This repository contains a simple packet decoder.

It takes a stream of bytes, decodes the bytes into valid packets, and calls some callback function each time it successfully decodes a packet.

The example.cpp file contains a few tests using the library.

To run the tests:

g++ -std=c++11 example.cpp pkt_decoder.cpp -o example.o

./example.o

Alternatively, there is a make file. To use the make file, build with make target and run with make run.

The packet decoder interface can be found in pkt_decoder.h. The implementation is in pkt_decoder.cpp.

The packet decoder stores the necessary state for processing streams of packets in a pkt_decoder struct.

There are three main functions for interacting with the packet decoder:

1. pkt_decoder_create :

   • Creates a new packet decoder.
   • Arguments:
     • callback - a function that is called each time a complete packet is decoded
     • callback_ctx - a pointer to some context that will be passed to callback

2. pkt_decoder_destroy

   • Cleans up memory used by the decoder.
   • Arguments:
     • decoder - decoder to destruct

3. pkt_decoder_write_bytes

   • Decodes packets from a byte stream. The function can be called repeatedly to process multiple streams.
   • Arguments:
     • decoder - the packet decoder currently being used
     • len - length of the byte stream to be decoded
     • data - pointer to the byte stream to be decoded
   • Notes:
     • Silently drops invalid packets.

Packets are expected to come in a stream of bytes.

Packets have the following constraints to be considered valid:

• Packets must start with 0x02
• Packets must end with 0x03
• Packets must be no longer than 512 bytes, including the starting and ending byte.
• The 0x10 byte is treated as an escape character. The byte that follows it is in the format 0x20 | value where value is the actual byte value that we want to add to our decoded packet. This byte value will be decoded with the operation value & ~0x20 and added to the decoded packet.

Invalid packets are discarded. The packet decoder will continue processing additional bytes in the stream after discarding an invalid packet.