let pos_of_numeric_grid c =
  match c with
  | '7' -> (0, 0)
  | '8' -> (1, 0)
  | '9' -> (2, 0)
  | '4' -> (0, 1)
  | '5' -> (1, 1)
  | '6' -> (2, 1)
  | '1' -> (0, 2)
  | '2' -> (1, 2)
  | '3' -> (2, 2)
  | '0' -> (1, 3)
  | 'A' -> (2, 3)
  | _ -> raise (invalid_arg "pos_of_numeric_grid")

let pos_of_dir_grid c =
  match c with
  | '^' -> (1, 3)
  | 'A' -> (2, 3)
  | '<' -> (0, 4)
  | 'v' -> (1, 4)
  | '>' -> (2, 4)
  | _ -> raise (invalid_arg "pos_of_dir_grid")

let invalid_x, invalid_y = (0, 3)

let rec move pos_of_grid builder (cx, cy) code =
  match Seq.uncons code with
  | None -> Buffer.contents builder
  | Some (c, t) ->
      let dx, dy = pos_of_grid c in
      if dx > cx && (cy = dy || cy <> invalid_y) then begin
        Buffer.add_string builder (String.make (dx - cx) '>');
        move pos_of_grid builder (dx, cy) code
      end
      else if dx < cx && (cy = dy || cy <> invalid_y) then begin
        Buffer.add_string builder (String.make (cx - dx) '<');
        move pos_of_grid builder (dx, cy) code
      end
      else if dy > cy && (cx = dx || cx <> invalid_x) then begin
        Buffer.add_string builder (String.make (dy - cy) 'v');
        move pos_of_grid builder (cx, dy) code
      end
      else if dy < cy && (cx = dx || cx <> invalid_x) then begin
        Buffer.add_string builder (String.make (cy - dy) '^');
        move pos_of_grid builder (cx, dy) code
      end
      else begin
        assert (cx = dx);
        assert (dy = dy);
        Buffer.add_char builder 'A';
        move pos_of_grid builder (cx, cy) t
      end

let process_code pos_of_grid code =
  let builder = Buffer.create 30 in
  move pos_of_grid builder (pos_of_grid 'A') (String.to_seq code)

let human_insns code =
  let first_robot_insns = process_code pos_of_numeric_grid code in
  let second_robot_insns = process_code pos_of_dir_grid first_robot_insns in
  let human = process_code pos_of_dir_grid second_robot_insns in
  print_endline human;
  print_endline second_robot_insns;
  print_endline first_robot_insns;
  print_endline code;
  human

let complexity code =
  let insns = human_insns code in
  let num = int_of_string (String.sub code 0 (String.length code - 1)) in
  Printf.printf "%s (%d): %s (%d) (%d) \n" code num insns (String.length insns)
    (num * String.length insns);
  num * String.length insns

let part1 codes = List.map complexity codes |> List.fold_left ( + ) 0
let _ = Aoc.main Aoc.strings_of_file [ (string_of_int, part1) ]

(* 175386 too high *)