diff --git a/aoc.m b/aoc.m
index cb54831..2996ed8 100644
--- a/aoc.m
+++ b/aoc.m
@@ -50,6 +50,7 @@ run_day(Day, Part, Lines, Out) :-
 :- import_module day13.
 :- import_module day14.
 :- import_module day16.
+:- import_module day18.
 
 :- pred solution(int::in, sol::out(sol)) is semidet.
 solution(1, day1.run).
@@ -66,3 +67,4 @@ solution(11, day11.run).
 solution(13, day13.run).
 solution(14, day14.run).
 solution(16, day16.run).
+solution(18, day18.run).
diff --git a/day18.m b/day18.m
new file mode 100644
index 0000000..ba61148
--- /dev/null
+++ b/day18.m
@@ -0,0 +1,130 @@
+:- module day18.
+:- interface.
+:- import_module basics.
+
+:- pred run(part::in, lines::in, answer::out) is cc_multi.
+
+:- implementation.
+:- import_module int.
+:- import_module char.
+:- import_module string.
+:- import_module list.
+:- import_module digraph.
+:- import_module ranges.
+:- import_module solutions.
+:- import_module set_tree234.
+
+:- type set(T) == set_tree234(T).
+
+:- type point3 ---> p(x :: int, y :: int, z :: int).
+
+:- type bounds ---> b(lo :: point3, hi :: point3).
+
+:- type dir ---> left; right; up; down; fore; back.
+
+:- type vertex ---> empty(point3) ; full(point3, dir).
+
+:- type points == set(point3).
+:- type graph  == digraph(vertex).
+:- type key    == digraph_key(vertex).
+
+
+:- pred lines(lines::in, list(point3)::out) is semidet.
+lines(Lines, Ps) :- map(line, Lines, Ps).
+
+:- pred line(string::in, point3::out) is semidet.
+line(Str, P) :- line(P, to_char_list(Str), []).
+
+:- pred line(point3::out, chars::in, chars::out) is semidet.
+line(p(X, Y, Z)) --> number(X), [','], number(Y), [','], number(Z).
+
+:- pred number(int::out, chars::in, chars::out) is semidet.
+number(N) --> digits(S), {to_int(from_char_list(S), N)}.
+
+:- pred digits(chars::out, chars::in, chars::out) is semidet.
+digits([C | Cs]) -->
+  [C], {is_digit(C)},
+  (if digits(Cs0) then {Cs = Cs0} else {Cs = []}).
+
+
+:- func pointwise(func(int) = int, point3) = point3.
+pointwise(F, p(X,Y,Z)) = p(F(X), F(Y), F(Z)).
+
+:- pred pointwise(pred(int, int, int), point3, point3, point3).
+:- mode pointwise(pred(in, in, out) is det, in, in, out) is det.
+pointwise(P, p(X1,Y1,Z1), p(X2,Y2,Z2), p(X,Y,Z)) :-
+  P(X1, X2, X), P(Y1, Y2, Y), P(Z1, Z2, Z).
+
+
+:- pred bounds(list(point3), bounds).
+:- mode bounds(in, out) is semidet.
+:- mode bounds(in(non_empty_list), out) is det.
+bounds([P | Ps], b(Lo, Hi)) :-
+  foldl(pointwise(min), Ps, P, Lo0), foldl(pointwise(max), Ps, P, Hi0),
+  Lo = pointwise(func(I) = I - 1, Lo0),
+  Hi = pointwise(func(I) = I + 1, Hi0).
+
+:- pred in_bounds(bounds::in, point3::in) is semidet.
+in_bounds(b(p(X1,Y1,Z1), p(X2,Y2,Z2)), p(X,Y,Z)) :-
+  X1 =< X, X =< X2, Y1 =< Y, Y =< Y2, Z1 =< Z, Z =< Z2.
+
+:- pred nondet_in_bounds(bounds::in, point3::out) is nondet.
+nondet_in_bounds(b(p(X1,Y1,Z1), p(X2,Y2,Z2)), p(X,Y,Z)) :-
+  Xs = range(X1, X2), Ys = range(Y1, Y2), Zs = range(Z1, Z2),
+  nondet_member(X, Xs), nondet_member(Y, Ys), nondet_member(Z, Zs).
+
+:- pred adj0(point3::in, point3::out, dir::out) is multi.
+adj0(p(X,Y,Z), p(X1,Y,Z), D) :- X1 = X - 1, D = left ; X1 = X + 1, D = right.
+adj0(p(X,Y,Z), p(X,Y1,Z), D) :- Y1 = Y - 1, D = up   ; Y1 = Y + 1, D = down.
+adj0(p(X,Y,Z), p(X,Y,Z1), D) :- Z1 = Z - 1, D = fore ; Z1 = Z + 1, D = back.
+
+:- pred adj(bounds::in, point3::in, {point3,dir}::out) is nondet.
+adj(Bounds, P, {Q, D}) :- adj0(P, Q, D), in_bounds(Bounds, Q).
+
+
+:- pred build_graph(list(point3)::in, graph::out, key::out) is cc_nondet.
+build_graph(PointList, Graph, StartKey) :-
+  bounds(PointList, Bounds),
+  list_to_set(PointList, Points),
+  build_graph0(Bounds, Points, Graph),
+  nondet_in_bounds(Bounds, Start),
+  not member(Start, Points),
+  search_key(Graph, empty(Start), StartKey).
+
+:- pred build_graph0(bounds::in, points::in, graph::out) is det.
+build_graph0(Bounds, Full, Graph) :-
+  solutions(nondet_in_bounds(Bounds), AllPoints),
+  foldl(add_adj_vertices(Bounds, Full), AllPoints, init, Graph).
+
+:- pred add_adj_vertices(bounds::in, points::in, point3::in,
+                         graph::in, graph::out) is det.
+add_adj_vertices(Bounds, Full, P, !G) :-
+  if member(P, Full) then true else
+  solutions(adj(Bounds, P), Adj),
+  foldl(add_vertex(Full, P), Adj, !G).
+
+:- pred add_vertex(points::in, point3::in, {point3,dir}::in,
+                   graph::in, graph::out) is det.
+add_vertex(Full, From, {To, Dir}, !G) :-
+  ToV = ite((pred) is semidet :- member(To, Full), full(To, Dir), empty(To)),
+  add_vertices_and_edge(empty(From), ToV, !G).
+
+
+:- func count_faces(graph, key) = int.
+count_faces(Graph, Start) = Count :-
+  dfs(Graph, Start, SeenKeys),
+  foldl(pred(K::in, Acc0::in, Acc::out) is det :-
+          (if lookup_vertex(Graph, K) = full(_, _) then Acc = Acc0 + 1
+          else Acc = Acc0),
+        SeenKeys, 0, Count).
+
+
+run(one, _, _) :- die("bqn").
+run(two, Lines, Out) :-
+  if
+    lines(Lines, PointList),
+    build_graph(PointList, Graph, Start)
+  then
+    Out = int(count_faces(Graph, Start))
+  else
+    die("bad input").