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   *  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
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   *    https://www.apache.org/licenses/LICENSE-2.0
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  package org.apache.cayenne.velocity;
  
  import java.io.IOException;
  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.Node;
  
  /**
   * A custom Velocity directive to describe a conditional chunk of a {@link ChainDirective chain}.
   * Usage of chunk is the following:
   * 
   * <pre>
   * #chunk()...#end - e.g. #chunk()A = 5#end
   * #chunk($paramKey)...#end - e.g. #chunk($a)A = $a#end
   * </pre>
   * <p>
   * If context contains paramKey and it's value isn't null, chunk is included in the
   * chain, and if it is not the first chunk, it is prefixed with chain join (OR/AND).
   * If context doesn't contain paramKey or it's value is null, chunk is skipped.
   * @since 1.1
   */
  public class ChunkDirective extends Directive {
  
      @Override
      public String getName() {
          return "chunk";
      }
  
      @Override
      public int getType() {
          return BLOCK;
      }
  
      @Override
      public boolean render(InternalContextAdapter context, Writer writer, Node node)
              throws IOException, ResourceNotFoundException, ParseErrorException,
              MethodInvocationException {
  
          // first child is an expression, second is BLOCK
          if (node.jjtGetNumChildren() > 1 && node.jjtGetChild(0).value(context) == null) {
              // skip this chunk
              return false;
          }
  
          // BLOCK is the last child
          Node block = node.jjtGetChild(node.jjtGetNumChildren() - 1);
          block.render(context, writer);
          return true;
      }
  
  }
  