Parsing binary data from an RS232 connection

[Russell Haley <russ.haley@...>]

Hi,

I am writing a little Lua program to parse binary data received via serial communications. I'm wondering if someone on the mailing list has a novel approach to parsing the data? The communications protocol is something like this: 

<HEADER> - 4 bytes

<MSG_TYPE> - 1 byte

<SEQUENCE> - 2 bytes

<PAYLOAD_LENGTH> - 2 bytes

<PAYLOAD> - X bytes (variable based on message type)

<CHECKSUM> - 4 bytes

I see two options for parsing messages received via serial:

1) Process each character.  Lots of examples here: https://stackoverflow.com/questions/829063/how-to-iterate-individual-characters-in-lua-string. Once the header is found, just count until I find the payload length (+ checksum) and then count that number of characters

2) Using pattern matching and captures to search for the <HEADER> + 5 bytes. Then parse out the payload length (+ checksum) and take that many characters. This seems potentially very fast, but creates a great deal of complexity if I don't get the entire message in one serial port read. Something like this (where 'ABCD' represents the header): 

   local m = input:match('ABCD(.+5)')
   if m then
   local typ, seq, pl_len = string.unpack('>B>H>H',m)
   local msg_len =
   if pl_len > 0 then

       ...

   end

Does anyone have a suggestion for parsing the data or even an existing project? Any and all input is welcome.

Thanks!

Russell