Day 8-1

day8.erl

@@ -0,0 +1,53 @@
+-module(day8).
+
+-export([solve/1]).
+
+-compile(export_all).
+
+solve(Input) ->
+    Grid = parse_grid(Input),
+    {map_size(visible_trees(Grid)), none}.
+
+visible_trees({_, H, W} = Grid) ->
+    %top
+    Visible0 = #{},
+    %Left
+    Visible1 = range_fold(0, H-1, fun(Row, Acc) -> element(2, grid_traverse(fun visible_path/3, {-1, Acc}, Grid, {0, Row}, {1, 0})) end, Visible0),
+    %Right
+    Visible2 = range_fold(0, H-1, fun(Row, Acc) -> element(2, grid_traverse(fun visible_path/3, {-1, Acc}, Grid, {W-1, Row}, {-1, 0})) end, Visible1),
+    %Top
+    Visible3 = range_fold(0, W-1, fun(Col, Acc) -> element(2, grid_traverse(fun visible_path/3, {-1, Acc}, Grid, {Col, 0}, {0, 1})) end, Visible2),
+    %Bottom
+    range_fold(0, W-1, fun(Col, Acc) -> element(2, grid_traverse(fun visible_path/3, {-1, Acc}, Grid, {Col, H-1}, {0, -1})) end, Visible3).
+
+visible_path(Tree, Position, {MaxHeight, Visible}) when Tree > MaxHeight -> {Tree, Visible#{Position => []}};
+visible_path(_Tree, _Position, State) -> State.
+
+grid_traverse(Fun, AccIn, Grid, {XPos, YPos}, {XVec, YVec}) ->
+    case grid_at(Grid, XPos, YPos) of
+        error -> AccIn;
+        {ok, Item} -> grid_traverse(Fun, Fun(Item, {XPos, YPos}, AccIn), Grid, {XPos+XVec, YPos+YVec}, {XVec, YVec})
+    end.
+
+parse_grid(Data) ->
+    {W, H} = dimensions(Data),
+    {Data, H, W}.
+
+dimensions(Grid) -> dimensions(Grid, 1, 0).
+dimensions(<<>>, Height, Width) -> {Width, Height};
+dimensions(<<$\n>>, Height, Width) -> {Width, Height};
+dimensions(<<$\n, Rest/binary>>, Height, _Width) ->
+    dimensions(Rest, Height+1, 0);
+dimensions(<<_Digit, Rest/binary>>, Height, Width) ->
+    dimensions(Rest, Height, Width+1).
+
+grid_at({_Data, _H, _W}, X, Y) when X < 0 orelse Y < 0 -> error;
+grid_at({_Data, H, W}, X, Y) when X >= W orelse Y >= H -> error;
+grid_at({Data, _H, W}, X, Y) ->
+    Index = X + Y * (W+1),
+    <<_:Index/binary, Tree, _/binary>> = Data,
+    {ok, Tree-$0}.
+
+range_fold(End, End, Fun, Acc) -> Fun(End, Acc);
+range_fold(Start, End, Fun, Acc) when Start < End -> range_fold(Start+1, End, Fun, Fun(Start, Acc));
+range_fold(Start, End, Fun, Acc) when Start > End -> range_fold(Start-1, End, Fun, Fun(Start, Acc)).