Day 9-2

day9.erl

@@ -3,9 +3,47 @@
 -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}.
+    %#{visited := Visited} = EndState = fold_moves(Input, fun simulate/2, State),
+    {simulate_rope(Input, 1), simulate_rope(Input, 9)}.
+
+make_rope(Length) ->
+    Rope0 = make_head(),
+    Rope1 = add_knots(Rope0, Length),
+    add_trace(Rope1).
+
+simulate_rope(Input, Length) ->
+    {RopeFn, RopeState} = make_rope(Length),
+    {Visited, _RopeState} = fold_moves(Input, RopeFn, RopeState),
+    map_size(Visited).
+
+make_head() ->
+    {
+     fun(Move, [Pos]) ->
+             [vec_add(Move, Pos)]
+     end,
+     [{0, 0}]
+    }.
+
+add_knot({HeadFn, Head0}) ->
+    {
+     fun(Move, [TailPos|HeadState]) ->
+            [HeadPosNew|_] = HeadStateNew = HeadFn(Move, HeadState),
+            [chase_tail(HeadPosNew, TailPos) | HeadStateNew]
+     end,
+     [{0,0}|Head0]
+    }.
+
+add_knots(Stream, 0) -> Stream;
+add_knots(Stream, N) -> add_knots(add_knot(Stream), N-1).
+
+add_trace({SimFn, Sim0}) ->
+    {
+     fun(Move, {Visited, SimAccIn}) ->
+             [Pos | _] = SimAccOut = SimFn(Move, SimAccIn),
+             {Visited#{Pos => []}, SimAccOut}
+     end,
+     {#{}, Sim0}
+    }.
 
 simulate(Move, #{head := Head, tail := Tail, visited := Visited}) ->
     NewHead = vec_add(Head, Move),