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Enhances extension capabilities of Yaml configuration files (~ to JCR 'extends' property), provides connection to the JCR config workspace and to all the registries as well
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/**
 * This file Copyright (c) 2016 Magnolia International
 * Ltd.  (http://www.magnolia-cms.com). All rights reserved.
 *
 *
 * This file is dual-licensed under both the Magnolia
 * Network Agreement and the GNU General Public License.
 * You may elect to use one or the other of these licenses.
 *
 * This file is distributed in the hope that it will be
 * useful, but AS-IS and WITHOUT ANY WARRANTY; without even the
 * implied warranty of MERCHANTABILITY or FITNESS FOR A
 * PARTICULAR PURPOSE, TITLE, or NONINFRINGEMENT.
 * Redistribution, except as permitted by whichever of the GPL
 * or MNA you select, is prohibited.
 *
 * 1. For the GPL license (GPL), you can redistribute and/or
 * modify this file under the terms of the GNU General
 * Public License, Version 3, as published by the Free Software
 * Foundation.  You should have received a copy of the GNU
 * General Public License, Version 3 along with this program;
 * if not, write to the Free Software Foundation, Inc., 51
 * Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * 2. For the Magnolia Network Agreement (MNA), this file
 * and the accompanying materials are made available under the
 * terms of the MNA which accompanies this distribution, and
 * is available at http://www.magnolia-cms.com/mna.html
 *
 * Any modifications to this file must keep this entire header
 * intact.
 *
 */
package org.mgnlconfig.yaml;

import org.mgnlconfig.yaml.constructs.EmptyMapConstruct;
import org.mgnlconfig.yaml.constructs.ExclusionConstruct;
import org.mgnlconfig.yaml.constructs.ExtensionConstruct;
import org.mgnlconfig.yaml.constructs.ListConstruct;
import org.mgnlconfig.yaml.constructs.OverridingConstruct;
import org.mgnlconfig.yaml.constructs.VerbatimDependencyInclusionConstruct;
import org.mgnlconfig.yaml.dependency.ConfigDependency;
import org.mgnlconfig.yaml.dependency.DependencyTracker;
import org.yaml.snakeyaml.constructor.Construct;
import org.yaml.snakeyaml.constructor.Constructor;
import org.yaml.snakeyaml.nodes.MappingNode;
import org.yaml.snakeyaml.nodes.Node;
import org.yaml.snakeyaml.nodes.ScalarNode;
import org.yaml.snakeyaml.nodes.SequenceNode;

import com.google.common.base.Optional;

public class MgnYamlConstructor extends Constructor {

    private final DependencyTracker dependencyTracker;

    public MgnYamlConstructor(DependencyTracker dependencyTracker) {
        this.dependencyTracker = dependencyTracker;
    }

    @Override
    protected Construct getConstructor(Node node) {
        Construct construct = super.getConstructor(node);

        final String tag = node.getTag().getValue();

        // handle the legacy case first
        if ("!include".equals(tag) && node instanceof ScalarNode) {
            final Optional legacyYamlDependency = dependencyTracker.resolveDependencyFromYamlTag(tag + ":" + ((ScalarNode) node).getValue());
            if (legacyYamlDependency.isPresent()) {
                return new VerbatimDependencyInclusionConstruct(legacyYamlDependency.get());
            }
        }

        // simply handle the exclusion case
        if ("!exclude".equals(tag)) {
            return new ExclusionConstruct();
        }

        final Optional dependency = dependencyTracker.resolveDependencyFromYamlTag(tag);

        // (re-)initialise base construct
        if (node instanceof MappingNode) {
            construct = new ConstructYamlMap();
        } else if (node instanceof SequenceNode) {
            construct = new ConstructYamlSeq();
        } else if (node instanceof ScalarNode && dependency.isPresent()) {
            construct = new EmptyMapConstruct();
        }

        // wrap base construct into an extending construct if a dependency is present
        if (dependency.isPresent()) {
            construct = new ExtensionConstruct(construct, dependency.get());
        }

        // wrap construct into an overriding construct if a dependency is present
        if ("!override".equals(tag)) {
            construct = new OverridingConstruct(construct);
        }

        // if a node represents a sequence - make sure we return a list eventually
        if (node instanceof SequenceNode) {
            construct = new ListConstruct(construct);
        }

        return construct;
    }

    public static class Exclusion {}

}