3 ms·
Hard to beat the Erlang "destructuring" bit syntax, for example [0] parsing an IPv4 datagram from `Dgram`: -define(IP_VERSION, 4). -define(IP_MIN_HDR_LEN,
by lsllc 5y ago
Hard to beat the Erlang "destructuring" bit syntax, for example [0] parsing an IPv4 datagram from `Dgram`:
-define(IP_VERSION, 4).
-define(IP_MIN_HDR_LEN, 5).
DgramSize = byte_size(Dgram),
case Dgram of
<<?IP_VERSION:4, HLen:4, SrvcType:8, TotLen:16,
ID:16, Flgs:3, FragOff:13,
TTL:8, Proto:8, HdrChkSum:16,
SrcIP:32,
DestIP:32, RestDgram/binary>> when HLen>=5, 4*HLen=<DgramSize ->
OptsLen = 4*(HLen - ?IP_MIN_HDR_LEN),
<<Opts:OptsLen/binary,Data/binary>> = RestDgram,
...
end.
[0] https://www.erlang.org/doc/programming_examples/bit_syntax.html https://www.erlang.org/doc/programming_examples/bit_syntax.h...
- rwmj 5y agoMy OCaml Bitstring was directly influenced by Erlang: https://bitstring.software/examples/ https://bitstring.software/examples/
- dhx 5y agoFrom the page you linked, it appears that the syntax can only handle cases where the size of a data structure is known ahead of reading the data? What about a more complex example of JFIF or ZIP where the parser has to read a data structure of unknown length until terminating bytes are discovered. Do you know of any more complex Erlang examples?
- lsllc 5y agoThe page covers this further down -- you can "bind" and then "use" a size [0]: <<Sz:8,Payload:Sz/binary-unit:8,Rest/binary>> This bit syntax binds `Sz` as an 8-bit length value, `Payload` as `Sz` 8-bit bytes with the remainder in `Rest`. [0] https://www.erlang.org/doc/programming_examples/bit_syntax.html#binding-and-using-a-size-variable https://www.erlang.org/doc/programming_examples/bit_syntax.h...
- dhx 5y agoThanks for the example. Is there a way to perform a positive and negative look-ahead instead of just reading the remaining data to end of input?
- eproxus 5y agoYou also have Erlang’s pattern matching at your disposal: decode(<<16#01, Size:8, Payload:Size/binary-unit:8, Rest/binary>>) -> [{field1, Payload}|decode(Rest)]; decode(<<16#02, Flags:16, Rest/binary>>) when Flags =:= 1 -> [{field2, true}|decode(Rest)]; decode(<<16#02, Flags:16, Rest/binary>>) -> [{field2, false}|decode(Rest)]. Anything after the when keyword is a guard condition allowing for some manner of positive/negative “look ahead” (Contrived example, but maybe you get the point).
- lsllc 5y agoYes, this is exactly how you do it, with pattern matching & guards!
- deleted 5y ago[deleted]