2021/day9/day9.erl

   1 %% to compile: erlc day3A.erl
   2 %% to run: erl -noshell -s day3 solve
   3 %%
   4 -module(day9).
   5
   6 -export ([solve/0, solve/1, solve/2]).
   7 -export ([read_input/0]).
   8
   9 solve() ->
  10     solve(['1']),
  11     solve(['2']),
  12     init:stop().
  13
  14 solve(A) ->
  15     solve(A, read_input()).
  16
  17 solve(['1'], D) ->
  18     io:format("The solution to ~p puzzle1 is: ~p~n", [?MODULE, solve(1, D)]);
  19 solve(1, D) ->
  20     solution1(D);
  21 solve(['2'], D) ->
  22     io:format("The solution to ~p puzzle2 is: ~p~n", [?MODULE, solve(2, D)]);
  23 solve(2, D) ->
  24     solution2(D).
  25
  26 read_input() ->
  27         {ok, IO} = file:open("input.txt", 'read'),
  28         Data = read_input(IO),
  29         file:close(IO),
  30         Data.
  31
  32 read_input(IO) ->
  33         read_input(IO, []).
  34
  35 read_input(IO, Input) ->
  36         case read_line(IO) of
  37                 'eof' -> Input;
  38                 Line -> read_input(IO, Input ++ [Line])
  39         end.
  40
  41 read_line(IO) ->
  42         case file:read_line(IO) of
  43                 'eof' -> 'eof';
  44                 {ok, Line} -> [ X - $0 || X <- Line, [X] /= "\n"]
  45         end.
  46
  47 solution1(Table) ->
  48   Low_points = low_points(Table),
  49   lists:foldl(fun(C, Acc) -> Acc + 1 + get_value(C, Table) end, 0, Low_points).
  50
  51 solution2(Table) ->
  52   Low_points = low_points(Table),
  53   Basins = get_basins(Low_points, Table),
  54   [B1,B2,B3|_] = lists:sort(fun(A, B) -> length(A) > length(B) end, Basins),
  55   length(B1) * length(B2) * length(B3).
  56
  57
  58 get_basins(Low_points, Table) ->
  59         get_basins(Low_points, Table, []).
  60
  61 get_basins([], _Table, Basins) -> Basins;
  62 get_basins([Low_point|Rest], Table, Basins) ->
  63   Basin = [Low_point|get_up_slopes([Low_point], Table)],
  64   get_basins(Rest, Table, [Basin|Basins]).
  65
  66 low_points([H|_] =  Table) ->
  67   X = length(H),
  68   Y = length(Table),
  69   Coords = lists:flatten([[{A,B} || A <- lists:seq(1,X)] || B <- lists:seq(1,Y)]),
  70   lists:filter(fun(C) -> is_low_point(C, Table) end, Coords).
  71
  72 get_up_slopes(Coords, Table) ->
  73         get_up_slopes(Coords, Table, []).
  74
  75 get_up_slopes([], _Table, Acc) -> Acc;
  76 get_up_slopes([{X, Y}|Rest], Table, Acc) ->
  77     V = get_value({X,Y}, Table),
  78     Neighbours = [{X+1, Y},{X-1, Y},{X, Y+1},{X, Y-1}],
  79     Up_slopes = lists:filter(fun({X1,Y1}) -> C = get_value({X1,Y1}, Table), C /= 9 andalso C > V end, Neighbours),
  80     get_up_slopes(lists:usort(Rest ++ Up_slopes), Table, lists:usort(Acc ++ Up_slopes)).
  81
  82 is_low_point({X,Y}, Table) ->
  83     V = get_value({X,Y}, Table),
  84     V < get_value({X+1, Y}, Table) andalso
  85     V < get_value({X-1, Y}, Table) andalso
  86     V < get_value({X, Y+1}, Table) andalso
  87     V < get_value({X, Y-1}, Table).
  88
  89 get_value({X,Y}, [H|_] = Table) when X == 0; Y == 0; Y > length(Table); X > length(H) -> 9;
  90
  91 get_value({X,Y}, Table) ->
  92         lists:nth(X, lists:nth(Y, Table)).
  93