module type DEVICE = sig
  type state

  val state : state
  val can_handle : int -> bool
  val dei : Machine.machine -> int -> int option
  val dei2 : Machine.machine -> int -> int option
  val deo : Machine.machine -> int -> int -> unit
end

module Compose (D1 : DEVICE) (D2 : DEVICE) : DEVICE = struct
  type state = D1.state * D2.state

  let state = (D1.state, D2.state)
  let can_handle port = D1.can_handle port || D2.can_handle port

  let dei mach port =
    match (D1.can_handle port, D2.can_handle port) with
    | true, false -> D1.dei mach port
    | false, true -> D2.dei mach port
    | _ -> None

  let dei2 mach port =
    match (D1.can_handle port, D2.can_handle port) with
    | true, false -> D1.dei2 mach port
    | false, true -> D2.dei2 mach port
    | _ -> None

  let deo mach port value =
    match (D1.can_handle port, D2.can_handle port) with
    | true, false -> D1.deo mach port value
    | false, true -> D2.deo mach port value
    | _ -> ()
end