/*
    * Copyright 2012 The Netty Project
    *
    * The Netty Project 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, software
   * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
   * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
   * License for the specific language governing permissions and limitations
   * under the License.
   */
  package io.netty.handler.codec.http;
  
  import io.netty.buffer.ByteBuf;
  import io.netty.buffer.ByteBufHolder;
  import io.netty.channel.ChannelHandlerContext;
  import io.netty.channel.embedded.EmbeddedChannel;
  import io.netty.handler.codec.MessageToMessageCodec;
  import io.netty.handler.codec.http.HttpHeaders.Names;
  import io.netty.handler.codec.http.HttpHeaders.Values;
  import io.netty.util.ReferenceCountUtil;
  
  import java.util.ArrayDeque;
  import java.util.List;
  import java.util.Queue;
  
  /**
   * Encodes the content of the outbound {@link HttpResponse} and {@link HttpContent}.
   * The original content is replaced with the new content encoded by the
   * {@link EmbeddedChannel}, which is created by {@link #beginEncode(HttpResponse, String)}.
   * Once encoding is finished, the value of the <tt>'Content-Encoding'</tt> header
   * is set to the target content encoding, as returned by
   * {@link #beginEncode(HttpResponse, String)}.
   * Also, the <tt>'Content-Length'</tt> header is updated to the length of the
   * encoded content.  If there is no supported or allowed encoding in the
   * corresponding {@link HttpRequest}'s {@code "Accept-Encoding"} header,
   * {@link #beginEncode(HttpResponse, String)} should return {@code null} so that
   * no encoding occurs (i.e. pass-through).
   * <p>
   * Please note that this is an abstract class.  You have to extend this class
   * and implement {@link #beginEncode(HttpResponse, String)} properly to make
   * this class functional.  For example, refer to the source code of
   * {@link HttpContentCompressor}.
   * <p>
   * This handler must be placed after {@link HttpObjectEncoder} in the pipeline
   * so that this handler can intercept HTTP responses before {@link HttpObjectEncoder}
   * converts them into {@link ByteBuf}s.
   */
  public abstract class HttpContentEncoder extends MessageToMessageCodec<HttpRequest, HttpObject> {
  
      private enum State {
          PASS_THROUGH,
          AWAIT_HEADERS,
          AWAIT_CONTENT
      }
  
      private static final CharSequence ZERO_LENGTH_HEAD = "HEAD";
      private static final CharSequence ZERO_LENGTH_CONNECT = "CONNECT";
      private static final int CONTINUE_CODE = HttpResponseStatus.CONTINUE.code();
  
      private final Queue<CharSequence> acceptEncodingQueue = new ArrayDeque<CharSequence>();
      private EmbeddedChannel encoder;
      private State state = State.AWAIT_HEADERS;
  
      @Override
      public boolean acceptOutboundMessage(Object msg) throws Exception {
          return msg instanceof HttpContent || msg instanceof HttpResponse;
      }
  
      @Override
      protected void decode(ChannelHandlerContext ctx, HttpRequest msg, List<Object> out)
              throws Exception {
          CharSequence acceptedEncoding = msg.headers().get(Names.ACCEPT_ENCODING);
          if (acceptedEncoding == null) {
              acceptedEncoding = Values.IDENTITY;
          }
  
          HttpMethod meth = msg.getMethod();
          if (meth == HttpMethod.HEAD) {
              acceptedEncoding = ZERO_LENGTH_HEAD;
          } else if (meth == HttpMethod.CONNECT) {
              acceptedEncoding = ZERO_LENGTH_CONNECT;
          }
  
          acceptEncodingQueue.add(acceptedEncoding);
          out.add(ReferenceCountUtil.retain(msg));
      }
  
      @Override
      protected void encode(ChannelHandlerContext ctx, HttpObject msg, List<Object> out) throws Exception {
          final boolean isFull = msg instanceof HttpResponse && msg instanceof LastHttpContent;
          switch (state) {
              case AWAIT_HEADERS: {
                  ensureHeaders(msg);
                  assert encoder == null;
 
                 final HttpResponse res = (HttpResponse) msg;
                 final int code = res.getStatus().code();
                 final CharSequence acceptEncoding;
                 if (code == CONTINUE_CODE) {
                     // We need to not poll the encoding when response with CONTINUE as another response will follow
                     // for the issued request. See https://github.com/netty/netty/issues/4079
                     acceptEncoding = null;
                 } else {
                     // Get the list of encodings accepted by the peer.
                     acceptEncoding = acceptEncodingQueue.poll();
                     if (acceptEncoding == null) {
                         throw new IllegalStateException("cannot send more responses than requests");
                     }
                 }
 
                 /*
                  * per rfc2616 4.3 Message Body
                  * All 1xx (informational), 204 (no content), and 304 (not modified) responses MUST NOT include a
                  * message-body. All other responses do include a message-body, although it MAY be of zero length.
                  *
                  * 9.4 HEAD
                  * The HEAD method is identical to GET except that the server MUST NOT return a message-body
                  * in the response.
                  *
                  * Also we should pass through HTTP/1.0 as transfer-encoding: chunked is not supported.
                  *
                  * See https://github.com/netty/netty/issues/5382
                  */
                 if (isPassthru(res.getProtocolVersion(), code, acceptEncoding)) {
                     if (isFull) {
                         out.add(ReferenceCountUtil.retain(res));
                     } else {
                         out.add(res);
                         // Pass through all following contents.
                         state = State.PASS_THROUGH;
                     }
                     break;
                 }
 
                 if (isFull) {
                     // Pass through the full response with empty content and continue waiting for the the next resp.
                     if (!((ByteBufHolder) res).content().isReadable()) {
                         out.add(ReferenceCountUtil.retain(res));
                         break;
                     }
                 }
 
                 // Prepare to encode the content.
                 final Result result = beginEncode(res, acceptEncoding.toString());
 
                 // If unable to encode, pass through.
                 if (result == null) {
                     if (isFull) {
                         out.add(ReferenceCountUtil.retain(res));
                     } else {
                         out.add(res);
                         // Pass through all following contents.
                         state = State.PASS_THROUGH;
                     }
                     break;
                 }
 
                 encoder = result.contentEncoder();
 
                 // Encode the content and remove or replace the existing headers
                 // so that the message looks like a decoded message.
                 res.headers().set(Names.CONTENT_ENCODING, result.targetContentEncoding());
 
                 // Output the rewritten response.
                 if (isFull) {
                     // Convert full message into unfull one.
                     HttpResponse newRes = new DefaultHttpResponse(res.getProtocolVersion(), res.getStatus());
                     newRes.headers().set(res.headers());
                     out.add(newRes);
 
                     ensureContent(res);
                     encodeFullResponse(newRes, (HttpContent) res, out);
                     break;
                 } else {
                     // Make the response chunked to simplify content transformation.
                     res.headers().remove(Names.CONTENT_LENGTH);
                     res.headers().set(Names.TRANSFER_ENCODING, Values.CHUNKED);
 
                     out.add(res);
                     state = State.AWAIT_CONTENT;
                     if (!(msg instanceof HttpContent)) {
                         // only break out the switch statement if we have not content to process
                         // See https://github.com/netty/netty/issues/2006
                         break;
                     }
                     // Fall through to encode the content
                 }
             }
             case AWAIT_CONTENT: {
                 ensureContent(msg);
                 if (encodeContent((HttpContent) msg, out)) {
                     state = State.AWAIT_HEADERS;
                 }
                 break;
             }
             case PASS_THROUGH: {
                 ensureContent(msg);
                 out.add(ReferenceCountUtil.retain(msg));
                 // Passed through all following contents of the current response.
                 if (msg instanceof LastHttpContent) {
                     state = State.AWAIT_HEADERS;
                 }
                 break;
             }
         }
     }
 
     private void encodeFullResponse(HttpResponse newRes, HttpContent content, List<Object> out) {
         int existingMessages = out.size();
         encodeContent(content, out);
 
         if (HttpHeaders.isContentLengthSet(newRes)) {
             // adjust the content-length header
             int messageSize = 0;
             for (int i = existingMessages; i < out.size(); i++) {
                 Object item = out.get(i);
                 if (item instanceof HttpContent) {
                     messageSize += ((HttpContent) item).content().readableBytes();
                 }
             }
             HttpHeaders.setContentLength(newRes, messageSize);
         } else {
             newRes.headers().set(Names.TRANSFER_ENCODING, Values.CHUNKED);
         }
     }
 
     private static boolean isPassthru(HttpVersion version, int code, CharSequence httpMethod) {
         return code < 200 || code == 204 || code == 304 ||
                (httpMethod == ZERO_LENGTH_HEAD || (httpMethod == ZERO_LENGTH_CONNECT && code == 200)) ||
                 version == HttpVersion.HTTP_1_0;
     }
 
     private static void ensureHeaders(HttpObject msg) {
         if (!(msg instanceof HttpResponse)) {
             throw new IllegalStateException(
                     "unexpected message type: " +
                     msg.getClass().getName() + " (expected: " + HttpResponse.class.getSimpleName() + ')');
         }
     }
 
     private static void ensureContent(HttpObject msg) {
         if (!(msg instanceof HttpContent)) {
             throw new IllegalStateException(
                     "unexpected message type: " +
                     msg.getClass().getName() + " (expected: " + HttpContent.class.getSimpleName() + ')');
         }
     }
 
     private boolean encodeContent(HttpContent c, List<Object> out) {
         ByteBuf content = c.content();
 
         encode(content, out);
 
         if (c instanceof LastHttpContent) {
             finishEncode(out);
             LastHttpContent last = (LastHttpContent) c;
 
             // Generate an additional chunk if the decoder produced
             // the last product on closure,
             HttpHeaders headers = last.trailingHeaders();
             if (headers.isEmpty()) {
                 out.add(LastHttpContent.EMPTY_LAST_CONTENT);
             } else {
                 out.add(new ComposedLastHttpContent(headers));
             }
             return true;
         }
         return false;
     }
 
     /**
      * Prepare to encode the HTTP message content.
      *
      * @param headers
      *        the headers
      * @param acceptEncoding
      *        the value of the {@code "Accept-Encoding"} header
      *
      * @return the result of preparation, which is composed of the determined
      *         target content encoding and a new {@link EmbeddedChannel} that
      *         encodes the content into the target content encoding.
      *         {@code null} if {@code acceptEncoding} is unsupported or rejected
      *         and thus the content should be handled as-is (i.e. no encoding).
      */
     protected abstract Result beginEncode(HttpResponse headers, String acceptEncoding) throws Exception;
 
     @Override
     public void handlerRemoved(ChannelHandlerContext ctx) throws Exception {
         cleanupSafely(ctx);
         super.handlerRemoved(ctx);
     }
 
     @Override
     public void channelInactive(ChannelHandlerContext ctx) throws Exception {
         cleanupSafely(ctx);
         super.channelInactive(ctx);
     }
 
     private void cleanup() {
         if (encoder != null) {
             // Clean-up the previous encoder if not cleaned up correctly.
             encoder.finishAndReleaseAll();
             encoder = null;
         }
     }
 
     private void cleanupSafely(ChannelHandlerContext ctx) {
         try {
             cleanup();
         } catch (Throwable cause) {
             // If cleanup throws any error we need to propagate it through the pipeline
             // so we don't fail to propagate pipeline events.
             ctx.fireExceptionCaught(cause);
         }
     }
 
     private void encode(ByteBuf in, List<Object> out) {
         // call retain here as it will call release after its written to the channel
         encoder.writeOutbound(in.retain());
         fetchEncoderOutput(out);
     }
 
     private void finishEncode(List<Object> out) {
         if (encoder.finish()) {
             fetchEncoderOutput(out);
         }
         encoder = null;
     }
 
     private void fetchEncoderOutput(List<Object> out) {
         for (;;) {
             ByteBuf buf = (ByteBuf) encoder.readOutbound();
             if (buf == null) {
                 break;
             }
             if (!buf.isReadable()) {
                 buf.release();
                 continue;
             }
             out.add(new DefaultHttpContent(buf));
         }
     }
 
     public static final class Result {
         private final String targetContentEncoding;
         private final EmbeddedChannel contentEncoder;
 
         public Result(String targetContentEncoding, EmbeddedChannel contentEncoder) {
             if (targetContentEncoding == null) {
                 throw new NullPointerException("targetContentEncoding");
             }
             if (contentEncoder == null) {
                 throw new NullPointerException("contentEncoder");
             }
 
             this.targetContentEncoding = targetContentEncoding;
             this.contentEncoder = contentEncoder;
         }
 
         public String targetContentEncoding() {
             return targetContentEncoding;
         }
 
         public EmbeddedChannel contentEncoder() {
             return contentEncoder;
         }
     }
 }