();
        }
    }

    /**
     * {@inheritDoc} This implementation checks whether a combined root node is available. If not, it is constructed by
     * requesting a write lock.
     */
    @Override
    protected void beginRead(final boolean optimize) {
        if (optimize) {
            // just need a lock, don't construct configuration
            super.beginRead(true);
            return;
        }

        boolean lockObtained = false;
        do {
            super.beginRead(false);
            if (isUpToDate()) {
                lockObtained = true;
            } else {
                // release read lock and try to obtain a write lock
                endRead();
                beginWrite(false); // this constructs the root node
                endWrite();
            }
        } while (!lockObtained);
    }

    /**
     * {@inheritDoc} This implementation checks whether a combined root node is available. If not, it is constructed now.
     */
    @Override
    protected void beginWrite(final boolean optimize) {
        super.beginWrite(true);
        if (optimize) {
            // just need a lock, don't construct configuration
            return;
        }

        boolean success = false;
        try {
            if (!isUpToDate()) {
                getSubConfigurationParentModel().replaceRoot(constructCombinedNode(), this);
                upToDate = true;
            }
            success = true;
        } finally {
            if (!success) {
                endWrite();
            }
        }
    }

    /**
     * Returns a flag whether this configuration has been invalidated. This means that the combined nodes structure has to
     * be rebuilt before the configuration can be accessed.
     *
     * @return a flag whether this configuration is invalid
     */
    private boolean isUpToDate() {
        return upToDate;
    }

    /**
     * Marks this configuration as invalid. This means that the next access re-creates the root node. An invalidate event is
     * also fired. Note: This implementation expects that an exclusive (write) lock is held on this instance.
     */
    private void invalidateInternal() {
        upToDate = false;
        fireEvent(COMBINED_INVALIDATE, null, null, false);
    }

    /**
     * Initializes internal data structures for storing information about child configurations.
     */
    private void initChildCollections() {
        configurations = new ArrayList<>();
        namedConfigurations = new HashMap<>();
    }

    /**
     * Creates the root node of this combined configuration.
     *
     * @return the combined root node
     */
    private ImmutableNode constructCombinedNode() {
        if (getNumberOfConfigurationsInternal() < 1) {
            if (getLogger().isDebugEnabled()) {
                getLogger().debug("No configurations defined for " + this);
            }
            return EMPTY_ROOT;
        }
        final Iterator<ConfigData> it = configurations.iterator();
        ImmutableNode node = it.next().getTransformedRoot();
        while (it.hasNext()) {
            node = nodeCombiner.combine(node, it.next().getTransformedRoot());
        }
        if (getLogger().isDebugEnabled()) {
            final ByteArrayOutputStream os = new ByteArrayOutputStream();
            final PrintStream stream = new PrintStream(os);
            TreeUtils.printTree(stream, node);
            getLogger().debug(os.toString());
        }
        return node;
    }

    /**
     * Determines the configurations to which the specified node belongs. This is done by inspecting the nodes structures of
     * all child configurations.
     *
     * @param node the node
     * @return a set with the owning configurations
     */
    private Set<Configuration> findSourceConfigurations(final ImmutableNode node) {
        final Set<Configuration> result = new HashSet<>();
        final FindNodeVisitor<ImmutableNode> visitor = new FindNodeVisitor<>(node);

        configurations.forEach(cd -> {
            NodeTreeWalker.INSTANCE.walkBFS(cd.getRootNode(), visitor, getModel().getNodeHandler());
            if (visitor.isFound()) {
                result.add(cd.getConfiguration());
                visitor.reset();
            }
        });

        return result;
    }

    /**
     * Registers this combined configuration as listener at the given child configuration.
     *
     * @param configuration the child configuration
     */
    private void registerListenerAt(final Configuration configuration) {
        if (configuration instanceof EventSource) {
            ((EventSource) configuration).addEventListener(ConfigurationEvent.ANY, this);
        }
    }

    /**
     * Removes this combined configuration as listener from the given child configuration.
     *
     * @param configuration the child configuration
     */
    private void unregisterListenerAt(final Configuration configuration) {
        if (configuration instanceof EventSource) {
            ((EventSource) configuration).removeEventListener(ConfigurationEvent.ANY, this);
        }
    }

    /**
     * Removes this combined configuration as listener from all child configurations. This method is called on a clear()
     * operation.
     */
    private void unregisterListenerAtChildren() {
        if (configurations != null) {
            configurations.forEach(child -> unregisterListenerAt(child.getConfiguration()));
        }
    }

    /**
     * Gets the number of child configurations in this combined configuration. The internal list of child configurations
     * is accessed without synchronization.
     *
     * @return the number of child configurations
     */
    private int getNumberOfConfigurationsInternal() {
        return configurations.size();
    }

    /**
     * An internal helper class for storing information about contained configurations.
     */
    private final class ConfigData {
        /** Stores a reference to the configuration. */
        private final Configuration configuration;

        /** Stores the name under which the configuration is stored. */
        private final String name;

        /** Stores the at information as path of nodes. */
        private final Collection<String> atPath;

        /** Stores the at string. */
        private final String at;

        /** Stores the root node for this child configuration. */
        private ImmutableNode rootNode;

        /**
         * Creates a new instance of {@code ConfigData} and initializes it.
         *
         * @param config the configuration
         * @param n the name
         * @param at the at position
         */
        public ConfigData(final Configuration config, final String n, final String at) {
            configuration = config;
            name = n;
            atPath = parseAt(at);
            this.at = at;
        }

        /**
         * Gets the stored configuration.
         *
         * @return the configuration
         */
        public Configuration getConfiguration() {
            return configuration;
        }

        /**
         * Gets the configuration's name.
         *
         * @return the name
         */
        public String getName() {
            return name;
        }

        /**
         * Gets the at position of this configuration.
         *
         * @return the at position
         */
        public String getAt() {
            return at;
        }

        /**
         * Gets the root node for this child configuration.
         *
         * @return the root node of this child configuration
         * @since 1.5
         */
        public ImmutableNode getRootNode() {
            return rootNode;
        }

        /**
         * Gets the transformed root node of the stored configuration. The term &quot;transformed&quot; means that an
         * eventually defined at path has been applied.
         *
         * @return the transformed root node
         */
        public ImmutableNode getTransformedRoot() {
            final ImmutableNode configRoot = getRootNodeOfConfiguration();
            return atPath == null ? configRoot : prependAtPath(configRoot);
        }

        /**
         * Prepends the at path to the given node.
         *
         * @param node the root node of the represented configuration
         * @return the new root node including the at path
         */
        private ImmutableNode prependAtPath(final ImmutableNode node) {
            final ImmutableNode.Builder pathBuilder = new ImmutableNode.Builder();
            final Iterator<String> pathIterator = atPath.iterator();
            prependAtPathComponent(pathBuilder, pathIterator.next(), pathIterator, node);
            return new ImmutableNode.Builder(1).addChild(pathBuilder.create()).create();
        }

        /**
         * Handles a single component of the at path. A corresponding node is created and added to the hierarchical path to the
         * original root node of the configuration.
         *
         * @param builder the current node builder object
         * @param currentComponent the name of the current path component
         * @param components an iterator with all components of the at path
         * @param orgRoot the original root node of the wrapped configuration
         */
        private void prependAtPathComponent(final ImmutableNode.Builder builder, final String currentComponent, final Iterator<String> components,
            final ImmutableNode orgRoot) {
            builder.name(currentComponent);
            if (components.hasNext()) {
                final ImmutableNode.Builder childBuilder = new ImmutableNode.Builder();
                prependAtPathComponent(childBuilder, components.next(), components, orgRoot);
                builder.addChild(childBuilder.create());
            } else {
                builder.addChildren(orgRoot.getChildren());
                builder.addAttributes(orgRoot.getAttributes());
                builder.value(orgRoot.getValue());
            }
        }

        /**
         * Obtains the root node of the wrapped configuration. If necessary, a hierarchical representation of the configuration
         * has to be created first.
         *
         * @return the root node of the associated configuration
         */
        private ImmutableNode getRootNodeOfConfiguration() {
            getConfiguration().lock(LockMode.READ);
            try {
                final ImmutableNode root = ConfigurationUtils.convertToHierarchical(getConfiguration(), conversionExpressionEngine).getNodeModel()
                    .getInMemoryRepresentation();
                rootNode = root;
                return root;
            } finally {
                getConfiguration().unlock(LockMode.READ);
            }
        }

        /**
         * Splits the at path into its components.
         *
         * @param at the at string
         * @return a collection with the names of the single components
         */
        private Collection<String> parseAt(final String at) {
            if (StringUtils.isEmpty(at)) {
                return null;
            }

            final Collection<String> result = new ArrayList<>();
            final DefaultConfigurationKey.KeyIterator it = new DefaultConfigurationKey(AT_ENGINE, at).iterator();
            while (it.hasNext()) {
                result.add(it.nextKey());
            }
            return result;
        }
    }
}