let sprintf = Printf.sprintf

type ('impl, 'intf) intf_or_impl =
  | Impl of 'impl
  | Intf of 'intf

module File = struct
  let of_filename s = if Filename.check_suffix s ".rei" then Intf s else Impl s

  let output_fn = function
    | Impl fn -> fn ^ ".ml"
    | Intf fn -> fn ^ ".mli"
  ;;
end

let () =
  let set_binary = function
    | "binary" -> ()
    | _ -> failwith "Only the value 'binary' is allowed for --parse / --print"
  in
  let args =
    [ "--print", Arg.String set_binary, ""
    ; "--parse", Arg.String set_binary, ""
    ; "-i", Arg.Bool ignore, ""
    ]
  in
  let source = ref None in
  let anon s =
    match !source with
    | None -> source := Some s
    | Some _ -> failwith "source may be set only once"
  in
  Arg.parse args anon "";
  let source =
    match !source with
    | None -> failwith "source file isn't set"
    | Some s -> s
  in
  let ic = open_in source in
  let source_file = File.of_filename source in
  let out_fn = File.output_fn source_file in
  let out = open_out_bin out_fn in
  output_string out (sprintf "# 1 %S\n" source);
  let rec loop () =
    match input_char ic with
    | exception End_of_file -> ()
    | s ->
      output_char out s;
      loop ()
  in
  loop ();
  close_out_noerr out;
  let inch = open_in_bin out_fn in
  let contents = really_input_string inch (in_channel_length inch) in
  close_in inch;
  Printf.printf "%s" contents
;;