package moreservlets.filters;

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.util.zip.*;

/** Filter that compresses output with gzip
 *  (assuming that browser supports gzip).
 *  <P>
 *  Taken from More Servlets and JavaServer Pages
 *  from Prentice Hall and Sun Microsystems Press,
 *  http://www.moreservlets.com/.
 *  &copy; 2002 Marty Hall; may be freely used or adapted.
 */

public class CompressionFilter implements Filter {
  private FilterConfig config;

  /** If browser does not support gzip, invoke resource
   *  normally. If browser <I>does</I> support gzip,
   *  set the Content-Encoding response header and
   *  invoke resource with a wrapped response that
   *  collects all the output. Extract the output
   *  and write it into a gzipped byte array. Finally,
   *  write that array to the client's output stream.
   */
  
  public void doFilter(ServletRequest request,
                       ServletResponse response,
                       FilterChain chain)
      throws ServletException, IOException {
    HttpServletRequest req = (HttpServletRequest)request;
    HttpServletResponse res = (HttpServletResponse)response;
    if (!isGzipSupported(req)) {
      // Invoke resource normally.
      chain.doFilter(req,res);
    } else {
      // Tell browser we are sending it gzipped data.
      res.setHeader("Content-Encoding", "gzip");
      
      // Invoke resource, accumulating output in the wrapper.
      CharArrayWrapper responseWrapper =
        new CharArrayWrapper(res);
      chain.doFilter(req,responseWrapper);
      
      // Get character array representing output.
      char[] responseChars = responseWrapper.toCharArray();

      // Make a writer that compresses data and puts
      // it into a byte array.
      ByteArrayOutputStream byteStream =
        new ByteArrayOutputStream();
      GZIPOutputStream zipOut =
        new GZIPOutputStream(byteStream);
      OutputStreamWriter tempOut =
        new OutputStreamWriter(zipOut);
      
      // Compress original output and put it into byte array.
      tempOut.write(responseChars);
      
      // Gzip streams must be explicitly closed.
      tempOut.close();
      
      // Update the Content-Length header.
      res.setContentLength(byteStream.size());

      // Send compressed result to client.
      OutputStream realOut = res.getOutputStream();
      byteStream.writeTo(realOut);
    }
  }

  /** Store the FilterConfig object in case subclasses
   *  want it.
   */
  
  public void init(FilterConfig config)
      throws ServletException {
    this.config = config;
  }

  protected FilterConfig getFilterConfig() {
    return(config);
  }
  
  public void destroy() {}

  private boolean isGzipSupported(HttpServletRequest req) {
    String browserEncodings =
      req.getHeader("Accept-Encoding");
    return((browserEncodings != null) &&
           (browserEncodings.indexOf("gzip") != -1));
  }
}