Decoding bitmask?

Per Gustafsson <>
Wed Dec 21 16:38:45 CET 2005


Hi

The problem is somewhat underspecified. That is do the optional values 
all have the same size? What kind of datastructure would you want to 
store the result in?

The solution below assumes that all options have the same size and the 
results are returned as a list of two tuples where the first element is 
the number of the bit that was set and the second element is the value 
of that option.

I have not tested the speed of the two different solutions but I suspect 
that the second solution is faster, though I feel that the first one is 
prettier.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% Solution 1 dealing with the bitmask as a binary
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

decode(<<Man1,Man2,BitMask:2/binary,Rest/binary>>) ->
   Options = decode_options(BitMask, Rest, 0),
   {Man1,Man2,Options}.

decode_options(BitMask, OptBin, N) when N < 16 ->
   Pad = 15-N,
   case BitMask of
     <<_:N,1:1,_:Pad>> ->
       <<OptVal,RestOpts/binary>> = OptBin,
       [{N+1, OptVal}|decode_options(BitMask,RestOpts,N+1)];
     _ ->
       decode_options(BitMask,OptBin,N+1)
   end;
decode_options(_BitMask, _OptBin, _N) ->
   [].

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% Solution 2 dealing with the bitmask as an integer 
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

decode(<<Man1,Man2,BitMask:16,Rest/binary>>) ->
   Options = decode_options(BitMask, Rest, 0),
   {Man1,Man2,Options}.

decode_options(BitMask, OptBin, N) when N < 16 ->
   Mask = 1 bsl (15-N),
   case BitMask band Mask of
     X when X > 0 ->
       <<OptVal,RestOpts/binary>> = OptBin,
       [{N+1, OptVal}|decode_options(BitMask,RestOpts,N+1)];
     _ ->
       decode_options(BitMask,OptBin,N+1)
   end;
decode_options(_BitMask, _OptBin, _N) ->
   [].


Ericsson wrote:
> Hello!
>  
> I'm working on decoding a binary message. The first variables are Mandatory, so they are easy to decode. The last of the mandatory variables is an 16 bit option mask where each bit corresponds to an optional variable that might follow directly after the OptionMask.
>  
> How can I decode this OptionMask, and then parse the optional variables, in the cleanest  and most "Erlang optimized" way?
>  
> <<Mandatory1, Mandatory2, OptionMask:16/binary, Rest/binary>> = Message,
> etc...
>  
> /BE
> 
> 
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