/*
    *  Licensed to the Apache Software Foundation (ASF) under one
    *  or more contributor license agreements.  See the NOTICE file
    *  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
    *  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 org.apache.mina.filter.codec.demux;
  
  import org.apache.mina.core.buffer.IoBuffer;
  import org.apache.mina.core.session.AttributeKey;
  import org.apache.mina.core.session.IoSession;
  import org.apache.mina.filter.codec.CumulativeProtocolDecoder;
  import org.apache.mina.filter.codec.ProtocolDecoder;
  import org.apache.mina.filter.codec.ProtocolDecoderException;
  import org.apache.mina.filter.codec.ProtocolDecoderOutput;
  
  /**
   * A composite {@link ProtocolDecoder} that demultiplexes incoming {@link IoBuffer}
   * decoding requests into an appropriate {@link MessageDecoder}.
   * 
   * <h2>Internal mechanism of {@link MessageDecoder} selection</h2>
   * <p>
   * <ol>
   * <li>{@link DemuxingProtocolDecoder} iterates the list of candidate
   * {@link MessageDecoder}s and calls {@link MessageDecoder#decodable(IoSession, IoBuffer)}.
   * Initially, all registered {@link MessageDecoder}s are candidates.</li>
   * <li>If {@link MessageDecoderResult#NOT_OK} is returned, it is removed from the candidate
   *     list.</li>
   * <li>If {@link MessageDecoderResult#NEED_DATA} is returned, it is retained in the candidate
   *     list, and its {@link MessageDecoder#decodable(IoSession, IoBuffer)} will be invoked
   *     again when more data is received.</li>
   * <li>If {@link MessageDecoderResult#OK} is returned, {@link DemuxingProtocolDecoder}
   *     found the right {@link MessageDecoder}.</li>
   * <li>If there's no candidate left, an exception is raised.  Otherwise, 
   *     {@link DemuxingProtocolDecoder} will keep iterating the candidate list.
   * </ol>
   * 
   * Please note that any change of position and limit of the specified {@link IoBuffer}
   * in {@link MessageDecoder#decodable(IoSession, IoBuffer)} will be reverted back to its
   * original value.
   * <p>
   * Once a {@link MessageDecoder} is selected, {@link DemuxingProtocolDecoder} calls
   * {@link MessageDecoder#decode(IoSession, IoBuffer, ProtocolDecoderOutput)} continuously
   * reading its return value:
   * <ul>
   * <li>{@link MessageDecoderResult#NOT_OK} - protocol violation; {@link ProtocolDecoderException}
   *                                           is raised automatically.</li>
   * <li>{@link MessageDecoderResult#NEED_DATA} - needs more data to read the whole message;
   *                                              {@link MessageDecoder#decode(IoSession, IoBuffer, ProtocolDecoderOutput)}
   *                                              will be invoked again when more data is received.</li>
   * <li>{@link MessageDecoderResult#OK} - successfully decoded a message; the candidate list will
   *                                       be reset and the selection process will start over.</li>
   * </ul>
   *
   * @author The Apache MINA Project (dev@mina.apache.org)
   *
   * @see MessageDecoderFactory
   * @see MessageDecoder
   */
  public class DemuxingProtocolDecoder extends CumulativeProtocolDecoder {
  
      private final AttributeKey STATE = new AttributeKey(getClass(), "state");
      
      private MessageDecoderFactory[] decoderFactories = new MessageDecoderFactory[0];
      private static final Class<?>[] EMPTY_PARAMS = new Class[0];
  
      public DemuxingProtocolDecoder() {
          // Do nothing
      }
  
      public void addMessageDecoder(Class<? extends MessageDecoder> decoderClass) {
          if (decoderClass == null) {
              throw new NullPointerException("decoderClass");
          }
  
          try {
              decoderClass.getConstructor(EMPTY_PARAMS);
          } catch (NoSuchMethodException e) {
              throw new IllegalArgumentException(
                      "The specified class doesn't have a public default constructor.");
          }
  
          boolean registered = false;
          if (MessageDecoder.class.isAssignableFrom(decoderClass)) {
              addMessageDecoder(new DefaultConstructorMessageDecoderFactory(decoderClass));
              registered = true;
         }
 
         if (!registered) {
             throw new IllegalArgumentException(
                     "Unregisterable type: " + decoderClass);
         }
     }
 
     public void addMessageDecoder(MessageDecoder decoder) {
         addMessageDecoder(new SingletonMessageDecoderFactory(decoder));
     }
 
     public void addMessageDecoder(MessageDecoderFactory factory) {
         if (factory == null) {
             throw new NullPointerException("factory");
         }
         MessageDecoderFactory[] decoderFactories = this.decoderFactories;
         MessageDecoderFactory[] newDecoderFactories = new MessageDecoderFactory[decoderFactories.length + 1];
         System.arraycopy(decoderFactories, 0, newDecoderFactories, 0,
                 decoderFactories.length);
         newDecoderFactories[decoderFactories.length] = factory;
         this.decoderFactories = newDecoderFactories;
     }
     
     @Override
     protected boolean doDecode(IoSession session, IoBuffer in,
             ProtocolDecoderOutput out) throws Exception {
         State state = getState(session);
         
         if (state.currentDecoder == null) {
             MessageDecoder[] decoders = state.decoders;
             int undecodables = 0;
         
             for (int i = decoders.length - 1; i >= 0; i--) {
                 MessageDecoder decoder = decoders[i];
                 int limit = in.limit();
                 int pos = in.position();
 
                 MessageDecoderResult result;
                 
                 try {
                     result = decoder.decodable(session, in);
                 } finally {
                     in.position(pos);
                     in.limit(limit);
                 }
 
                 if (result == MessageDecoder.OK) {
                     state.currentDecoder = decoder;
                     break;
                 } else if (result == MessageDecoder.NOT_OK) {
                     undecodables++;
                 } else if (result != MessageDecoder.NEED_DATA) {
                     throw new IllegalStateException(
                             "Unexpected decode result (see your decodable()): "
                                     + result);
                 }
             }
 
             if (undecodables == decoders.length) {
                 // Throw an exception if all decoders cannot decode data.
                 String dump = in.getHexDump();
                 in.position(in.limit()); // Skip data
                 ProtocolDecoderException e = new ProtocolDecoderException(
                         "No appropriate message decoder: " + dump);
                 e.setHexdump(dump);
                 throw e;
             }
 
             if (state.currentDecoder == null) {
                 // Decoder is not determined yet (i.e. we need more data)
                 return false;
             }
         }
 
         MessageDecoderResult result = state.currentDecoder.decode(session, in,
                 out);
         if (result == MessageDecoder.OK) {
             state.currentDecoder = null;
             return true;
         } else if (result == MessageDecoder.NEED_DATA) {
             return false;
         } else if (result == MessageDecoder.NOT_OK) {
             state.currentDecoder = null;
             throw new ProtocolDecoderException(
                     "Message decoder returned NOT_OK.");
         } else {
             state.currentDecoder = null;
             throw new IllegalStateException(
                     "Unexpected decode result (see your decode()): "
                             + result);
         }
     }
 
     @Override
     public void finishDecode(IoSession session, ProtocolDecoderOutput out)
             throws Exception {
         super.finishDecode(session, out);
         State state = getState(session);
         MessageDecoder currentDecoder = state.currentDecoder;
         if (currentDecoder == null) {
             return;
         }
 
         currentDecoder.finishDecode(session, out);
     }
 
     @Override
     public void dispose(IoSession session) throws Exception {
         super.dispose(session);
         session.removeAttribute(STATE);
     }
     
     private State getState(IoSession session) throws Exception {
         State state = (State) session.getAttribute(STATE);
         
         if (state == null) {
             state = new State();
             State oldState = (State) session.setAttributeIfAbsent(STATE, state);
             
             if (oldState != null) {
                 state = oldState;
             }
         }
         
         return state;
     }
     
     private class State {
         private final MessageDecoder[] decoders;
         private MessageDecoder currentDecoder;
         
         private State() throws Exception {
             MessageDecoderFactory[] decoderFactories = DemuxingProtocolDecoder.this.decoderFactories;
             decoders = new MessageDecoder[decoderFactories.length];
             for (int i = decoderFactories.length - 1; i >= 0; i--) {
                 decoders[i] = decoderFactories[i].getDecoder();
             }
         }
     }
 
     private static class SingletonMessageDecoderFactory implements
             MessageDecoderFactory {
         private final MessageDecoder decoder;
 
         private SingletonMessageDecoderFactory(MessageDecoder decoder) {
             if (decoder == null) {
                 throw new NullPointerException("decoder");
             }
             this.decoder = decoder;
         }
 
         public MessageDecoder getDecoder() {
             return decoder;
         }
     }
 
     private static class DefaultConstructorMessageDecoderFactory implements
             MessageDecoderFactory {
         private final Class<?> decoderClass;
 
         private DefaultConstructorMessageDecoderFactory(Class<?> decoderClass) {
             if (decoderClass == null) {
                 throw new NullPointerException("decoderClass");
             }
 
             if (!MessageDecoder.class.isAssignableFrom(decoderClass)) {
                 throw new IllegalArgumentException(
                         "decoderClass is not assignable to MessageDecoder");
             }
             this.decoderClass = decoderClass;
         }
 
         public MessageDecoder getDecoder() throws Exception {
             return (MessageDecoder) decoderClass.newInstance();
         }
     }
 }