module type KEY_IMPL = sig
  type t

  exception Store of t

  val id : int
end

module Key = struct
  type 'a t = (module KEY_IMPL with type t = 'a)

  let _n = ref 0

  let create (type k) () =
    incr _n;
    let id = !_n in
    let module K = struct
      type t = k

      let id = id

      exception Store of k
    end in
    (module K : KEY_IMPL with type t = k)

  let[@inline] id (type k) (module K : KEY_IMPL with type t = k) = K.id

  let equal : type a b. a t -> b t -> bool =
   fun (module K1) (module K2) -> K1.id = K2.id
end

type 'a key = 'a Key.t
type binding = B : 'a Key.t * 'a -> binding

open struct
  type exn_pair = E_pair : 'a Key.t * exn -> exn_pair

  let pair_of_e_pair (E_pair (k, e)) =
    let module K = (val k) in
    match e with
    | K.Store v -> B (k, v)
    | _ -> assert false
end

module M = Map.Make (struct
  type t = int

  let compare (i : int) j = Stdlib.compare i j
end)

type t = exn_pair M.t

let empty = M.empty
let[@inline] mem k (self : t) = M.mem (Key.id k) self

let find_exn (type a) (k : a Key.t) (self : t) : a =
  let module K = (val k) in
  let (E_pair (_, e)) = M.find K.id self in
  match e with
  | K.Store v -> v
  | _ -> assert false

let find k (self : t) = try Some (find_exn k self) with Not_found -> None

open struct
  let add_e_pair_ p t =
    let (E_pair ((module K), _)) = p in
    M.add K.id p t

  let add_pair_ p (self : t) : t =
    let (B (((module K) as k), v)) = p in
    let p = E_pair (k, K.Store v) in
    M.add K.id p self
end

let add (type a) (k : a Key.t) v (self : t) : t =
  let module K = (val k) in
  add_e_pair_ (E_pair (k, K.Store v)) self

let remove (type a) (k : a Key.t) (self : t) : t =
  let module K = (val k) in
  M.remove K.id self

let cardinal : t -> int = M.cardinal
let length = cardinal
let iter f (self : t) = M.iter (fun _ p -> f (pair_of_e_pair p)) self

let to_list (self : t) : binding list =
  M.fold (fun _ p l -> pair_of_e_pair p :: l) self []

let add_list (self : t) l = List.fold_right add_pair_ l self