diff --git a/day9.erl b/day9.erl
new file mode 100644
index 0000000..067febd
--- /dev/null
+++ b/day9.erl
@@ -0,0 +1,59 @@
+-module(day9).
+
+-export([solve/1]).
+
+solve(Input) ->
+    State = #{head => {0, 0}, tail => {0, 0}, visited => #{{0, 0} => []}},
+    #{visited := Visited} = EndState = fold_moves(Input, fun simulate/2, State),
+    {map_size(Visited), none}.
+
+simulate(Move, #{head := Head, tail := Tail, visited := Visited}) ->
+    NewHead = vec_add(Head, Move),
+    NewTail = chase_tail(NewHead, Tail),
+    NewVisited = Visited#{NewTail => []},
+    #{head => NewHead, tail => NewTail, visited => NewVisited}.
+
+chase_tail(Head, Tail) ->
+    Diff = vec_sub(Head, Tail),
+    case vec_norm(Diff) of
+        Diff -> Tail;
+        Norm -> vec_add(Norm, Tail)
+    end.
+
+fold_instructions(<<Letter, $\s, RestIn/binary>>, Fun, Acc) ->
+    {Count, RestOut} = parse_int(RestIn, 0),
+    Direction = 
+        case Letter of
+            $U -> up;
+            $D -> down;
+            $L -> left;
+            $R -> right
+        end,
+    fold_instructions(RestOut, Fun, Fun({Direction, Count}, Acc));
+fold_instructions(<<$\n, Rest/binary>>, Fun, Acc) -> fold_instructions(Rest, Fun, Acc);
+fold_instructions(<<>>, _Fun, Acc) -> Acc.
+
+fold_moves(Stream, Fun, Acc) -> element(2, fold_instructions(Stream, fun per_move/2, {Fun, Acc})).
+
+per_move({Direction, Count}, {Fun, Acc}) -> {Fun, per_move0(direction_vec(Direction), Count, Fun, Acc)}.
+
+per_move0(_Move, 0, _Fun, Acc) -> Acc;
+per_move0(Move, Count, Fun, Acc) -> per_move0(Move, Count-1, Fun, Fun(Move, Acc)).
+
+direction_vec(up) -> {0, 1};
+direction_vec(down) -> {0, -1};
+direction_vec(left) -> {-1, 0};
+direction_vec(right) -> {1, 0}.
+
+vec_add({X1, Y1}, {X2, Y2}) -> {X1+X2, Y1+Y2}.
+
+vec_sub({X1, Y1}, {X2, Y2}) -> {X1-X2, Y1-Y2}.
+
+vec_norm({X, Y}) -> {norm(X), norm(Y)}.
+
+norm(X) when X > 0 -> 1;
+norm(X) when X < 0 -> -1;
+norm(0) -> 0.
+
+parse_int(<<Char, Rest/binary>>, Acc) when Char >= $0 andalso Char =< $9 -> parse_int(Rest, Acc*10+(Char - $0));
+parse_int(Rest, Acc) -> {Acc, Rest}.