let load_file fname =
  match In_channel.with_open_text fname In_channel.input_line with
  | Some x -> x
  | None -> failwith "load_file"

let log10i i =
  let rec impl acc = function 0 -> acc | x -> impl (acc + 1) (x / 10) in
  assert (i > 0);
  impl ~-1 i

let digits10 i = 1 + log10i i

(** [pow10 n] returns [10] raised to the [n]th power. [n] must be non-negative.
*)
let pow10 n =
  let rec impl acc = function 0 -> acc | x -> impl (acc * 10) (x - 1) in
  assert (n >= 0);
  impl 1 n

let rec apply_n n fn arg = if n <= 0 then arg else apply_n (n - 1) fn (fn arg)

(*
let print_int_list lst =
  List.iter
    (fun i ->
      print_int i;
      print_char ' ')
    lst;
  print_newline ();
  ()
*)
let calc n input =
  let rec step_rec acc = function
    | [] -> acc
    | 0 :: t -> step_rec (1 :: acc) t
    | x :: t when digits10 x mod 2 = 0 ->
        let pow = pow10 (digits10 x / 2) in
        let left = x / pow in
        let right = x mod pow in
        step_rec (right :: left :: acc) t
    | x :: t -> step_rec ((x * 2024) :: acc) t
  in
  apply_n n (step_rec []) input

let part1 str = Aoc.ints_of_string str |> calc 25 |> List.length
let part2 str = Aoc.ints_of_string str |> calc 75 |> List.length
let _ = Aoc.main load_file [ (string_of_int, part1); (string_of_int, part2) ]