/*

  * 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.configuration.tree;

 import java.util.List;

 /**

  * <p>

  * Definition of an interface for evaluating keys for hierarchical

  * configurations.

  * </p>

  * <p>

  * An <em>expression engine</em> knows how to map a key for a configuration's

  * property to a single or a set of configuration nodes. Thus it defines the way

  * how properties are addressed in this configuration. Methods of a

  * configuration that have to handle property key (e.g.

  * <code>getProperty()</code> or <code>addProperty()</code> do not interpret

  * the passed in keys on their own, but delegate this task to an associated

  * expression engine. This expression engine will then find out, which

  * configuration nodes are addressed by the key.

  * </p>

  * <p>

  * Seperating the task of evaluating property keys from the configuration object

  * has the advantage that many different expression languages (i.e. ways for

  * querying or setting properties) can be supported. Just set a suitable

  * implementation of this interface as the configuration's expression engine,

  * and you can use the syntax provided by this implementation.

  * </p>

  *

  * @since 1.3

  * @author Oliver Heger

  */

 public interface ExpressionEngine

 {

     /**

      * Finds the node(s) that is (are) matched by the specified key. This is the

      * main method for interpreting property keys. An implementation must

      * traverse the given root node and its children to find all nodes that are

      * matched by the given key. If the key is not correct in the syntax

      * provided by that implementation, it is free to throw a (runtime)

      * exception indicating this error condition.

      *

      * @param root the root node of a hierarchy of configuration nodes

      * @param key the key to be evaluated

      * @return a list with the nodes that are matched by the key (should never

      * be <b>null</b>)

      */

     List query(ConfigurationNode root, String key);

     /**

      * Returns the key for the specified node in the expression language

      * supported by an implementation. This method is called whenever a property

      * key for a node has to be constructed, e.g. by the

      * <code>{@link org.apache.commons.configuration.Configuration#getKeys() getKeys()}</code>

      * method.

      *

      * @param node the node, for which the key must be constructed

      * @param parentKey the key of this node's parent (can be <b>null</b> for

      * the root node)

      * @return this node's key

      */

     String nodeKey(ConfigurationNode node, String parentKey);

     /**

      * Returns information needed for an add operation. This method gets called

      * when new properties are to be added to a configuration. An implementation

      * has to interpret the specified key, find the parent node for the new

      * elements, and provide all information about new nodes to be added.

      *

      * @param root the root node

      * @param key the key for the new property

      * @return an object with all information needed for the add operation

      */

     NodeAddData prepareAdd(ConfigurationNode root, String key);

 }