pkt

%% Copyright (c) 2009-2015, Michael Santos <michael.santos@gmail.com>
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-module(pkt_ipv4).

-include("pkt_ipproto.hrl").
-include("pkt_ipv4.hrl").

-export([codec/1]).

codec(
    <<4:4, HL:4, ToS:8, Len:16,
    Id:16, 0:1, DF:1, MF:1, %% RFC791 states it's a MUST
    Off:13, TTL:8, P:8, Sum:16,
    SA1:8, SA2:8, SA3:8, SA4:8,
    DA1:8, DA2:8, DA3:8, DA4:8,
    Rest/binary>>
) when HL >= 5 ->
    OptLen = (HL - 5) * 4,
    <<Opt:OptLen/binary, Payload/binary>> = Rest,
    {#ipv4{
        hl = HL, tos = ToS, len = Len,
        id = Id, df = DF, mf = MF,
        off = Off, ttl = TTL, p = P, sum = Sum,
        saddr = {SA1,SA2,SA3,SA4},
        daddr = {DA1,DA2,DA3,DA4},
        opt = Opt
    }, Payload};
codec(#ipv4{
        hl = HL, tos = ToS, len = Len,
        id = Id, df = DF, mf = MF,
        off = Off, ttl = TTL, p = P, sum = Sum,
        saddr = {SA1,SA2,SA3,SA4},
        daddr = {DA1,DA2,DA3,DA4},
        opt = Opt
    }) ->
    Pad = ((HL - 5) * 4 - byte_size(Opt)) * 8,
    <<4:4, HL:4, ToS:8, Len:16,
    Id:16, 0:1, DF:1, MF:1, %% RFC791 states it's a MUST
    Off:13, TTL:8, P:8, Sum:16,
    SA1:8, SA2:8, SA3:8, SA4:8,
    DA1:8, DA2:8, DA3:8, DA4:8,
    Opt/binary, 0:Pad>>.