read_file(Stream, []) :-
    at_end_of_stream(Stream).

read_file(Stream, [(Cmd, N)|L]) :-
    \+ at_end_of_stream(Stream),
    read_line_to_codes(Stream, C),
    string_chars(S, C),
    split_string(S, " ", "", [S0, S1]),
    atom_string(Cmd, S0),
    number_string(N, S1),
    read_file(Stream, L).

main :-
    open('../input/02', read, Stream),
    read_file(Stream, Lines), !,
    close(Stream),
    step(Lines, X, Y),
    Z is X * Y,
    print((X, Y, Z)).

step(Commands, X, Y) :-
    step_(Commands, 0, 0, 0, X, Y).

step_([], _, X0, Y0, X0, Y0).

step_([(forward, N)|Tail], Aim, X0, Y0, X, Y) :-
    X1 is X0 + N,
    Y1 is Y0 + N * Aim,
    step_(Tail, Aim, X1, Y1, X, Y).

step_([(up, N)|Tail], Aim, X0, Y0, X, Y) :-
    Aim1 is Aim - N,
    step_(Tail, Aim1, X0, Y0, X, Y).

step_([(down, N)|Tail], Aim, X0, Y0, X, Y) :-
    Aim1 is Aim + N,
    step_(Tail, Aim1, X0, Y0, X, Y).