[Date Prev][Date Next][Thread Prev][Thread Next]
- Subject: Re: An operator syntax for bitfields using metatables
- From: Todd Coram <todd@...>
- Date: Tue, 16 Jul 2013 16:08:33 -0400
On Tue, Jul 16, 2013, at 11:50 AM, Sean Conner wrote:
> It was thus said that the Great Todd Coram once stated:
> > Erlang's bit syntax is a joy to work with and I miss it every time I use
> > another programming language to decode protocols.
> > In particular, I like the inline binding to variables as opposed to the
> > regex/scanf like approach taken by other languages.
> > An efficient analogue (to Erlang's bit syntax) in Lua would be
> > tremendously useful.
> > Lua tables would work great for something like this. :)
> A good test for this would be DNS packets. The header portion isn't
> query:1, -- boolean, 0=query, 1=response
> opcode:4, -- type of query
> aa:1, -- boolean, authoritative answer
> tc:1, -- boolean, truncation
> rd:1, -- boolean, recursion desired
> ra:1, -- boolean, recursion available
> z:1, -- not used, must be 0
> ad:1, -- boolean, authentic bit
> cd:1, -- boolean, checking disabled
> rcode:4, -- return code
> qcount:16, -- query count
> ancount:16, -- answer count
> nscount:16, -- nameserver count
> adcount:16 -- additional count
> But the record types are variable length:
> name:??, -- key for record, example: www.conman.org
> type:16, -- type of record
> class:16, -- class of record
> ttl:16, -- Time-To-Live
> rdlength:16, -- length of rdata
> rdata:?? -- rest of data
> And the name portion isn't easy to skip---you *have* to parse it to find
> end (check RFC-1035 for the gory details).
I don't see the problem. You do have to do some parsing, but the header
is indeed a no-brainer. In Erlang i'd use the bit syntax parser for the
header, write a parser for "name", then grab the rest of the record
with bit syntax (rdata can be captured by an addition bit string by
using the captured rdlength).
I am not suggesting that Lua add a "half baked" implementation of Erlang
bit syntax or that the syntax magically solves all parsing problems.
For sufficiently complex protocols you are going to do some hand
I just think that the declarative portion provided by Erlang bit syntax
is very nice and is worthy of study when considering add bit parser
support to Lua.