-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)).