(* registry keys *)
module type KEY = sig
  type elt

  val id : int

  exception E of elt
end

type 'a key = (module KEY with type elt = 'a)
type t = { tbl: exn Util.Int_tbl.t } [@@unboxed]

let create () : t = { tbl = Util.Int_tbl.create 8 }

(* TODO: use atomic *)
let n_ = ref 0

let create_key (type a) () : a key =
  let id = !n_ in
  incr n_;
  let module K = struct
    type elt = a

    exception E of a

    let id = id
  end in
  (module K)

let get (type a) (self : t) (k : a key) : _ option =
  let (module K : KEY with type elt = a) = k in
  match Util.Int_tbl.get self.tbl K.id with
  | Some (K.E x) -> Some x
  | _ -> None

let set (type a) (self : t) (k : a key) (v : a) : unit =
  let (module K) = k in
  Util.Int_tbl.replace self.tbl K.id (K.E v)