chars_digits([], []).
chars_digits([A|As], [B|Bs]) :-
    char_type(A, digit(B)),
    chars_digits(As, Bs).

read_file(Stream, []) :-
    at_end_of_stream(Stream).
read_file(Stream, [X|Xs]) :-
    \+ at_end_of_stream(Stream),
    read_line_to_codes(Stream, C),
    chars_digits(C, X),
    read_file(Stream, Xs).

main :-
    open('../input/09', read, Stream),
    read_file(Stream, Lines), !,
    close(Stream),
    low_points(Lines, Res),
    point_sum(Lines, Res, N), print(N).
    %print(Res).

point_sum(_, [], 0).
point_sum(A, [[X, Y]|Ps], S) :-
    point_sum(A, Ps, S0),
    nth0(Y, A, Row),
    nth0(X, Row, P),
    S is S0 + P + 1.

low_point([[A,B|_]|[[C|_]|_]], 0, 0) :-
    A < B, A < C.

low_point([A|As], X, 0) :-
    X > 0,
    nth0(X, A, R0),
    X0 is X - 1,
    X1 is X + 1,
    nth0(X0, A, R1),
    R0 < R1,
    (
        (length(A, X1), length([A|As], 1));
        (length([A|As], 1), nth0(X1, A, R2), R0 < R2);
        (length(A, X1), nth0(1, [A|As], E), nth0(X, E, E0), R0 < E0);
        (nth0(X1, A, R2), R0 < R2, nth0(1, [A|As], E), nth0(X, E, E0), R0 < E0)
    ).

low_point(A, 0, Y) :-
    Y > 0,
    nth0(Y, A, [R0,R1|_]),
    R0 < R1,
    Y0 is Y - 1,
    Y1 is Y + 1,
    nth0(Y0, A, [T0|_]),
    R0 < T0,
    (
        length(A, Y1);
        (nth0(Y1, A, [E0|_]), R0 < E0)
    ).

low_point(A, X, Y) :-
    Y > 0,
    X > 0,
    nth0(Y, A, R),
    nth0(X, R, R0),
    X0 is X - 1,
    Y0 is Y - 1,
    X1 is X + 1,
    Y1 is Y + 1,
    nth0(X0, R, R1),
    R0 < R1,
    nth0(Y0, A, E),
    nth0(X, E, E0),
    R0 < E0,
    (
        (length(A, Y1), length(R, X1));
        (length(A, Y1), nth0(X1, R, R2), R0 < R2);
        (length(R, X1), nth0(Y1, A, T), nth0(X, T, T0), R0 < T0);
        (nth0(X1, R, R2), R0 < R2, nth0(Y1, A, T), nth0(X, T, T0), R0 < T0)
    ).

low_points([A|As], Ret) :-
    length([A|As], Y),
    length(A, X),
    X0 is X - 1,
    Y0 is Y - 1,
    low_points([A|As], X0, Y0, Ret).
low_points(A, 0, 0, [[0, 0]]) :-
    low_point(A, 0, 0).
low_points(A, 0, 0, []) :-
    \+ low_point(A, 0, 0).
low_points(A, X, Y, [[X, Y]|Rs]) :-
    X > 0,
    low_point(A, X, Y),
    X0 is X - 1,
    low_points(A, X0, Y, Rs).
low_points(A, X, Y, Rs) :-
    X > 0,
    \+ low_point(A, X, Y),
    X0 is X - 1,
    low_points(A, X0, Y, Rs).
low_points([A|As], 0, Y, [[0, Y]|Rs]) :-
    Y > 0,
    length(A, Len),
    low_point([A|As], 0, Y),
    Y0 is Y - 1,
    X0 is Len - 1,
    low_points([A|As], X0, Y0, Rs).
low_points([A|As], 0, Y, Rs) :-
    Y > 0,
    length(A, Len),
    \+ low_point([A|As], 0, Y),
    Y0 is Y - 1,
    X0 is Len - 1,
    low_points([A|As], X0, Y0, Rs).