:- use_module(library(dcg/basics)).
:- use_module(library(ugraphs)).
:- use_module(library(assoc)).

char(X, L, C0, L, C) -->
  [X0], {X0 \= 0'\n, char_code(X, X0), C is C0 + 1}, !.

newline(L0, _, L, 0) --> "\n", {L is L0 + 1}.

empty(L, C0, L, C)    --> char('.', L, C0, L, C).
pipe(Ds, L, C0, L, C) --> char(X, L, C0, L, C), {is_pipe(X, Ds)}.
start(L, C0, L, C)    --> char('S', L, C0, L, C).

is_pipe('F', [down, right]).
is_pipe('L', [up,   right]).
is_pipe('7', [down, left]).
is_pipe('J', [up,   left]).
is_pipe('-', [left, right]).
is_pipe('|', [up,   down]).


nonthing(L0, C0, L, C) --> empty(L0, C0, L, C).
nonthing(L0, C0, L, C) --> newline(L0, C0, L, C).

thing(s(L0, C0),    L0, C0, L, C) --> start(L0, C0, L, C).
thing(p(P, L0, C0), L0, C0, L, C) --> pipe(P, L0, C0, L, C).

map([], L, C, L, C)       --> [].
map(Xs, L0, C0, L, C)     --> nonthing(L0, C0, L1, C1), map(Xs, L1, C1, L, C).
map([X|Xs], L0, C0, L, C) --> thing(X, L0, C0, L1, C1), map(Xs, L1, C1, L, C).

map(Xs) --> map(Xs, 0, 0, _, _).

insert(s(L, C),    A0, A) :- put_assoc(L-C, A0, s,    A).
insert(p(P, L, C), A0, A) :- put_assoc(L-C, A0, p(P), A).

make_assoc(Map, A) :-
  empty_assoc(Start),
  foldl(insert, Map, Start, A).

parse(File, A) :-
  phrase_from_file(map(Map), File), !,
  make_assoc(Map, A).

converse(up,    down).
converse(down,  up).
converse(left,  right).
converse(right, left).

compat(p(Ds1), p(Ds2)) :- member(D1, Ds1), member(D2, Ds2), converse(D1, D2), !.
compat(s,      p(_)).
compat(p(_),   s).


% vim: set ft=prolog :