gistfile1.erl

% list of {type, flag} tuples
pack(List) -> pack(List, <<>>).

pack([], Ready) -> Ready;
pack([{Type, Flag}|Rest], Ready) ->
    Bit   = ?MODULE:Type(Flag) - 1,
    Size  = size(Ready) * 8,
    io:format("switch bit ~p in ~p bits~n", [Bit, Size]),
    switch(Bit, Size, Rest, Ready).

switch(Bit, Size, Rest, Ready) when Bit >= Size ->
    BB       = (Bit - Size) rem 8,
    Zeros    = ((Bit - Size) div 8) * 8,
    NewReady = <<Ready/binary,
                 0:Zeros/integer,
                 (1 bsl BB):8/integer>>,
    pack(Rest, NewReady);
switch(Bit, _, Rest, Ready) ->
    ByteNum  = max(Bit div 8, 0),
    <<PreByte:ByteNum/binary,
      Byte:8/integer,
      PostByte/binary>> = Ready,
    BB       = Bit - (Bit div 8) * 8,
    NewByte  = Byte bor (1 bsl BB),
    NewReady = <<PreByte/binary,
                 NewByte:8/integer,
                 PostByte/binary>>,
    pack(Rest, NewReady).