type 'a t = { mutable cur: 'a; stack: 'a Vec.t; copy: ('a -> 'a) option }

let create ?copy x : _ t = { cur = x; stack = Vec.create (); copy }
let[@inline] get self = self.cur
let[@inline] set self x = self.cur <- x
let[@inline] update self f = self.cur <- f self.cur
let[@inline] n_levels self = Vec.size self.stack

let[@inline] push_level self : unit =
  let x = self.cur in
  let x =
    match self.copy with
    | None -> x
    | Some f -> f x
  in
  Vec.push self.stack x

let pop_levels self n : unit =
  assert (n >= 0);
  if n > Vec.size self.stack then invalid_arg "Backtrackable_ref.pop_levels";
  let i = Vec.size self.stack - n in
  let x = Vec.get self.stack i in
  self.cur <- x;
  Vec.shrink self.stack i;
  ()