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Tools to assist in the reading of configuration/preferences files in various formats
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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;

import java.util.LinkedList;
import java.util.List;

/**
 * 
 * A class providing different algorithms for traversing a hierarchy of configuration nodes.
 * 
 * 
 * The methods provided by this class accept a {@link ConfigurationNodeVisitor} and visit all nodes in a hierarchy
 * starting from a given root node. Because a {@link NodeHandler} has to be passed in, too, arbitrary types of nodes can
 * be processed. The {@code walk()} methods differ in the order in which nodes are visited. Details can be found in the
 * method documentation.
 * 
 * 
 * An instance of this class does not define any state; therefore, it can be shared and used concurrently. The
 * {@code INSTANCE} member field can be used for accessing a default instance. If desired (e.g. for testing purposes),
 * new instances can be created.
 * 
 *
 * @since 2.0
 */
public class NodeTreeWalker {
    /** The default instance of this class. */
    public static final NodeTreeWalker INSTANCE = new NodeTreeWalker();

    /**
     * Helper method for performing a BFS traversal. Implementation node: This method organizes the nodes to be visited in
     * structures on the heap. Therefore, it can deal with larger structures than would be the case in a recursive approach
     * (where the stack size limits the size of the structures which can be traversed).
     *
     * @param root the root node to be navigated
     * @param visitor the visitor
     * @param handler the handler
     * @param  the type of the nodes involved
     */
    private static  void bfs(final T root, final ConfigurationNodeVisitor visitor, final NodeHandler handler) {
        final List pendingNodes = new LinkedList<>();
        pendingNodes.add(root);
        boolean cancel = false;

        while (!pendingNodes.isEmpty() && !cancel) {
            final T node = pendingNodes.remove(0);
            visitor.visitBeforeChildren(node, handler);
            cancel = visitor.terminate();
            pendingNodes.addAll(handler.getChildren(node));
        }
    }

    /**
     * Helper method for checking the parameters for the walk() methods. If mandatory parameters are missing, an exception
     * is thrown. The return value indicates whether an operation can be performed.
     *
     * @param root the root node
     * @param visitor the visitor
     * @param handler the handler
     * @param  the type of the nodes involved
     * @return true if a walk operation can be performed, false otherwise
     * @throws IllegalArgumentException if a required parameter is missing
     */
    private static  boolean checkParameters(final T root, final ConfigurationNodeVisitor visitor, final NodeHandler handler) {
        if (visitor == null) {
            throw new IllegalArgumentException("Visitor must not be null!");
        }
        if (handler == null) {
            throw new IllegalArgumentException("NodeHandler must not be null!");
        }
        return root != null;
    }

    /**
     * Recursive helper method for performing a DFS traversal.
     *
     * @param node the current node
     * @param visitor the visitor
     * @param handler the handler
     * @param  the type of the nodes involved
     */
    private static  void dfs(final T node, final ConfigurationNodeVisitor visitor, final NodeHandler handler) {
        if (!visitor.terminate()) {
            visitor.visitBeforeChildren(node, handler);
            handler.getChildren(node).forEach(c -> dfs(c, visitor, handler));
            if (!visitor.terminate()) {
                visitor.visitAfterChildren(node, handler);
            }
        }
    }

    /**
     * Visits all nodes in the hierarchy represented by the given root node in breadth first search manner. This
     * means that the nodes are visited in an order corresponding to the distance from the root node: first the root node is
     * visited, then all direct children of the root node, then all direct children of the first child of the root node,
     * etc. In this mode of traversal, there is no direct connection between the encounter of a node and its children.
     * Therefore, on the visitor object only the {@code visitBeforeChildren()} method gets called!.
     *
     * @param root the root node of the hierarchy to be processed (may be null, then this call has no effect)
     * @param visitor the {@code ConfigurationNodeVisitor} (must not be null)
     * @param handler the {@code NodeHandler} (must not be null)
     * @param  the type of the nodes involved
     * @throws IllegalArgumentException if a required parameter is null
     */
    public  void walkBFS(final T root, final ConfigurationNodeVisitor visitor, final NodeHandler handler) {
        if (checkParameters(root, visitor, handler)) {
            bfs(root, visitor, handler);
        }
    }

    /**
     * Visits all nodes in the hierarchy represented by the given root node in depth first search manner. This
     * means that first {@link ConfigurationNodeVisitor#visitBeforeChildren(Object, NodeHandler)} is called on a node, then
     * recursively all of its children are processed, and eventually
     * {@link ConfigurationNodeVisitor#visitAfterChildren(Object, NodeHandler)} gets invoked.
     *
     * @param root the root node of the hierarchy to be processed (may be null, then this call has no effect)
     * @param visitor the {@code ConfigurationNodeVisitor} (must not be null)
     * @param handler the {@code NodeHandler} (must not be null)
     * @param  the type of the nodes involved
     * @throws IllegalArgumentException if a required parameter is null
     */
    public  void walkDFS(final T root, final ConfigurationNodeVisitor visitor, final NodeHandler handler) {
        if (checkParameters(root, visitor, handler)) {
            dfs(root, visitor, handler);
        }
    }
}