diff --git a/aoc.m b/aoc.m
index f1664b1..cb54831 100644
--- a/aoc.m
+++ b/aoc.m
@@ -47,9 +47,9 @@ run_day(Day, Part, Lines, Out) :-
 :- import_module day9.
 :- import_module day10.
 :- import_module day11.
-% :- import_module day12.
 :- import_module day13.
 :- import_module day14.
+:- import_module day16.
 
 :- pred solution(int::in, sol::out(sol)) is semidet.
 solution(1, day1.run).
@@ -63,6 +63,6 @@ solution(8, day8.run).
 solution(9, day9.run).
 solution(10, day10.run).
 solution(11, day11.run).
-% solution(12, day12.run).
 solution(13, day13.run).
 solution(14, day14.run).
+solution(16, day16.run).
diff --git a/day16.m b/day16.m
new file mode 100644
index 0000000..50032d6
--- /dev/null
+++ b/day16.m
@@ -0,0 +1,116 @@
+:- module day16.
+:- 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 map.
+:- import_module digraph.
+:- import_module set.
+
+:- type label == string.
+:- type key   == digraph_key(label).
+:- type rates == map(key, int).
+:- type graph == digraph(label).
+:- type open  == set(key).
+
+
+:- pred spaces(chars::in, chars::out) is det.
+spaces --> if [' '] then spaces else [].
+
+:- pred word0(chars::out, chars::in, chars::out) is semidet.
+word0([C | Cs]) -->
+  [C], {is_alpha(C)},
+  (if word0(Cs0) then {Cs = Cs0} else {Cs = []}).
+
+:- pred word(string::out, chars::in, chars::out) is semidet.
+word(W) --> word0(Cs), {W = from_char_list(Cs)}, spaces.
+
+:- pred words(list(string)::in, chars::in, chars::out) is semidet.
+words([])       --> [].
+words([W | Ws]) --> word(W), words(Ws).
+
+:- pred sym(char::in, chars::in, chars::out) is semidet.
+sym(S) --> [S], spaces.
+
+:- pred tunnels(list(string)::out, chars::in, chars::out) is semidet.
+tunnels([T | Ts]) -->
+  word(T), (if sym(',') then tunnels(Ts) else {Ts = []}).
+
+:- 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(Cs2) then {Cs = Cs2} else {Cs = []}).
+
+:- pred line(graph::in, graph::out, rates::in, rates::out,
+             chars::in, chars::out) is nondet.
+line(!Graph, !Rates) -->
+  word("Valve"), word(Label),
+  words(["has", "flow", "rate"]), sym('='),
+  number(Rate), sym(';'),
+  (words(["tunnels", "lead",  "to", "valves"]) ;
+   words(["tunnel",  "leads", "to", "valve"])),
+  tunnels(Tunnels),
+  {add_vertex(Label, Key, !Graph)},
+  {insert(Key, Rate, !Rates)},
+  {foldl(add_vertices_and_edge(Label), Tunnels, !Graph)}.
+
+:- pred line(string::in, graph::in, graph::out, rates::in, rates::out)
+    is nondet.
+line(Str, !G, !R) :- line(!G, !R, to_char_list(Str), []).
+
+:- pred lines(lines::in, graph::out, rates::out) is cc_nondet.
+lines(Lines, G, R) :- foldl2(line, Lines, digraph.init, G, map.init, R).
+
+
+:- pred max_release(int::in, label::in, graph::in, rates::in, int::out) is det.
+max_release(Time, Start, Graph, Rates, Max) :-
+  max_release(Time, lookup_key(Graph, Start), Graph, Rates, init, _, Max).
+
+:- pred max_release(int, key, graph, rates, open, open, int).
+:- mode max_release(in, in, in, in, in, out, out) is det.
+:- pragma memo(max_release/7,
+    [specified([value, value, addr, addr, value, output, output])]).
+max_release(Time, Here, Graph, Rates, !Open, Max) :-
+  if Time = 0 then Max = 0 else
+  HereRate = lookup(Rates, Here),
+  StartOpen = !.Open,
+  (if
+    HereRate \= 0,
+    insert_new(Here, !Open)
+  then
+    max_release(Time - 1, Here, Graph, Rates, !Open, Max0),
+    MaxHere = Max0 + HereRate * (Time - 1)
+  else
+    MaxHere = 0),
+  Nexts = [Here | set.to_sorted_list(lookup_from(Graph, Here))],
+  max_releases(Time - 1, Nexts, Graph, Rates, StartOpen, Outs),
+  get_max([{!.Open, MaxHere} | Outs], !:Open, Max).
+
+:- type out == {open, int}.
+
+:- pred max_releases(int, list(key), graph, rates, open, list(out)).
+:- mode max_releases(in, in, in, in, in, out) is det.
+max_releases(_, [], _, _, _, []).
+max_releases(Time, [Here|Theres], Graph, Rates, Open0, [{Open, Max} | Rest]) :-
+  max_release(Time, Here, Graph, Rates, Open0, Open, Max) &
+  max_releases(Time, Theres, Graph, Rates, Open0, Rest).
+
+:- pred get_max(list(out)::in(non_empty_list), open::out, int::out) is det.
+get_max([{O, M}], O, M).
+get_max([{O, M}, R | Rest], O0, M0) :-
+  get_max([R | Rest], OR, MR),
+  (if M >= MR then O0 = O, M0 = M else O0 = OR, M0 = MR).
+
+run(one, Lines, int(Max)) :-
+  if lines(Lines, G, R) then max_release(30, "AA", G, R, Max)
+  else die("bad input").
+run(two, _Lines, _Out) :- die("idk").