Day 8-2

day8.erl

@@ -2,11 +2,22 @@
 
 -export([solve/1]).
 
--compile(export_all).
-
 solve(Input) ->
     Grid = parse_grid(Input),
-    {map_size(visible_trees(Grid)), none}.
+    {map_size(visible_trees(Grid)), scenery(Grid)}.
+
+
+scenery({_, H, W} = Grid) ->
+    Visible0 = #{},
+    %Left
+    Visible1 = range_fold(0, H-1, fun(Row, Acc) -> element(2, grid_traverse(fun scenery_score/3, {[], Acc}, Grid, {0, Row}, {1, 0})) end, Visible0),
+    %Right
+    Visible2 = range_fold(0, H-1, fun(Row, Acc) -> element(2, grid_traverse(fun scenery_score/3, {[], Acc}, Grid, {W-1, Row}, {-1, 0})) end, Visible1),
+    %Top
+    Visible3 = range_fold(0, W-1, fun(Col, Acc) -> element(2, grid_traverse(fun scenery_score/3, {[], Acc}, Grid, {Col, 0}, {0, 1})) end, Visible2),
+    %Bottom
+    Visible4 = range_fold(0, W-1, fun(Col, Acc) -> element(2, grid_traverse(fun scenery_score/3, {[], Acc}, Grid, {Col, H-1}, {0, -1})) end, Visible3),
+    lists:max(maps:values(Visible4)).
 
 visible_trees({_, H, W} = Grid) ->
     %top
@@ -23,6 +34,16 @@ visible_trees({_, H, W} = Grid) ->
 visible_path(Tree, Position, {MaxHeight, Visible}) when Tree > MaxHeight -> {Tree, Visible#{Position => []}};
 visible_path(_Tree, _Position, State) -> State.
 
+% horrible name, calculate the view distance in one direction
+tree_stack(_Tree, []) -> 0;
+tree_stack(Tree, [Other|_Rest]) when Other >= Tree -> 1;
+tree_stack(Tree, [_Other|Rest]) -> tree_stack(Tree, Rest) + 1.
+
+scenery_score(Tree, Position, {TreeStack, Trees}) ->
+    Distance = tree_stack(Tree, TreeStack),
+    NewTrees = Trees#{Position => Distance * maps:get(Position, Trees, 1)},
+    {[Tree|TreeStack], NewTrees}.
+
 grid_traverse(Fun, AccIn, Grid, {XPos, YPos}, {XVec, YVec}) ->
     case grid_at(Grid, XPos, YPos) of
         error -> AccIn;