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Pure Java implementation of libzmq
There is a newer version: 0.6.0
/*
    Copyright (c) 2009-2011 250bpm s.r.o.
    Copyright (c) 2007-2009 iMatix Corporation
    Copyright (c) 2007-2011 Other contributors as noted in the AUTHORS file

    This file is part of 0MQ.

    0MQ is free software; you can redistribute it and/or modify it under
    the terms of the GNU Lesser General Public License as published by
    the Free Software Foundation; either version 3 of the License, or
    (at your option) any later version.

    0MQ is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU Lesser General Public License for more details.

    You should have received a copy of the GNU Lesser General Public License
    along with this program.  If not, see .
*/

package zmq;

import java.nio.ByteBuffer;

//  Helper base class for decoders that know the amount of data to read
//  in advance at any moment. Knowing the amount in advance is a property
//  of the protocol used. 0MQ framing protocol is based size-prefixed
//  paradigm, which qualifies it to be parsed by this class.
//  On the other hand, XML-based transports (like XMPP or SOAP) don't allow
//  for knowing the size of data to read in advance and should use different
//  decoding algorithms.
//
//  This class implements the state machine that parses the incoming buffer.
//  Derived class should implement individual state machine actions.

public class Decoder extends DecoderBase {
    
    private final static int one_byte_size_ready = 0;
    private final static int eight_byte_size_ready = 1;
    private final static int flags_ready = 2;
    private final static int message_ready = 3;
    
    private final byte[] tmpbuf;
    private Msg in_progress;
    private final long maxmsgsize;
    private IMsgSink msg_sink;

    public Decoder (int bufsize_, long maxmsgsize_)
    {
        super(bufsize_);
        maxmsgsize = maxmsgsize_;
        tmpbuf = new byte[8];
        
    
        //  At the beginning, read one byte and go to one_byte_size_ready state.
        next_step (tmpbuf, 1, one_byte_size_ready);
    }
    

    //  Set the receiver of decoded messages.
    @Override
    public void set_msg_sink (IMsgSink msg_sink_) 
    {
        msg_sink = msg_sink_;
    }

    @Override
    protected boolean next() {
        switch(state()) {
        case one_byte_size_ready:
            return one_byte_size_ready ();
        case eight_byte_size_ready:
            return eight_byte_size_ready ();
        case flags_ready:
            return flags_ready ();
        case message_ready:
            return message_ready ();
        default:
            return false;
        }
    }



    private boolean one_byte_size_ready() {
        //  First byte of size is read. If it is 0xff(-1 for java byte) read 8-byte size.
        //  Otherwise allocate the buffer for message data and read the
        //  message data into it.
        byte first = tmpbuf[0];
        if (first == -1) {
            next_step (tmpbuf, 8, eight_byte_size_ready);
        } else {

            //  There has to be at least one byte (the flags) in the message).
            if (first == 0) {
                decoding_error ();
                return false;
            }
            
            int size = (int) first;
            if (size < 0) {
                size = (0xFF) & first;
            }

            //  in_progress is initialised at this point so in theory we should
            //  close it before calling zmq_msg_init_size, however, it's a 0-byte
            //  message and thus we can treat it as uninitialised...
            if (maxmsgsize >= 0 && (long) (size - 1) > maxmsgsize) {
                decoding_error ();
                return false;

            }
            else {
                in_progress = new Msg(size-1);
            }

            next_step (tmpbuf, 1, flags_ready);
        }
        return true;

    }
    
    private boolean eight_byte_size_ready() {
        //  8-byte payload length is read. Allocate the buffer
        //  for message body and read the message data into it.
        final long payload_length = ByteBuffer.wrap(tmpbuf).getLong();

        //  There has to be at least one byte (the flags) in the message).
        if (payload_length <= 0) {
            decoding_error ();
            return false;
        }

        //  Message size must not exceed the maximum allowed size.
        if (maxmsgsize >= 0 && payload_length - 1 > maxmsgsize) {
            decoding_error ();
            return false;
        }

        //  Message size must fit within range of size_t data type.
        if (payload_length - 1 > Integer.MAX_VALUE) {
            decoding_error ();
            return false;
        }

        final int msg_size =  (int)(payload_length - 1);
        //  in_progress is initialized at this point so in theory we should
        //  close it before calling init_size, however, it's a 0-byte
        //  message and thus we can treat it as uninitialized...
        in_progress = new Msg(msg_size);
        
        next_step (tmpbuf, 1, flags_ready);
        
        return true;

    }
    
    private boolean flags_ready() {

        //  Store the flags from the wire into the message structure.
        
        int first = tmpbuf[0];
        
        in_progress.setFlags (first & Msg.MORE);

        next_step (in_progress,
            message_ready);

        return true;

    }
    
    private boolean message_ready() {
        //  Message is completely read. Push it further and start reading
        //  new message. (in_progress is a 0-byte message after this point.)
        
        if (msg_sink == null)
            return false;
        
        int rc = msg_sink.push_msg (in_progress);
        if (rc != 0) {
            return false;
        }
        
        next_step (tmpbuf, 1, one_byte_size_ready);
        
        return true;
    }


}