package com.atlassian.plugin.webresource.transformer;
   
   import com.atlassian.plugin.servlet.DownloadException;
   import com.atlassian.plugin.servlet.DownloadableResource;
   import com.google.common.base.Function;
   import org.apache.commons.io.IOUtils;
   import org.apache.commons.io.input.CharSequenceReader;
   
   import java.io.IOException;
  import java.io.OutputStream;
  import java.io.StringWriter;
  import java.nio.charset.Charset;
  
  /**
   * Utility class for transforming resources
   */
  public class TransformerUtils
  {
      public static final Charset UTF8 = Charset.forName("UTF-8");
  
      /**
       * Write apply a given transform a resource and then write the transformed content
       * to the supplied OutputStream.
       * Note that the OutputStream will not be closed by this method.
       * @param originalResource - the resource to transform
       * @param encoding - the encoding to use for writing
       * @param out - the output stream
       * @param transform - a function for transforming the content
       * @throws DownloadException - thrown if it is not possible to stream the output
       * @since 2.9.0
       */
      public static void transformAndStreamResource(final DownloadableResource originalResource, final Charset encoding, final OutputStream out, final Function<CharSequence, CharSequence> transform) throws DownloadException
      {
          try
          {
              final StringWriter writer = new StringWriter();
              final WriterOutputStream output = new WriterOutputStream(writer, encoding);
              originalResource.streamResource(output);
              output.flush();
              IOUtils.copy(new CharSequenceReader(transform.apply(writer.getBuffer().toString())), out, encoding.name());
          }
          catch (final IOException e)
          {
              throw new DownloadException("Unable to stream to the output", e);
          }
      }
  }