diff --git a/aoc.m b/aoc.m
index 99f7a3e..2e96b18 100644
--- a/aoc.m
+++ b/aoc.m
@@ -44,6 +44,7 @@ run_day(Day, Part, Lines, Out) :-
 :- import_module day4.
 :- import_module day5.
 :- import_module day6.
+:- import_module day7.
 
 :- pred solution(int::in, sol::out(sol)) is semidet.
 solution(1, day1.run).
@@ -52,3 +53,4 @@ solution(3, day3.run).
 solution(4, day4.run).
 solution(5, day5.run).
 solution(6, day6.run).
+solution(7, day7.run).
diff --git a/day7.m b/day7.m
new file mode 100644
index 0000000..9aff4a0
--- /dev/null
+++ b/day7.m
@@ -0,0 +1,80 @@
+:- module day7.
+:- interface.
+:- import_module basics.
+:- import_module univ.
+
+:- pred run(part::in, lines::in, univ::out) is cc_multi.
+
+:- implementation.
+:- import_module int.
+:- import_module string.
+:- import_module list.
+
+
+:- type words == list(string).
+
+:- type file ---> file(name :: string, size :: int).
+:- type files == list(file).
+
+:- type dir --->
+    dir(name :: string,
+        immed :: int, total :: int,
+        files :: files, dirs :: dirs).
+:- type dirs == list(dir).
+:- func total(dir) = int.
+
+:- pred line(words::out, lines::in, lines::out) is semidet.
+line(words(Line)) --> [Line].
+
+:- pred dir(dir::out, lines::in, lines::out) is nondet.
+dir(dir(Name, Immed, Total, Files, Dirs)) -->
+  cd_down(Name),
+  ["$ ls"], files(Files),
+  {Immed = foldl(func(File, Acc) = size(File) + Acc, Files, 0)},
+  dirs(Dirs),
+  {Total = foldl(func(Dir, Acc) = total(Dir) + Acc, Dirs, Immed)},
+  (["$ cd .."] ; []).
+
+:- pred cd_down(string::out, lines::in, lines::out) is semidet.
+cd_down(D) --> line(["$", "cd", D]).
+
+:- pred files(files::out, lines::in, lines::out) is multi.
+files([file(Name, Size) | Files]) -->
+  line([SSize, Name]), {to_int(SSize, Size)},
+  files(Files).
+files(Files) -->
+  line([Dir, _]), {Dir = "dir"},
+  files(Files).
+files([]) --> [].
+
+:- pred dirs(dirs::out, lines::in, lines::out) is multi.
+dirs([D | Ds]) --> dir(D), dirs(Ds).
+dirs([])       --> [].
+
+:- pred find(pred(dir)::(pred(in) is semidet), dir::in, list(dir)::out) is det.
+find(P, Dir, Out) :-
+  map(find(P), dirs(Dir), Outs),
+  Children = condense(Outs),
+  (if P(Dir) then Out = [Dir | Children] else Out = Children).
+
+:- pred size_lte(int::in, dir::in) is semidet.
+size_lte(Size, Dir) :- total(Dir) =< Size.
+
+:- pred size_gte(int::in, dir::in) is semidet.
+size_gte(Size, Dir) :- total(Dir) >= Size.
+
+:- func sort_size(list(dir)) = list(dir).
+sort_size(Dirs) = sort(func(D1, D2) = ordering(total(D1), total(D2)), Dirs).
+
+run(one, Lines, univ(Out)) :-
+  if dir(Top, Lines, []) then
+    find(size_lte(100_000), Top, Found),
+    Out = sum(map(total, Found))
+  else
+    die("bad input").
+run(two, Lines, univ(Out)) :-
+  if dir(Top, Lines, []) then
+    find(size_gte(total(Top) - 40_000_000), Top, Found),
+    Out = det_head(sort(map(total, Found)))
+  else
+    die("bad input").