AbstractYAMLBasedConfiguration

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 * 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
 *
 *     https://www.apache.org/licenses/LICENSE-2.0
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package org.apache.commons.configuration2;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.stream.Collectors;

import org.apache.commons.configuration2.ex.ConfigurationException;
import org.apache.commons.configuration2.io.ConfigurationLogger;
import org.apache.commons.configuration2.tree.ImmutableNode;
import org.apache.commons.lang3.StringUtils;

/**
 * <p>
 * A base class for configuration implementations based on YAML structures.
 * </p>
 * <p>
 * This base class offers functionality related to YAML-like data structures based on maps. Such a map has strings as
 * keys and arbitrary objects as values. The class offers methods to transform such a map into a hierarchy of
 * {@link ImmutableNode} objects and vice versa.
 * </p>
 *
 * @since 2.2
 */
public class AbstractYAMLBasedConfiguration extends BaseHierarchicalConfiguration {

    /**
     * Adds a key value pair to a map, taking list structures into account. If a key is added which is already present in
     * the map, this method ensures that a list is created.
     *
     * @param map the map
     * @param key the key
     * @param value the value
     */
    private static void addEntry(final Map<String, Object> map, final String key, final Object value) {
        final Object oldValue = map.get(key);
        if (oldValue == null) {
            map.put(key, value);
        } else if (oldValue instanceof Collection) {
            // safe case because the collection was created by ourselves
            @SuppressWarnings("unchecked")
            final Collection<Object> values = (Collection<Object>) oldValue;
            values.add(value);
        } else {
            final Collection<Object> values = new ArrayList<>();
            values.add(oldValue);
            values.add(value);
            map.put(key, values);
        }
    }

    /**
     * Creates a part of the hierarchical nodes structure of the resulting configuration. The passed in element is converted into one or multiple configuration
     * nodes. (If list structures are involved, multiple nodes are returned.)
     *
     * @param key     the key of the new node(s).
     * @param elem    the element to be processed.
     * @param visited the set of visited objects.
     * @return a list with configuration nodes representing the element
     */
    private static List<ImmutableNode> constructHierarchy(final String key, final Object elem, final Set<Object> visited) {
        if (elem instanceof Map) {
            return isVisisted(elem, visited) ? Collections.emptyList() : parseMap((Map<String, Object>) elem, key, visited);
        }
        if (elem instanceof Collection) {
            return isVisisted(elem, visited) ? Collections.emptyList() : parseCollection((Collection<Object>) elem, key, visited);
        }
        return Collections.singletonList(new ImmutableNode.Builder().name(key).value(elem).create());
    }

    private static boolean isVisisted(final Object elem, final Set<Object> visited) {
        return !visited.add(System.identityHashCode(elem));
    }

    /**
     * Parses a collection structure. The elements of the collection are processed recursively.
     *
     * @param col     the collection to be processed.
     * @param key     the key under which this collection is to be stored.
     * @param visited the set of visited objects.
     * @return a node representing this collection.
     */
    private static List<ImmutableNode> parseCollection(final Collection<Object> col, final String key, final Set<Object> visited) {
        return col.stream().flatMap(elem -> constructHierarchy(key, elem, visited).stream()).collect(Collectors.toList());
    }

    /**
     * Parses a map structure. The single keys of the map are processed recursively.
     *
     * @param map     the map to be processed.
     * @param key     the key under which this map is to be stored.
     * @param visited the set of visited objects.
     * @return a node representing this map
     */
    private static List<ImmutableNode> parseMap(final Map<String, Object> map, final String key, final Set<Object> visited) {
        final ImmutableNode.Builder subtree = new ImmutableNode.Builder().name(key);
        map.forEach((k, v) -> constructHierarchy(k, v, visited).forEach(subtree::addChild));
        return Collections.singletonList(subtree.create());
    }

    /**
     * Internal helper method to wrap an exception in a {@code ConfigurationException}.
     *
     * @param e the exception to be wrapped
     * @throws ConfigurationException the resulting exception
     */
    static void rethrowException(final Exception e) throws ConfigurationException {
        if (e instanceof ClassCastException) {
            throw new ConfigurationException("Error parsing", e);
        }
        throw new ConfigurationException("Unable to load the configuration", e);
    }

    /**
     * Creates a new instance of {@code AbstractYAMLBasedConfiguration}.
     */
    protected AbstractYAMLBasedConfiguration() {
        initLogger(new ConfigurationLogger(getClass()));
    }

    /**
     * Creates a new instance of {@code AbstractYAMLBasedConfiguration} as a copy of the specified configuration.
     *
     * @param c the configuration to be copied
     */
    protected AbstractYAMLBasedConfiguration(final HierarchicalConfiguration<ImmutableNode> c) {
        super(c);
        initLogger(new ConfigurationLogger(getClass()));
    }

    /**
     * Constructs a YAML map, i.e. String -&gt; Object from a given configuration node.
     *
     * @param node The configuration node to create a map from.
     * @return A Map that contains the configuration node information.
     */
    protected Map<String, Object> constructMap(final ImmutableNode node) {
        final Map<String, Object> map = new HashMap<>(node.getChildren().size());
        node.forEach(cNode -> addEntry(map, cNode.getNodeName(), cNode.getChildren().isEmpty() ? cNode.getValue() : constructMap(cNode)));
        return map;
    }

    /**
     * Loads this configuration from the content of the specified map. The data in the map is transformed into a hierarchy
     * of {@link ImmutableNode} objects.
     *
     * @param map the map to be processed
     */
    protected void load(final Map<String, Object> map) {
        final List<ImmutableNode> roots = constructHierarchy(StringUtils.EMPTY, map, new HashSet<>());
        if (!roots.isEmpty()) {
            getNodeModel().setRootNode(roots.get(0));
        }
    }
}