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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,
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 *  KIND, either express or implied.  See the License for the
 *  specific language governing permissions and limitations
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package org.apache.cayenne.velocity;

import java.io.IOException;
import java.io.StringWriter;
import java.io.Writer;

import org.apache.velocity.context.InternalContextAdapter;
import org.apache.velocity.exception.MethodInvocationException;
import org.apache.velocity.exception.ParseErrorException;
import org.apache.velocity.exception.ResourceNotFoundException;
import org.apache.velocity.runtime.directive.Directive;
import org.apache.velocity.runtime.parser.node.ASTDirective;
import org.apache.velocity.runtime.parser.node.Node;

/**
 * A custom Velocity directive to conditionally join a number of {@link ChunkDirective chunks}.
 * Usage of chain is the following:
 * 
 * <pre>
 * #chain(operator) - e.g. #chain(' AND ')
 * #chain(operator prefix) - e.g. #chain(' AND ' 'WHERE ')</pre>
 * 
 * <p><code>operator</code> (e.g. AND, OR, etc.) is used to join chunks that are included
 * in a chain. <code>prefix</code> is inserted if a chain contains at least one chunk.
 * </p>
 * 
 * @since 1.1
 */
public class ChainDirective extends Directive {

    @Override
    public String getName() {
        return "chain";
    }

    @Override
    public int getType() {
        return BLOCK;
    }

    @Override
    public boolean render(InternalContextAdapter context, Writer writer, Node node)
        throws
            IOException,
            ResourceNotFoundException,
            ParseErrorException,
            MethodInvocationException {

        int size = node.jjtGetNumChildren();
        if (size == 0) {
            return true;
        }

        // BLOCK is the last child
        Node block = node.jjtGetChild(node.jjtGetNumChildren() - 1);
        String join = (size > 1) ? (String) node.jjtGetChild(0).value(context) : "";
        String prefix = (size > 2) ? (String) node.jjtGetChild(1).value(context) : "";

        // if there is a conditional prefix, use a separate buffer for children
        StringWriter childWriter = new StringWriter(30);

        int len = block.jjtGetNumChildren();
        int includedChunks = 0;
        for (int i = 0; i < len; i++) {
            Node child = block.jjtGetChild(i);

            // if this is a "chunk", evaluate its expression and prepend join if included...
            if (child instanceof ASTDirective
                && "chunk".equals(((ASTDirective) child).getDirectiveName())) {

                if (child.jjtGetNumChildren() < 2
                    || child.jjtGetChild(0).value(context) != null) {

                    if (includedChunks > 0) {
                        childWriter.write(join);
                    }

                    includedChunks++;
                }
            }

            child.render(context, childWriter);
        }

        if (includedChunks > 0) {
            childWriter.flush();
            writer.write(prefix);
            writer.write(childWriter.toString());
        }

        return true;
    }
}