package yarfraw.io;

 import java.io.IOException;

 import java.io.InputStream;

 import javax.xml.bind.ValidationEventHandler;

 import org.apache.commons.httpclient.Header;

 import org.apache.commons.httpclient.HttpClient;

 import org.apache.commons.httpclient.HttpURL;

 import org.apache.commons.httpclient.methods.GetMethod;

 import org.apache.commons.httpclient.params.HttpClientParams;

 import yarfraw.core.datamodel.ChannelFeed;

 import yarfraw.core.datamodel.YarfrawException;

 /**

  * This is a special {@link FeedReader} that supports "conditional get".

  * <p>

  * When a client first asks this reader to read from a remote source, this

  * reader automatically holds a reference to the returned {@link ChannelFeed}.

  * <br/>

  * When a client asks this reader to read again from the remote source, this reader

  * first queries the server to check if there's any new changes (by checking its last

  * modified date). If there is new changes, it reads and parse the updated feed and updates

  * the cache {@link ChannelFeed}. If there is no new changes since the last read, it returns

  * the cached {@link ChannelFeed} since there's no new changes.

  * </p>

  * <p>

  * Since this reader only need to perform parsing when there's new changes, it will

  * have better performance than the normal {@link FeedReader} class.

  * </p> 

  * Note that this class is only useful for reading from a remote source. 

  * 

  * <p>

  * for more information about conditional get, see 

  * <a href="http://fishbowl.pastiche.org/2002/10/21/http_conditional_get_for_rss_hackers" />

  * </p>

  * @author jliang

  *

  */

 public class CachedFeedReader extends FeedReader{

   private static final String ETAG = "ETag";

   private static final String LAST_MODIFIED = "Last-Modified";

   private static final int NOT_MODIFIED_STATUS_CODE = 304;

   private static final String IF_NONE_MATCH = "If-None-Match";

   private static final String IF_MODIFIED_SINCE = "If-Modified-Since";

   private ChannelFeed _cachedChannelFeed;

   private String _lastModified;

   private String _eTag;

   /**

    * Reads a channel from a local or remote feed.

    * <br/>

    * This method performs conditional get, if the remote is not modified

    * since last request, the cached feed will be returned.

    * 

    * @return a {@link ChannelFeed} object

    * @throws YarfrawException if read operation failed.

    */

   @Override

   public ChannelFeed readChannel() throws YarfrawException{

     try {

       _cachedChannelFeed = super.readChannel(null);

     } catch (NotModfiedException e) {

       //no changes since last request, no need to update cache

       //just return the cached feed

     }

     return _cachedChannelFeed;

   }

   /**

    * Reads a channel from a local or remote feed with a custom {@link ValidationEventHandler}

    *<br/>

    * This method performs conditional get, if the remote is not modified

    * since last request, the cached feed will be returned.

    * 

    * @param validationEventHandler a custom {@link javax.xml.bind.ValidationEventHandler}

    * @return a {@link ChannelFeed} object

    * @throws YarfrawException if read operation failed.

    */

   public ChannelFeed readChannel(ValidationEventHandler validationEventHandler) throws YarfrawException{

     try {

       _cachedChannelFeed = super.readChannel(validationEventHandler);

     } catch (NotModfiedException e) {

       //no changes since last request, no need to update cache

       //just return the cached feed

     }

     return _cachedChannelFeed;

   }

   //a special exception to signal there's no need changes to be parsed

   //the reader can just return the cached version

   class NotModfiedException extends RuntimeException{

     private static final long serialVersionUID = 1L;}

   @Override

   protected InputStream getStream() throws IOException{

     InputStream stream;

     GetMethod get = new GetMethod(_httpUrl.toString());

     HttpClient client = new HttpClient();

     if(_httpClientParams != null){

       client.setParams(_httpClientParams);

     }

     //there's a cached version of the feed, add the request

     //header to perform conditional get

     if(_cachedChannelFeed != null){

       get.addRequestHeader(new Header(IF_MODIFIED_SINCE, _lastModified));

       get.addRequestHeader(new Header(IF_NONE_MATCH, _eTag));

     }

     client.executeMethod(get);

     //get the headers

     Header lastModified = get.getResponseHeader(LAST_MODIFIED);

     _lastModified = lastModified == null ? null : lastModified.getValue();

     Header eTag = get.getResponseHeader(ETAG);

     _eTag = eTag == null ? null : eTag.getValue();

     if(get.getStatusCode()==NOT_MODIFIED_STATUS_CODE){

       throw new NotModfiedException();

     }

     stream = get.getResponseBodyAsStream();

     return stream; 

   }

   /**

    * 

    * @return - The value of the "Last-Modified" response header from the last read.

    */

   public String getLastModified() {

     return _lastModified;

   }

   /**

    * 

    * @return The value of the "ETag" response header from the last read.

    */

   public String getETag() {

     return _eTag;

   }

   /**

    * Setter for the "ETag" response header. 

    * <br/>This method is used to let the caller set an initial value

    * for this header. <br/> 

    * This value will be automatically updated by this class for every

    * {@link #readChannel()} method call.

    * @param tag

    */

   public void setETag(String tag) {

         _eTag = tag;

   }

   /**

    * Setter for the "Last-Modified" response header.

    * <br/>This method is used to let the caller set an initial value

    * for this header. <br/> 

    * This value will be automatically updated by this class for every

    * {@link #readChannel()} method call.

    * @param tag

    */

   public void setLastModified(String lastModified) {

         _lastModified = lastModified;

   }

   public ChannelFeed getCachedChannelFeed() {

     return _cachedChannelFeed;

   }

   /**

    * Constructs a {@link CachedFeedReader} to read from a remote source using Http.

    * <br/>

    * Format detection will be automatically performed. 

    * @param httpUrl - the {@link HttpURL} of the remote source

    * @param params - any {@link HttpClientParams}

    * @throws YarfrawException - if parse failed

    * @throws IOException - if format detection failed

    */

   @SuppressWarnings("deprecation")

   public CachedFeedReader(HttpURL httpUrl, HttpClientParams params) throws YarfrawException, IOException{

     super(httpUrl, params);

   }

   /**

    * Constructs a {@link CachedFeedReader} to read from a remote source using Http.

    * <br/>

    * Format detection will be automatically performed.

    * @param httpUrl - the {@link HttpURL} of the remote source

    * @throws YarfrawException - if parse failed

    * @throws IOException - if format detection failed

    */

   @SuppressWarnings("deprecation")

   public CachedFeedReader(HttpURL httpUrl) throws YarfrawException, IOException{

     super(httpUrl, null);

   }

 }