001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *     https://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.commons.configuration2.tree;
018
019import java.util.LinkedList;
020import java.util.List;
021
022/**
023 * <p>
024 * A class providing different algorithms for traversing a hierarchy of configuration nodes.
025 * </p>
026 * <p>
027 * The methods provided by this class accept a {@link ConfigurationNodeVisitor} and visit all nodes in a hierarchy
028 * starting from a given root node. Because a {@link NodeHandler} has to be passed in, too, arbitrary types of nodes can
029 * be processed. The {@code walk()} methods differ in the order in which nodes are visited. Details can be found in the
030 * method documentation.
031 * </p>
032 * <p>
033 * An instance of this class does not define any state; therefore, it can be shared and used concurrently. The
034 * {@code INSTANCE} member field can be used for accessing a default instance. If desired (for example for testing purposes),
035 * new instances can be created.
036 * </p>
037 *
038 * @since 2.0
039 */
040public class NodeTreeWalker {
041
042    /** The default instance of this class. */
043    public static final NodeTreeWalker INSTANCE = new NodeTreeWalker();
044
045    /**
046     * Helper method for performing a BFS traversal. Implementation node: This method organizes the nodes to be visited in
047     * structures on the heap. Therefore, it can deal with larger structures than would be the case in a recursive approach
048     * (where the stack size limits the size of the structures which can be traversed).
049     *
050     * @param root the root node to be navigated
051     * @param visitor the visitor
052     * @param handler the handler
053     * @param <T> the type of the nodes involved
054     */
055    private static <T> void bfs(final T root, final ConfigurationNodeVisitor<T> visitor, final NodeHandler<T> handler) {
056        final List<T> pendingNodes = new LinkedList<>();
057        pendingNodes.add(root);
058        boolean cancel = false;
059
060        while (!pendingNodes.isEmpty() && !cancel) {
061            final T node = pendingNodes.remove(0);
062            visitor.visitBeforeChildren(node, handler);
063            cancel = visitor.terminate();
064            pendingNodes.addAll(handler.getChildren(node));
065        }
066    }
067
068    /**
069     * Helper method for checking the parameters for the walk() methods. If mandatory parameters are missing, an exception
070     * is thrown. The return value indicates whether an operation can be performed.
071     *
072     * @param root the root node
073     * @param visitor the visitor
074     * @param handler the handler
075     * @param <T> the type of the nodes involved
076     * @return <strong>true</strong> if a walk operation can be performed, <strong>false</strong> otherwise
077     * @throws IllegalArgumentException if a required parameter is missing
078     */
079    private static <T> boolean checkParameters(final T root, final ConfigurationNodeVisitor<T> visitor, final NodeHandler<T> handler) {
080        if (visitor == null) {
081            throw new IllegalArgumentException("Visitor must not be null!");
082        }
083        if (handler == null) {
084            throw new IllegalArgumentException("NodeHandler must not be null!");
085        }
086        return root != null;
087    }
088
089    /**
090     * Recursive helper method for performing a DFS traversal.
091     *
092     * @param node the current node
093     * @param visitor the visitor
094     * @param handler the handler
095     * @param <T> the type of the nodes involved
096     */
097    private static <T> void dfs(final T node, final ConfigurationNodeVisitor<T> visitor, final NodeHandler<T> handler) {
098        if (!visitor.terminate()) {
099            visitor.visitBeforeChildren(node, handler);
100            handler.getChildren(node).forEach(c -> dfs(c, visitor, handler));
101            if (!visitor.terminate()) {
102                visitor.visitAfterChildren(node, handler);
103            }
104        }
105    }
106
107    /**
108     * Constructs a new instance.
109     */
110    public NodeTreeWalker() {
111        // empty
112    }
113
114    /**
115     * Visits all nodes in the hierarchy represented by the given root node in <em>breadth first search</em> manner. This
116     * means that the nodes are visited in an order corresponding to the distance from the root node: first the root node is
117     * visited, then all direct children of the root node, then all direct children of the first child of the root node,
118     * etc. In this mode of traversal, there is no direct connection between the encounter of a node and its children.
119     * <strong>Therefore, on the visitor object only the {@code visitBeforeChildren()} method gets called!</strong>.
120     *
121     * @param root the root node of the hierarchy to be processed (may be <strong>null</strong>, then this call has no effect)
122     * @param visitor the {@code ConfigurationNodeVisitor} (must not be <strong>null</strong>)
123     * @param handler the {@code NodeHandler} (must not be <strong>null</strong>)
124     * @param <T> the type of the nodes involved
125     * @throws IllegalArgumentException if a required parameter is <strong>null</strong>
126     */
127    public <T> void walkBFS(final T root, final ConfigurationNodeVisitor<T> visitor, final NodeHandler<T> handler) {
128        if (checkParameters(root, visitor, handler)) {
129            bfs(root, visitor, handler);
130        }
131    }
132
133    /**
134     * Visits all nodes in the hierarchy represented by the given root node in <em>depth first search</em> manner. This
135     * means that first {@link ConfigurationNodeVisitor#visitBeforeChildren(Object, NodeHandler)} is called on a node, then
136     * recursively all of its children are processed, and eventually
137     * {@link ConfigurationNodeVisitor#visitAfterChildren(Object, NodeHandler)} gets invoked.
138     *
139     * @param root the root node of the hierarchy to be processed (may be <strong>null</strong>, then this call has no effect)
140     * @param visitor the {@code ConfigurationNodeVisitor} (must not be <strong>null</strong>)
141     * @param handler the {@code NodeHandler} (must not be <strong>null</strong>)
142     * @param <T> the type of the nodes involved
143     * @throws IllegalArgumentException if a required parameter is <strong>null</strong>
144     */
145    public <T> void walkDFS(final T root, final ConfigurationNodeVisitor<T> visitor, final NodeHandler<T> handler) {
146        if (checkParameters(root, visitor, handler)) {
147            dfs(root, visitor, handler);
148        }
149    }
150}