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    * 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.xpath;
  
  import java.util.Collections;
  import java.util.LinkedList;
  import java.util.List;
  import java.util.StringTokenizer;
  import java.util.stream.Collectors;
  
  import org.apache.commons.configuration2.tree.ExpressionEngine;
  import org.apache.commons.configuration2.tree.NodeAddData;
  import org.apache.commons.configuration2.tree.NodeHandler;
  import org.apache.commons.configuration2.tree.QueryResult;
  import org.apache.commons.jxpath.JXPathContext;
  import org.apache.commons.jxpath.ri.JXPathContextReferenceImpl;
  import org.apache.commons.lang3.StringUtils;
  
  /**
   * <p>
   * A specialized implementation of the {@code ExpressionEngine} interface that is able to evaluate XPATH expressions.
   * </p>
   * <p>
   * This class makes use of <a href="https://commons.apache.org/jxpath/"> Commons JXPath</a> for handling XPath
   * expressions and mapping them to the nodes of a hierarchical configuration. This makes the rich and powerful XPATH
   * syntax available for accessing properties from a configuration object.
   * </p>
   * <p>
   * For selecting properties arbitrary XPATH expressions can be used, which select single or multiple configuration
   * nodes. The associated {@code Configuration} instance will directly pass the specified property keys into this engine.
   * If a key is not syntactically correct, an exception will be thrown.
   * </p>
   * <p>
   * For adding new properties, this expression engine uses a specific syntax: the &quot;key&quot; of a new property must
   * consist of two parts that are separated by whitespace:
   * </p>
   * <ol>
   * <li>An XPATH expression selecting a single node, to which the new element(s) are to be added. This can be an
   * arbitrary complex expression, but it must select exactly one node, otherwise an exception will be thrown.</li>
   * <li>The name of the new element(s) to be added below this parent node. Here either a single node name or a complete
   * path of nodes (separated by the &quot;/&quot; character or &quot;@&quot; for an attribute) can be specified.</li>
   * </ol>
   * <p>
   * Some examples for valid keys that can be passed into the configuration's {@code addProperty()} method follow:
   * </p>
   *
   * <pre>
   * &quot;/tables/table[1] type&quot;
   * </pre>
   *
   * <p>
   * This will add a new {@code type} node as a child of the first {@code table} element.
   * </p>
   *
   * <pre>
   * &quot;/tables/table[1] @type&quot;
   * </pre>
   *
   * <p>
   * Similar to the example above, but this time a new attribute named {@code type} will be added to the first
   * {@code table} element.
   * </p>
   *
   * <pre>
   * &quot;/tables table/fields/field/name&quot;
   * </pre>
   *
   * <p>
   * This example shows how a complex path can be added. Parent node is the {@code tables} element. Here a new branch
   * consisting of the nodes {@code table}, {@code fields}, {@code field}, and {@code name} will be added.
   * </p>
   *
   * <pre>
   * &quot;/tables table/fields/field@type&quot;
   * </pre>
   *
   * <p>
   * This is similar to the last example, but in this case a complex path ending with an attribute is defined.
   * </p>
   * <p>
   * <strong>Note:</strong> This extended syntax for adding properties only works with the {@code addProperty()} method.
   * {@code setProperty()} does not support creating new nodes this way.
   * </p>
   * <p>
   * From version 1.7 on, it is possible to use regular keys in calls to {@code addProperty()} (i.e. keys that do not have
  * to contain a whitespace as delimiter). In this case the key is evaluated, and the biggest part pointing to an
  * existing node is determined. The remaining part is then added as new path. As an example consider the key
  * </p>
  *
  * <pre>
  * &quot;tables/table[last()]/fields/field/name&quot;
  * </pre>
  *
  * <p>
  * If the key does not point to an existing node, the engine will check the paths
  * {@code "tables/table[last()]/fields/field"}, {@code "tables/table[last()]/fields"}, {@code "tables/table[last()]"},
  * and so on, until a key is found which points to a node. Let's assume that the last key listed above can be resolved
  * in this way. Then from this key the following key is derived: {@code "tables/table[last()] fields/field/name"} by
  * appending the remaining part after a whitespace. This key can now be processed using the original algorithm. Keys of
  * this form can also be used with the {@code setProperty()} method. However, it is still recommended to use the old
  * format because it makes explicit at which position new nodes should be added. For keys without a whitespace delimiter
  * there may be ambiguities.
  * </p>
  *
  * @since 1.3
  */
 public class XPathExpressionEngine implements ExpressionEngine {
     /** Constant for the path delimiter. */
     static final String PATH_DELIMITER = "/";
 
     /** Constant for the attribute delimiter. */
     static final String ATTR_DELIMITER = "@";
 
     /** Constant for the delimiters for splitting node paths. */
     private static final String NODE_PATH_DELIMITERS = PATH_DELIMITER + ATTR_DELIMITER;
 
     /**
      * Constant for a space which is used as delimiter in keys for adding properties.
      */
     private static final String SPACE = " ";
 
     /** Constant for a default size of a key buffer. */
     private static final int BUF_SIZE = 128;
 
     /** Constant for the start of an index expression. */
     private static final char START_INDEX = '[';
 
     /** Constant for the end of an index expression. */
     private static final char END_INDEX = ']';
 
     /** The internally used context factory. */
     private final XPathContextFactory contextFactory;
 
     /**
      * Creates a new instance of {@code XPathExpressionEngine} with default settings.
      */
     public XPathExpressionEngine() {
         this(new XPathContextFactory());
     }
 
     /**
      * Creates a new instance of {@code XPathExpressionEngine} and sets the context factory. This constructor is mainly used
      * for testing purposes.
      *
      * @param factory the {@code XPathContextFactory}
      */
     XPathExpressionEngine(final XPathContextFactory factory) {
         contextFactory = factory;
     }
 
     /**
      * {@inheritDoc} This implementation interprets the passed in key as an XPATH expression.
      */
     @Override
     public <T> List<QueryResult<T>> query(final T root, final String key, final NodeHandler<T> handler) {
         if (StringUtils.isEmpty(key)) {
             final QueryResult<T> result = createResult(root);
             return Collections.singletonList(result);
         }
         final JXPathContext context = createContext(root, handler);
         List<?> results = context.selectNodes(key);
         if (results == null) {
             results = Collections.emptyList();
         }
         return convertResults(results);
     }
 
     /**
      * {@inheritDoc} This implementation creates an XPATH expression that selects the given node (under the assumption that
      * the passed in parent key is valid). As the {@code nodeKey()} implementation of
      * {@link org.apache.commons.configuration2.tree.DefaultExpressionEngine DefaultExpressionEngine} this method does not
      * return indices for nodes. So all child nodes of a given parent with the same name have the same key.
      */
     @Override
     public <T> String nodeKey(final T node, final String parentKey, final NodeHandler<T> handler) {
         if (parentKey == null) {
             // name of the root node
             return StringUtils.EMPTY;
         }
         if (handler.nodeName(node) == null) {
             // paranoia check for undefined node names
             return parentKey;
         }
         final StringBuilder buf = new StringBuilder(parentKey.length() + handler.nodeName(node).length() + PATH_DELIMITER.length());
         if (!parentKey.isEmpty()) {
             buf.append(parentKey);
             buf.append(PATH_DELIMITER);
         }
         buf.append(handler.nodeName(node));
         return buf.toString();
     }
 
     @Override
     public String attributeKey(final String parentKey, final String attributeName) {
         final StringBuilder buf = new StringBuilder(
             StringUtils.length(parentKey) + StringUtils.length(attributeName) + PATH_DELIMITER.length() + ATTR_DELIMITER.length());
         if (StringUtils.isNotEmpty(parentKey)) {
             buf.append(parentKey).append(PATH_DELIMITER);
         }
         buf.append(ATTR_DELIMITER).append(attributeName);
         return buf.toString();
     }
 
     /**
      * {@inheritDoc} This implementation works similar to {@code nodeKey()}, but always adds an index expression to the
      * resulting key.
      */
     @Override
     public <T> String canonicalKey(final T node, final String parentKey, final NodeHandler<T> handler) {
         final T parent = handler.getParent(node);
         if (parent == null) {
             // this is the root node
             return StringUtils.defaultString(parentKey);
         }
 
         final StringBuilder buf = new StringBuilder(BUF_SIZE);
         if (StringUtils.isNotEmpty(parentKey)) {
             buf.append(parentKey).append(PATH_DELIMITER);
         }
         buf.append(handler.nodeName(node));
         buf.append(START_INDEX);
         buf.append(determineIndex(parent, node, handler));
         buf.append(END_INDEX);
         return buf.toString();
     }
 
     /**
      * {@inheritDoc} The expected format of the passed in key is explained in the class comment.
      */
     @Override
     public <T> NodeAddData<T> prepareAdd(final T root, final String key, final NodeHandler<T> handler) {
         if (key == null) {
             throw new IllegalArgumentException("prepareAdd: key must not be null!");
         }
 
         String addKey = key;
         int index = findKeySeparator(addKey);
         if (index < 0) {
             addKey = generateKeyForAdd(root, addKey, handler);
             index = findKeySeparator(addKey);
         } else if (index >= addKey.length() - 1) {
             invalidPath(addKey, " new node path must not be empty.");
         }
 
         final List<QueryResult<T>> nodes = query(root, addKey.substring(0, index).trim(), handler);
         if (nodes.size() != 1) {
             throw new IllegalArgumentException("prepareAdd: key '" + key + "' must select exactly one target node!");
         }
 
         return createNodeAddData(addKey.substring(index).trim(), nodes.get(0));
     }
 
     /**
      * Creates the {@code JXPathContext} to be used for executing a query. This method delegates to the context factory.
      *
      * @param root the configuration root node
      * @param handler the node handler
      * @return the new context
      */
     private <T> JXPathContext createContext(final T root, final NodeHandler<T> handler) {
         return getContextFactory().createContext(root, handler);
     }
 
     /**
      * Creates a {@code NodeAddData} object as a result of a {@code prepareAdd()} operation. This method interprets the
      * passed in path of the new node.
      *
      * @param path the path of the new node
      * @param parentNodeResult the parent node
      * @param <T> the type of the nodes involved
      */
     <T> NodeAddData<T> createNodeAddData(final String path, final QueryResult<T> parentNodeResult) {
         if (parentNodeResult.isAttributeResult()) {
             invalidPath(path, " cannot add properties to an attribute.");
         }
         final List<String> pathNodes = new LinkedList<>();
         String lastComponent = null;
         boolean attr = false;
         boolean first = true;
 
         final StringTokenizer tok = new StringTokenizer(path, NODE_PATH_DELIMITERS, true);
         while (tok.hasMoreTokens()) {
             final String token = tok.nextToken();
             if (PATH_DELIMITER.equals(token)) {
                 if (attr) {
                     invalidPath(path, " contains an attribute" + " delimiter at a disallowed position.");
                 }
                 if (lastComponent == null) {
                     invalidPath(path, " contains a '/' at a disallowed position.");
                 }
                 pathNodes.add(lastComponent);
                 lastComponent = null;
             } else if (ATTR_DELIMITER.equals(token)) {
                 if (attr) {
                     invalidPath(path, " contains multiple attribute delimiters.");
                 }
                 if (lastComponent == null && !first) {
                     invalidPath(path, " contains an attribute delimiter at a disallowed position.");
                 }
                 if (lastComponent != null) {
                     pathNodes.add(lastComponent);
                 }
                 attr = true;
                 lastComponent = null;
             } else {
                 lastComponent = token;
             }
             first = false;
         }
 
         if (lastComponent == null) {
             invalidPath(path, "contains no components.");
         }
 
         return new NodeAddData<>(parentNodeResult.getNode(), lastComponent, attr, pathNodes);
     }
 
     /**
      * Gets the {@code XPathContextFactory} used by this instance.
      *
      * @return the {@code XPathContextFactory}
      */
     XPathContextFactory getContextFactory() {
         return contextFactory;
     }
 
     /**
      * Tries to generate a key for adding a property. This method is called if a key was used for adding properties which
      * does not contain a space character. It splits the key at its single components and searches for the last existing
      * component. Then a key compatible key for adding properties is generated.
      *
      * @param root the root node of the configuration
      * @param key the key in question
      * @param handler the node handler
      * @return the key to be used for adding the property
      */
     private <T> String generateKeyForAdd(final T root, final String key, final NodeHandler<T> handler) {
         int pos = key.lastIndexOf(PATH_DELIMITER, key.length());
 
         while (pos >= 0) {
             final String keyExisting = key.substring(0, pos);
             if (!query(root, keyExisting, handler).isEmpty()) {
                 final StringBuilder buf = new StringBuilder(key.length() + 1);
                 buf.append(keyExisting).append(SPACE);
                 buf.append(key.substring(pos + 1));
                 return buf.toString();
             }
             pos = key.lastIndexOf(PATH_DELIMITER, pos - 1);
         }
 
         return SPACE + key;
     }
 
     /**
      * Determines the index of the given child node in the node list of its parent.
      *
      * @param parent the parent node
      * @param child the child node
      * @param handler the node handler
      * @param <T> the type of the nodes involved
      * @return the index of this child node
      */
     private static <T> int determineIndex(final T parent, final T child, final NodeHandler<T> handler) {
         return handler.getChildren(parent, handler.nodeName(child)).indexOf(child) + 1;
     }
 
     /**
      * Helper method for throwing an exception about an invalid path.
      *
      * @param path the invalid path
      * @param msg the exception message
      */
     private static void invalidPath(final String path, final String msg) {
         throw new IllegalArgumentException("Invalid node path: \"" + path + "\" " + msg);
     }
 
     /**
      * Determines the position of the separator in a key for adding new properties. If no delimiter is found, result is -1.
      *
      * @param key the key
      * @return the position of the delimiter
      */
     private static int findKeySeparator(final String key) {
         int index = key.length() - 1;
         while (index >= 0 && !Character.isWhitespace(key.charAt(index))) {
             index--;
         }
         return index;
     }
 
     /**
      * Converts the objects returned as query result from the JXPathContext to query result objects.
      *
      * @param results the list with results from the context
      * @param <T> the type of results to be produced
      * @return the result list
      */
     private static <T> List<QueryResult<T>> convertResults(final List<?> results) {
         return results.stream().map(res -> (QueryResult<T>) createResult(res)).collect(Collectors.toList());
     }
 
     /**
      * Creates a {@code QueryResult} object from the given result object of a query. Because of the node pointers involved
      * result objects can only be of two types:
      * <ul>
      * <li>nodes of type T</li>
      * <li>attribute results already wrapped in {@code QueryResult} objects</li>
      * </ul>
      * This method performs a corresponding cast. Warnings can be suppressed because of the implementation of the query
      * functionality.
      *
      * @param resObj the query result object
      * @param <T> the type of the result to be produced
      * @return the {@code QueryResult}
      */
     @SuppressWarnings("unchecked")
     private static <T> QueryResult<T> createResult(final Object resObj) {
         if (resObj instanceof QueryResult) {
             return (QueryResult<T>) resObj;
         }
         return QueryResult.createNodeResult((T) resObj);
     }
 
     // static initializer: registers the configuration node pointer factory
     static {
         JXPathContextReferenceImpl.addNodePointerFactory(new ConfigurationNodePointerFactory());
     }
 }