type vtable = {
  n_slots : int;
  elems : (string, elem) Hashtbl.t;
}

and slotidx = int

and elem =
  | Field of slotidx
  | Method (* of callable *)

let make_vtable fields =
  {
    n_slots = List.length fields;
    elems = List.to_seq fields |> Seq.mapi (fun i name -> name, Field i) |> Hashtbl.of_seq;
  }

type t =
  | Nil
  | True
  | False
  | Int of int64
  | Obj of vtable * t array

let make_obj vtable = Obj (vtable, Array.make vtable.n_slots Nil)

let bool = function
  | true -> True
  | false -> False

let rec pp ppf = function
  | Obj (vtable, slots) -> pp_obj ppf vtable slots
  | Int n -> Fmt.string ppf (Int64.to_string n)
  | v -> Fmt.pf ppf "%S" (to_string v)

and to_string = function
  | Nil -> "nil"
  | True -> "true"
  | False -> "false"
  | v -> Fmt.str "%a" pp v

and pp_obj ppf vtable slots =
  Fmt.pf ppf "{";
  let sep = ref "" in
  Hashtbl.iter
    (fun name -> function
      | Method -> ()
      | Field idx ->
          Fmt.pf ppf "%s%S:%a" !sep name pp slots.(idx);
          sep := ",")
    vtable.elems;
  Fmt.pf ppf "}"