CoAP Message Parsing in Erlang

coap.erl

coap(<<Version:2, Type:2, TKL: 4, Code:8, MID:16, Token:TKL/bytes, Tail/bytes>>) when Version =:= 1->
        {Options, Payload} = coap_parse_options(Tail),
        {coap, {Type, Code, MID, Token, Options, Payload}}.

coap_parse_options(OptionBin) ->
        coap_parse_options(OptionBin, 0, []).

coap_parse_options(<<>>, _LastNum, OptionList) ->
        {OptionList, <<>>};
coap_parse_options(<<16#FF, Payload/bytes>>, _LastNum, OptionList) ->
        {OptionList, Payload};
coap_parse_options(<<BaseOptNum:4, BaseOptLen:4, Tail/bytes>>, LastNum, OptionList) ->
        {Tail1, OptNum} = case BaseOptNum of
                X when X < 13 ->
                        {Tail, BaseOptNum + LastNum};
                X when X =:= 13 ->
                        <<ExtOptNum, NewTail/bytes>> = Tail,
                        {NewTail, ExtOptNum + 13 + LastNum};
                X when X =:= 14 ->
                        <<ExtOptNum:2/bytes, NewTail/bytes>> = Tail,
                        {NewTail, ExtOptNum + 269 + LastNum}
        end,

        case BaseOptLen of
                X when X < 13 ->
                        Tail2 = Tail1,
                        OptLen = BaseOptLen;
                X when X =:= 13 ->
                        <<ExtOptLen, Tail2/bytes>> = Tail1,
                        OptLen = ExtOptLen + 13;
                X when X =:= 14 ->
                        <<ExtOptLen:2/bytes, Tail2/bytes>> = Tail1,
                        OptLen = ExtOptLen + 269
        end,

        <<OptVal:OptLen/bytes, NextOpt/bytes>> = Tail2,
        coap_parse_options([{coap_option, OptNum, OptVal} |OptionList], OptNum, NextOpt).