/* align */
align(p, 1).
align(b1, 1).
align(b2, 2).
align(b4, 4).
align(b8, 8).
align(struct(Fs), A) :-
    phrase(encode_struct(Fs, 0), E),
    maplist(align, E, As),
    max_list(As, A).

/* size */
size(M, L) :-
    phrase(encode(M), E),
    length(E, L).

/* pad */
pad(N) --> { N =:= 0 } -> [].
pad(N) --> { N > 0 } -> [p], pad(N - 1).

/* encode_struct */
encode_struct([], N) --> { N =:= 0 } -> encode(b1).
encode_struct([], N) --> { N > 0} -> [].
encode_struct([H|T], Offset) -->
    {
        align(H, Alignment),
        Padding is (Alignment - (Offset mod Alignment)) mod Alignment,
        size(H, Size)
    },
    pad(Padding),
    encode(H),
    encode_struct(T, Offset + Size).

/* encode */
encode(p)  --> [ p  ].
encode(b1) --> [ b1 ].
encode(b2) --> [ b2 ].
encode(b4) --> [ b4 ].
encode(b8) --> [ b8 ].
encode(struct(Fs)) --> encode_struct(Fs, 0).