Source code in the ocaml programming language

let max_doors = 100

let show_doors =
  Array.iteri (fun i x -> Printf.printf "Door %d is %s\n" (i+1)
                                        (if x then "open" else "closed"))

let flip_doors doors =
  for i = 1 to max_doors do
    let rec flip idx =
      if idx < max_doors then begin
        doors.(idx) <- not doors.(idx);
        flip (idx + i)
      end
    in flip (i - 1)
  done;
  doors

let () =
  show_doors (flip_doors (Array.make max_doors false))

let optimised_flip_doors doors =
  for i = 1 to int_of_float (sqrt (float_of_int max_doors)) do
    doors.(i*i - 1) <- true
  done;
  doors

let () =
  show_doors (optimised_flip_doors (Array.make max_doors false))

type door = Open | Closed    (* human readable code *)

let flipdoor = function Open -> Closed | Closed -> Open

let string_of_door =       
  function Open -> "is open." | Closed -> "is closed."

let printdoors ls =
  let f i d = Printf.printf "Door %i %s\n" (i + 1) (string_of_door d)
  in List.iteri f ls

let outerlim = 100
let innerlim = 100

let rec outer cnt accu =
  let rec inner i door = match i > innerlim with (* define inner loop *)
    | true  -> door                         
    | false -> inner (i + 1) (if (cnt mod i) = 0 then flipdoor door else door)
  in (* define and do outer loop *)
  match cnt > outerlim with
  | true  -> List.rev accu
  | false -> outer  (cnt + 1)  (inner 1 Closed :: accu) (* generate new entries with inner *)

let () = printdoors (outer 1 [])