:- use_module(library(pio)).
:- use_module(library(dcg/basics)).
:- initialization(main, main).

main([FileName|_]) :-
    input(FileName, Map),
    nth1(1, Map, Row1), nth1(StartY, Row1, '.'),
    findall(N, route(Map, visited{}, 1-StartY, N), Ns),
    max_list(Ns, Answer),
    format('~w <- ~w', [Answer, Ns]), nl.

route(Map, _, X-_, 0) :- length(Map, Height), X =:= Height.
route(Map, Visiteds, X-Y, N) :-
    Key is X*1000 + Y, NextVisiteds = Visiteds.put(Key, true),
    neighbor(Map, X-Y, X1-Y1, Dist),
    NeighborKey is X1*1000 + Y1, \+ _= Visiteds.get(NeighborKey),
    route(Map, NextVisiteds, X1-Y1, N1),
    N is N1 + Dist.

neighbor(Map, X-Y, NextX-NextY, Dist) :-
    (   X1 is X + 1, Y1 = Y;
        X1 is X - 1, Y1 = Y;
        X1 = X, Y1 is Y + 1;
        X1 = X, Y1 is Y - 1
    ),
    nth1(X1, Map, Row1), nth1(Y1, Row1, Cell1),
    (   Cell1 = '.' -> NextX = X1, NextY = Y1, Dist = 1
    ;   Cell1 = '>' -> NextX = X1, NextY is Y1 + 1, Dist = 2
    ;   Cell1 = 'v' -> NextX is X1 + 1, NextY = Y1, Dist = 2
    ), 
    ( NextX =\= X; NextY =\= Y ).

% input parsing stuff below. Brick indexing is for debugging.
input(FileName, Map) :- phrase_from_file(lines(Map), FileName).

lines([]) --> eos, !.
lines([Line|Lines]) --> line(Line), lines(Lines).

line([]) --> ("\n"; eos), !.
line([C|Chars]) --> [Ascii], line(Chars), {atom_codes(C, [Ascii])}.

% debug
print(Map) :-
    findall(
        X,
        (   nth1(X, Map, Line),
            format('~3d', [X]), write(" "),
            atomic_list_concat(Line, Str), write(Str), nl
        ),
        _),
    nl.