(** SpaceOS fixed-size frame codec.

    Every read/write is exactly 256 bytes. The [payload_length] field tells how
    many of the 248 payload bytes are meaningful.

    {v
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Version     |     Type      |         APID (uint16 BE)      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      Payload Length (uint16)  |        Reserved (uint16)      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                   Payload (248 bytes, zero-padded)            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    v} *)

val frame_size : int
(** Fixed frame size: 256 bytes. *)

val header_size : int
(** Header size: 8 bytes. *)

val max_payload : int
(** Maximum payload: 248 bytes. *)

(** {1 Message types} *)

type kind =
  | TM
  | TC
  | EVR
  | PRM_GET
  | PRM_SET
  | PRM_RSP
  | DP
  | HEALTH
  | LOG
  | ERROR

val kind_to_int : kind -> int
(** Convert a message type to its wire representation. *)

val kind_of_int : int -> kind option
(** Parse a message type from its wire representation. *)

val pp_kind : kind Fmt.t
(** Pretty-print a message type. *)

(** {1 Frame} *)

type t = {
  version : int;
  msg_type : int;
  apid : int;
  payload_length : int;
  reserved : int;
  payload : string;
}

val codec : t Wire.Codec.t
(** Wire codec for the full 256-byte frame. *)

val v : kind -> apid:int -> string -> t
(** [v typ ~apid payload] builds a frame. Payload is truncated to 248 bytes. *)

val payload_bytes : t -> string
(** [payload_bytes t] returns the meaningful payload bytes (first
    [payload_length] bytes). *)

val pp : t Fmt.t
(** Pretty-print a frame. *)

val equal : t -> t -> bool
(** Structural equality. *)