/**
 * Copyright 2007-2016, Kaazing Corporation. All rights reserved.
 *
 * Licensed 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 <a href="http://mina.apache.org">Apache MINA Project</a>
 *
 * @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();
        }
    }
}
