HierarchicalConfigurationConverter

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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;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.Set;

import org.apache.commons.configuration2.tree.DefaultConfigurationKey;
import org.apache.commons.configuration2.tree.DefaultExpressionEngine;

/**
 * <p>
 * A base class for converters that transform a normal configuration object into a hierarchical configuration.
 * </p>
 * <p>
 * This class provides a default mechanism for iterating over the keys in a configuration and to throw corresponding
 * element start and end events. By handling these events a hierarchy can be constructed that is equivalent to the keys
 * in the original configuration.
 * </p>
 * <p>
 * Concrete sub classes will implement event handlers that generate SAX events for XML processing or construct a
 * {@code HierarchicalConfiguration} root node. All in all with this class it is possible to treat a default
 * configuration as if it was a hierarchical configuration, which can be sometimes useful.
 * </p>
 *
 * @see HierarchicalConfiguration
 */
abstract class HierarchicalConfigurationConverter {
    /**
     * Processes the specified configuration object. This method implements the iteration over the configuration's keys. All
     * defined keys are translated into a set of element start and end events represented by calls to the
     * {@code elementStart()} and {@code elementEnd()} methods.
     *
     * @param config the configuration to be processed
     */
    public void process(final Configuration config) {
        if (config != null) {
            final DefaultExpressionEngine exprEngine = DefaultExpressionEngine.INSTANCE;
            final DefaultConfigurationKey keyEmpty = new DefaultConfigurationKey(exprEngine);
            DefaultConfigurationKey keyLast = keyEmpty;
            final Set<String> keySet = new HashSet<>();

            for (final Iterator<String> it = config.getKeys(); it.hasNext();) {
                final String key = it.next();
                if (keySet.contains(key)) {
                    // this key has already been processed by openElements
                    continue;
                }
                final DefaultConfigurationKey keyAct = new DefaultConfigurationKey(exprEngine, key);
                closeElements(keyLast, keyAct);
                final String elem = openElements(keyLast, keyAct, config, keySet);
                fireValue(elem, config.getProperty(key));
                keyLast = keyAct;
            }

            // close all open
            closeElements(keyLast, keyEmpty);
        }
    }

    /**
     * An event handler method that is called when an element starts. Concrete sub classes must implement it to perform a
     * proper event handling.
     *
     * @param name the name of the new element
     * @param value the element's value; can be <b>null</b> if the element does not have any value
     */
    protected abstract void elementStart(String name, Object value);

    /**
     * An event handler method that is called when an element ends. For each call of {@code elementStart()} there will be a
     * corresponding call of this method. Concrete sub classes must implement it to perform a proper event handling.
     *
     * @param name the name of the ending element
     */
    protected abstract void elementEnd(String name);

    /**
     * Fires all necessary element end events for the specified keys. This method is called for each key obtained from the
     * configuration to be converted. It calculates the common part of the actual and the last processed key and thus
     * determines how many elements must be closed.
     *
     * @param keyLast the last processed key
     * @param keyAct the actual key
     */
    protected void closeElements(final DefaultConfigurationKey keyLast, final DefaultConfigurationKey keyAct) {
        final DefaultConfigurationKey keyDiff = keyAct.differenceKey(keyLast);
        final Iterator<String> it = reverseIterator(keyDiff);
        if (it.hasNext()) {
            // Skip first because it has already been closed by fireValue()
            it.next();
        }

        while (it.hasNext()) {
            elementEnd(it.next());
        }
    }

    /**
     * Helper method for determining a reverse iterator for the specified key. This implementation returns an iterator that
     * returns the parts of the given key in reverse order, ignoring indices.
     *
     * @param key the key
     * @return a reverse iterator for the parts of this key
     */
    protected Iterator<String> reverseIterator(final DefaultConfigurationKey key) {
        final List<String> list = new ArrayList<>();
        for (final DefaultConfigurationKey.KeyIterator it = key.iterator(); it.hasNext();) {
            list.add(it.nextKey());
        }
        Collections.reverse(list);
        return list.iterator();
    }

    /**
     * Fires all necessary element start events for the specified key. This method is called for each key obtained from the
     * configuration to be converted. It ensures that all elements "between" the last key and the actual key are opened and
     * their values are set.
     *
     * @param keyLast the last processed key
     * @param keyAct the actual key
     * @param config the configuration to process
     * @param keySet the set with the processed keys
     * @return the name of the last element on the path
     */
    protected String openElements(final DefaultConfigurationKey keyLast, final DefaultConfigurationKey keyAct, final Configuration config,
        final Set<String> keySet) {
        final DefaultConfigurationKey.KeyIterator it = keyLast.differenceKey(keyAct).iterator();
        final DefaultConfigurationKey k = keyLast.commonKey(keyAct);
        for (it.nextKey(); it.hasNext(); it.nextKey()) {
            k.append(it.currentKey(true));
            elementStart(it.currentKey(true), config.getProperty(k.toString()));
            keySet.add(k.toString());
        }
        return it.currentKey(true);
    }

    /**
     * Fires all necessary element start events with the actual element values. This method is called for each key obtained
     * from the configuration to be processed with the last part of the key as argument. The value can be either a single
     * value or a collection.
     *
     * @param name the name of the actual element
     * @param value the element's value
     */
    protected void fireValue(final String name, final Object value) {
        if (value instanceof Collection) {
            final Collection<?> valueCol = (Collection<?>) value;
            valueCol.forEach(v -> fireValue(name, v));
        } else {
            elementStart(name, value);
            elementEnd(name);
        }
    }
}