/*
    * 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.Collection;
  import java.util.LinkedList;
  import java.util.List;
  
  import org.apache.commons.lang3.StringUtils;
  
  /**
   * <p>
   * A default implementation of the {@code ExpressionEngine} interface providing the &quot;native&quot; expression
   * language for hierarchical configurations.
   * </p>
   * <p>
   * This class implements a rather simple expression language for navigating through a hierarchy of configuration nodes.
   * It supports the following operations:
   * </p>
   * <ul>
   * <li>Navigating from a node to one of its children using the child node delimiter, which is by the default a dot
   * (&quot;.&quot;).</li>
   * <li>Navigating from a node to one of its attributes using the attribute node delimiter, which by default follows the
   * XPATH like syntax {@code [@&lt;attributeName&gt;]}.</li>
   * <li>If there are multiple child or attribute nodes with the same name, a specific node can be selected using a
   * numerical index. By default indices are written in parenthesis.</li>
   * </ul>
   * <p>
   * As an example consider the following XML document:
   * </p>
   *
   * <pre>
   *  &lt;database&gt;
   *    &lt;tables&gt;
   *      &lt;table type=&quot;system&quot;&gt;
   *        &lt;name&gt;users&lt;/name&gt;
   *        &lt;fields&gt;
   *          &lt;field&gt;
   *            &lt;name&gt;lid&lt;/name&gt;
   *            &lt;type&gt;long&lt;/name&gt;
   *          &lt;/field&gt;
   *          &lt;field&gt;
   *            &lt;name&gt;usrName&lt;/name&gt;
   *            &lt;type&gt;java.lang.String&lt;/type&gt;
   *          &lt;/field&gt;
   *         ...
   *        &lt;/fields&gt;
   *      &lt;/table&gt;
   *      &lt;table&gt;
   *        &lt;name&gt;documents&lt;/name&gt;
   *        &lt;fields&gt;
   *          &lt;field&gt;
   *            &lt;name&gt;docid&lt;/name&gt;
   *            &lt;type&gt;long&lt;/type&gt;
   *          &lt;/field&gt;
   *          ...
   *        &lt;/fields&gt;
   *      &lt;/table&gt;
   *      ...
   *    &lt;/tables&gt;
   *  &lt;/database&gt;
   * </pre>
   *
   * <p>
   * If this document is parsed and stored in a hierarchical configuration object, for instance the key
   * {@code tables.table(0).name} can be used to find out the name of the first table. In opposite
   * {@code tables.table.name} would return a collection with the names of all available tables. Similarly the key
   * {@code tables.table(1).fields.field.name} returns a collection with the names of all fields of the second table. If
   * another index is added after the {@code field} element, a single field can be accessed:
   * {@code tables.table(1).fields.field(0).name}. The key {@code tables.table(0)[@type]} would select the type attribute
   * of the first table.
   * </p>
   * <p>
   * This example works with the default values for delimiters and index markers. It is also possible to set custom values
   * for these properties so that you can adapt a {@code DefaultExpressionEngine} to your personal needs.
   * </p>
   * <p>
   * The concrete symbols used by an instance are determined by a {@link DefaultExpressionEngineSymbols} object passed to
   * the constructor. By providing a custom symbols object the syntax for querying properties in a hierarchical
   * configuration can be altered.
   * </p>
   * <p>
   * Instances of this class are thread-safe and can be shared between multiple hierarchical configuration objects.
   * </p>
   *
  * @since 1.3
  */
 public class DefaultExpressionEngine implements ExpressionEngine {
     /**
      * A default instance of this class that is used as expression engine for hierarchical configurations per default.
      */
     public static final DefaultExpressionEngine INSTANCE = new DefaultExpressionEngine(DefaultExpressionEngineSymbols.DEFAULT_SYMBOLS);
 
     /** The symbols used by this instance. */
     private final DefaultExpressionEngineSymbols symbols;
 
     /** The matcher for node names. */
     private final NodeMatcher<String> nameMatcher;
 
     /**
      * Creates a new instance of {@code DefaultExpressionEngine} and initializes its symbols.
      *
      * @param syms the object with the symbols (must not be <b>null</b>)
      * @throws IllegalArgumentException if the symbols are <b>null</b>
      */
     public DefaultExpressionEngine(final DefaultExpressionEngineSymbols syms) {
         this(syms, null);
     }
 
     /**
      * Creates a new instance of {@code DefaultExpressionEngine} and initializes its symbols and the matcher for comparing
      * node names. The passed in matcher is always used when the names of nodes have to be matched against parts of
      * configuration keys.
      *
      * @param syms the object with the symbols (must not be <b>null</b>)
      * @param nodeNameMatcher the matcher for node names; can be <b>null</b>, then a default matcher is used
      * @throws IllegalArgumentException if the symbols are <b>null</b>
      */
     public DefaultExpressionEngine(final DefaultExpressionEngineSymbols syms, final NodeMatcher<String> nodeNameMatcher) {
         if (syms == null) {
             throw new IllegalArgumentException("Symbols must not be null!");
         }
 
         symbols = syms;
         nameMatcher = nodeNameMatcher != null ? nodeNameMatcher : NodeNameMatchers.EQUALS;
     }
 
     /**
      * Gets the {@code DefaultExpressionEngineSymbols} object associated with this instance.
      *
      * @return the {@code DefaultExpressionEngineSymbols} used by this engine
      * @since 2.0
      */
     public DefaultExpressionEngineSymbols getSymbols() {
         return symbols;
     }
 
     /**
      * {@inheritDoc} This method supports the syntax as described in the class comment.
      */
     @Override
     public <T> List<QueryResult<T>> query(final T root, final String key, final NodeHandler<T> handler) {
         final List<QueryResult<T>> results = new LinkedList<>();
         findNodesForKey(new DefaultConfigurationKey(this, key).iterator(), root, results, handler);
         return results;
     }
 
     /**
      * {@inheritDoc} This implementation takes the given parent key, adds a property delimiter, and then adds the node's
      * name. The name of the root node is a blank string. Note that no indices are returned.
      */
     @Override
     public <T> String nodeKey(final T node, final String parentKey, final NodeHandler<T> handler) {
         if (parentKey == null) {
             // this is the root node
             return StringUtils.EMPTY;
         }
         final DefaultConfigurationKey key = new DefaultConfigurationKey(this, parentKey);
         key.append(handler.nodeName(node), true);
         return key.toString();
     }
 
     @Override
     public String attributeKey(final String parentKey, final String attributeName) {
         final DefaultConfigurationKey key = new DefaultConfigurationKey(this, parentKey);
         key.appendAttribute(attributeName);
         return key.toString();
     }
 
     /**
      * {@inheritDoc} This implementation works similar to {@code nodeKey()}; however, each key returned by this method has
      * an index (except for the root node). The parent key is prepended to the name of the current node in any case and
      * without further checks. If it is <b>null</b>, only the name of the current node with its index is returned.
      */
     @Override
     public <T> String canonicalKey(final T node, final String parentKey, final NodeHandler<T> handler) {
         final String nodeName = handler.nodeName(node);
         final T parent = handler.getParent(node);
         final DefaultConfigurationKey key = new DefaultConfigurationKey(this, parentKey);
         key.append(StringUtils.defaultString(nodeName));
 
         if (parent != null) {
             // this is not the root key
             key.appendIndex(determineIndex(node, parent, nodeName, handler));
         }
         return key.toString();
     }
 
     /**
      * <p>
      * Prepares Adding the property with the specified key.
      * </p>
      * <p>
      * To be able to deal with the structure supported by hierarchical configuration implementations the passed in key is of
      * importance, especially the indices it might contain. The following example should clarify this: Suppose the current
      * node structure looks like the following:
      * </p>
      *
      * <pre>
      *  tables
      *     +-- table
      *             +-- name = user
      *             +-- fields
      *                     +-- field
      *                             +-- name = uid
      *                     +-- field
      *                             +-- name = firstName
      *                     ...
      *     +-- table
      *             +-- name = documents
      *             +-- fields
      *                    ...
      * </pre>
      * <p>
      * In this example a database structure is defined, e.g. all fields of the first table could be accessed using the key
      * {@code tables.table(0).fields.field.name}. If now properties are to be added, it must be exactly specified at which
      * position in the hierarchy the new property is to be inserted. So to add a new field name to a table it is not enough
      * to say just
      * </p>
      *
      * <pre>
      * config.addProperty(&quot;tables.table.fields.field.name&quot;, &quot;newField&quot;);
      * </pre>
      * <p>
      * The statement given above contains some ambiguity. For instance it is not clear, to which table the new field should
      * be added. If this method finds such an ambiguity, it is resolved by following the last valid path. Here this would be
      * the last table. The same is true for the {@code field}; because there are multiple fields and no explicit index is
      * provided, a new {@code name} property would be added to the last field - which is probably not what was desired.
      * </p>
      * <p>
      * To make things clear explicit indices should be provided whenever possible. In the example above the exact table
      * could be specified by providing an index for the {@code table} element as in {@code tables.table(1).fields}. By
      * specifying an index it can also be expressed that at a given position in the configuration tree a new branch should
      * be added. In the example above we did not want to add an additional {@code name} element to the last field of the
      * table, but we want a complete new {@code field} element. This can be achieved by specifying an invalid index (like
      * -1) after the element where a new branch should be created. Given this our example would run:
      * </p>
      *
      * <pre>
      * config.addProperty(&quot;tables.table(1).fields.field(-1).name&quot;, &quot;newField&quot;);
      * </pre>
      * <p>
      * With this notation it is possible to add new branches everywhere. We could for instance create a new {@code table}
      * element by specifying
      * </p>
      *
      * <pre>
      * config.addProperty(&quot;tables.table(-1).fields.field.name&quot;, &quot;newField2&quot;);
      * </pre>
      * <p>
      * (Note that because after the {@code table} element a new branch is created indices in following elements are not
      * relevant; the branch is new so there cannot be any ambiguities.)
      * </p>
      *
      * @param <T> the type of the nodes to be dealt with
      * @param root the root node of the nodes hierarchy
      * @param key the key of the new property
      * @param handler the node handler
      * @return a data object with information needed for the add operation
      */
     @Override
     public <T> NodeAddData<T> prepareAdd(final T root, final String key, final NodeHandler<T> handler) {
         final DefaultConfigurationKey.KeyIterator it = new DefaultConfigurationKey(this, key).iterator();
         if (!it.hasNext()) {
             throw new IllegalArgumentException("Key for add operation must be defined!");
         }
 
         final T parent = findLastPathNode(it, root, handler);
         final List<String> pathNodes = new LinkedList<>();
 
         while (it.hasNext()) {
             if (!it.isPropertyKey()) {
                 throw new IllegalArgumentException("Invalid key for add operation: " + key + " (Attribute key in the middle.)");
             }
             pathNodes.add(it.currentKey());
             it.next();
         }
 
         return new NodeAddData<>(parent, it.currentKey(), !it.isPropertyKey(), pathNodes);
     }
 
     /**
      * Recursive helper method for evaluating a key. This method processes all facets of a configuration key, traverses the
      * tree of properties and fetches the results of all matching properties.
      *
      * @param <T> the type of nodes to be dealt with
      * @param keyPart the configuration key iterator
      * @param node the current node
      * @param results here the found results are stored
      * @param handler the node handler
      */
     protected <T> void findNodesForKey(final DefaultConfigurationKey.KeyIterator keyPart, final T node, final Collection<QueryResult<T>> results,
         final NodeHandler<T> handler) {
         if (!keyPart.hasNext()) {
             results.add(QueryResult.createNodeResult(node));
         } else {
             final String key = keyPart.nextKey(false);
             if (keyPart.isPropertyKey()) {
                 processSubNodes(keyPart, findChildNodesByName(handler, node, key), results, handler);
             }
             if (keyPart.isAttribute() && !keyPart.hasNext() && handler.getAttributeValue(node, key) != null) {
                 results.add(QueryResult.createAttributeResult(node, key));
             }
         }
     }
 
     /**
      * Finds the last existing node for an add operation. This method traverses the node tree along the specified key. The
      * last existing node on this path is returned.
      *
      * @param <T> the type of the nodes to be dealt with
      * @param keyIt the key iterator
      * @param node the current node
      * @param handler the node handler
      * @return the last existing node on the given path
      */
     protected <T> T findLastPathNode(final DefaultConfigurationKey.KeyIterator keyIt, final T node, final NodeHandler<T> handler) {
         final String keyPart = keyIt.nextKey(false);
 
         if (keyIt.hasNext()) {
             if (!keyIt.isPropertyKey()) {
                 // Attribute keys can only appear as last elements of the path
                 throw new IllegalArgumentException("Invalid path for add operation: " + "Attribute key in the middle!");
             }
             final int idx = keyIt.hasIndex() ? keyIt.getIndex() : handler.getMatchingChildrenCount(node, nameMatcher, keyPart) - 1;
             if (idx < 0 || idx >= handler.getMatchingChildrenCount(node, nameMatcher, keyPart)) {
                 return node;
             }
             return findLastPathNode(keyIt, findChildNodesByName(handler, node, keyPart).get(idx), handler);
         }
         return node;
     }
 
     /**
      * Called by {@code findNodesForKey()} to process the sub nodes of the current node depending on the type of the current
      * key part (children, attributes, or both).
      *
      * @param <T> the type of the nodes to be dealt with
      * @param keyPart the key part
      * @param subNodes a list with the sub nodes to process
      * @param nodes the target collection
      * @param handler the node handler
      */
     private <T> void processSubNodes(final DefaultConfigurationKey.KeyIterator keyPart, final List<T> subNodes, final Collection<QueryResult<T>> nodes,
         final NodeHandler<T> handler) {
         if (keyPart.hasIndex()) {
             if (keyPart.getIndex() >= 0 && keyPart.getIndex() < subNodes.size()) {
                 findNodesForKey((DefaultConfigurationKey.KeyIterator) keyPart.clone(), subNodes.get(keyPart.getIndex()), nodes, handler);
             }
         } else {
             subNodes.forEach(node -> findNodesForKey((DefaultConfigurationKey.KeyIterator) keyPart.clone(), node, nodes, handler));
         }
     }
 
     /**
      * Determines the index of the given node based on its parent node.
      *
      * @param node the current node
      * @param parent the parent node
      * @param nodeName the name of the current node
      * @param handler the node handler
      * @param <T> the type of the nodes to be dealt with
      * @return the index of this node
      */
     private <T> int determineIndex(final T node, final T parent, final String nodeName, final NodeHandler<T> handler) {
         return findChildNodesByName(handler, parent, nodeName).indexOf(node);
     }
 
     /**
      * Returns a list with all child nodes of the given parent node which match the specified node name. The match is done
      * using the current node name matcher.
      *
      * @param handler the {@code NodeHandler}
      * @param parent the parent node
      * @param nodeName the name of the current node
      * @param <T> the type of the nodes to be dealt with
      * @return a list with all matching child nodes
      */
     private <T> List<T> findChildNodesByName(final NodeHandler<T> handler, final T parent, final String nodeName) {
         return handler.getMatchingChildren(parent, nameMatcher, nodeName);
     }
 }