d23p2

23/part2.pl

@@ -1,36 +1,75 @@
 :- use_module(library(pio)).
 :- use_module(library(dcg/basics)).
 :- initialization(main, main).
-:- table neighbor/3.
+:- op(700, xfx, is_key_of).
 
 main([FileName|_]) :-
-    input(FileName, Map),
-    nth1(1, Map, Row1), nth1(StartY, Row1, '.'),
+    input(FileName, Map, Start, End),
+    graph(Map, Start, End, Graph),
+    StartKey is_key_of Start, EndKey is_key_of End,
+    findall(Dist, walk(Graph, visited{}, StartKey, EndKey, Dist), Dists),
+    max_list(Dists, Answer),
+    write(Answer), nl.
+
+walk(_, _, Node, Node, 0).
+walk(Graph, Visited, Start, End, Dist) :-
+    member(Next-NextDist, Graph.get(Start)), \+ _ = Visited.get(Next),
+    NextVisited = Visited.put(Next, true),
+    walk(Graph, NextVisited, Next, End, RemainingDist),
+    Dist is NextDist + RemainingDist.
+    
+graph(Map, Start, End, Graph) :-
+    branch_points(Map, Branches), Nodes = [Start, End|Branches],
     findall(
-        N,
-        (route(Map, visited{}, 1-StartY, N), format('~w, ', [N]), flush_output),
-        Ns),
-    max_list(Ns, Answer),
-    nl, format('Answer = ~w', [Answer]), nl.
+        Key-Dists,
+        (   member(Node, Nodes),
+            dists_from(Map, Nodes, Node, Dists),
+            Key is_key_of Node),
+        GraphList),
+    dict_pairs(Graph, edge, GraphList).
+        
+branch_points(Map, Points) :- findall(Point, branch_point(Map, Point), Points).
+branch_point(Map, X-Y) :-
+    nth1(X, Map, Row), nth1(Y, Row, Cell), Cell = '.',
+    findall(X1-Y1, cell_neighbor(Map, 0-0, X-Y, X1-Y1), Neighbors),
+    length(Neighbors, N), N > 2.
 
-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),
-    NeighborKey is X1*1000 + Y1, \+ _= Visiteds.get(NeighborKey),
-    route(Map, NextVisiteds, X1-Y1, N1),
-    N is N1 + 1.
+dists_from(Map, Nodes, N1, Dists) :-
+    findall(
+        Key-Dist, (
+            member(N2, Nodes), \+ N1 = N2,
+            nobranch_dist(Map, 0-0, N1, N2, Dist),
+            Key is_key_of N2),
+        Dists).
 
-neighbor(Map, X-Y, X1-Y1) :-
+is_key_of(Key, X-Y) :- Key is X*1000 + Y.
+    
+% nobranch_dist(X1-Y1, X2-Y2, Dist) :- true.
+nobranch_dist(_, _, X-Y, X-Y, 0).
+nobranch_dist(Map, FromX-FromY, X1-Y1, X2-Y2, Dist) :-
+    findall(Xn-Yn, cell_neighbor(Map, FromX-FromY, X1-Y1, Xn-Yn), Neighbors),
+    length(Neighbors, NeighborCount),
+    \+ (NeighborCount =\= 1, FromX =\= 0),  % on a route or at beginning
+    member(Neighbor, Neighbors),
+    nobranch_dist(Map, X1-Y1, Neighbor, X2-Y2, NextDist),
+    Dist is NextDist + 1.
+    
+% cell_neighbor finds an adjacent neighbor
+cell_neighbor(Map, FromX-FromY, X-Y, X1-Y1) :-
     (   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, '.').
+    nth1(X1, Map, Row1), nth1(Y1, Row1, '.'),
+    \+ (X1 = FromX, Y1 = FromY).
 
 % input parsing stuff below. Brick indexing is for debugging.
-input(FileName, Map) :- phrase_from_file(lines(Map), FileName).
+input(FileName, Map, 1-StartY, EndX-EndY) :-
+    phrase_from_file(lines(Map), FileName),
+    length(Map, EndX),
+    nth1(1, Map, StartRow), nth1(StartY, StartRow, '.'),
+    nth1(EndX, Map, EndRow), nth1(EndY, EndRow, '.').
 
 lines([]) --> eos, !.
 lines([Line|Lines]) --> line(Line), lines(Lines).