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74 lines
3.2 KiB
74 lines
3.2 KiB
-module(day8).
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-export([solve/1]).
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solve(Input) ->
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Grid = parse_grid(Input),
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{map_size(visible_trees(Grid)), scenery(Grid)}.
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scenery({_, H, W} = Grid) ->
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Visible0 = #{},
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%Left
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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),
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%Right
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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),
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%Top
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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),
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%Bottom
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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),
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lists:max(maps:values(Visible4)).
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visible_trees({_, H, W} = Grid) ->
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%top
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Visible0 = #{},
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%Left
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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),
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%Right
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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),
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%Top
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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),
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%Bottom
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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).
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visible_path(Tree, Position, {MaxHeight, Visible}) when Tree > MaxHeight -> {Tree, Visible#{Position => []}};
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visible_path(_Tree, _Position, State) -> State.
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% horrible name, calculate the view distance in one direction
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tree_stack(_Tree, []) -> 0;
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tree_stack(Tree, [Other|_Rest]) when Other >= Tree -> 1;
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tree_stack(Tree, [_Other|Rest]) -> tree_stack(Tree, Rest) + 1.
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scenery_score(Tree, Position, {TreeStack, Trees}) ->
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Distance = tree_stack(Tree, TreeStack),
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NewTrees = Trees#{Position => Distance * maps:get(Position, Trees, 1)},
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{[Tree|TreeStack], NewTrees}.
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grid_traverse(Fun, AccIn, Grid, {XPos, YPos}, {XVec, YVec}) ->
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case grid_at(Grid, XPos, YPos) of
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error -> AccIn;
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{ok, Item} -> grid_traverse(Fun, Fun(Item, {XPos, YPos}, AccIn), Grid, {XPos+XVec, YPos+YVec}, {XVec, YVec})
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end.
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parse_grid(Data) ->
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{W, H} = dimensions(Data),
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{Data, H, W}.
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dimensions(Grid) -> dimensions(Grid, 1, 0).
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dimensions(<<>>, Height, Width) -> {Width, Height};
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dimensions(<<$\n>>, Height, Width) -> {Width, Height};
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dimensions(<<$\n, Rest/binary>>, Height, _Width) ->
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dimensions(Rest, Height+1, 0);
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dimensions(<<_Digit, Rest/binary>>, Height, Width) ->
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dimensions(Rest, Height, Width+1).
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grid_at({_Data, _H, _W}, X, Y) when X < 0 orelse Y < 0 -> error;
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grid_at({_Data, H, W}, X, Y) when X >= W orelse Y >= H -> error;
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grid_at({Data, _H, W}, X, Y) ->
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Index = X + Y * (W+1),
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<<_:Index/binary, Tree, _/binary>> = Data,
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{ok, Tree-$0}.
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range_fold(End, End, Fun, Acc) -> Fun(End, Acc);
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range_fold(Start, End, Fun, Acc) when Start < End -> range_fold(Start+1, End, Fun, Fun(Start, Acc));
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range_fold(Start, End, Fun, Acc) when Start > End -> range_fold(Start-1, End, Fun, Fun(Start, Acc)).
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