2021/day17/day17.erl

   1 %% to compile: erlc day3A.erl
   2 %% to run: erl -noshell -s day5 solve
   3 %%
   4 -module(day17).
   5
   6 -export ([solve/0, solve/1, solve/2]).
   7 -compile ([export_all]).
   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 solution1({X,Y}) ->
  27     Xs = find_x(X),
  28     Ys = find_y(Y),
  29     {T, X1, Y1} = lists:max([{Key, X1, proplists:get_value(Key,Ys)} || {Key, X1} <- Xs, lists:keysearch(Key, 1, Ys) /= false]),
  30     y_max(T,Y1,0).
  31
  32 solution2({X,Y}) ->
  33     Xs = find_x(X),
  34     Ys = find_y(Y),
  35     length(lists:usort([{X,Y} || {Ty, Y} <- Ys, {T,X} <- lists:filter(fun({T,X}) -> T == Ty end, Xs)])).
  36
  37 read_input() ->
  38     {{124,174},{-123, -86}}.
  39 %%    {{20,30},{-10, -5}}.
  40
  41 find_x({Xmin, Xmax}) ->
  42     [ {T, Xstart}  || Xstart <- lists:seq(0,400), T <- lists:seq(1,1000),  x(Xstart, T) >= Xmin, x(Xstart, T) =< Xmax ].
  43
  44 find_y({Ymin, Ymax}) ->
  45     [ {T, Ystart}  || Ystart <- lists:seq(-400,400), T <- lists:seq(1,1000), y(Ystart, T) >= Ymin, y(Ystart, T) =< Ymax ].
  46
  47 x(Xstart, T) ->
  48     Last =
  49     case (Xstart - T + 1) of
  50         L when L < 0 -> 0;
  51         L -> L
  52     end,
  53     (Xstart - Last + 1) * (Xstart + Last) div 2.
  54
  55 y(Ystart, T) ->
  56     Last = (Ystart - T + 1),
  57     (Ystart - Last + 1) * (Ystart + Last) div 2.
  58
  59 y_max(0, _, Ymax) -> Ymax;
  60 y_max(T, Y, Ymax)  ->
  61     NewY = y(Y, T),
  62     NewMax =
  63     case  NewY of
  64         M when M > Ymax -> M;
  65         _ -> Ymax
  66     end,
  67     y_max(T-1, Y, NewMax).
  68