d23p2

23/part2.pl

@@ -0,0 +1,90 @@
+:- use_module(library(pio)).
+:- use_module(library(dcg/basics)).
+:- initialization(main, main).
+:- op(700, xfx, is_key_of).
+
+main([FileName|_]) :-
+    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(
+        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.
+
+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).
+
+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, '.'),
+    \+ (X1 = FromX, Y1 = FromY).
+
+% input parsing stuff below. Brick indexing is for debugging.
+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).
+
+line([]) --> ("\n"; eos), !.
+line(['#'|Chars]) --> "#", line(Chars).
+line(['.'|Chars]) --> ("."; ">"; "v"), line(Chars).
+
+% debug
+print(Map) :-
+    findall(
+        X,
+        (   nth1(X, Map, Line),
+            format('~3d', [X]), write(" "),
+            atomic_list_concat(Line, Str), write(Str), nl
+        ),
+        _),
+    nl.