let page_size = 4096

(* Field offsets *)
let off_heartbeat = 0x000
let off_heartbeat_ack = 0x008
let off_time_version = 0x010
let off_time_seconds = 0x014
let off_time_nanos = 0x01C
let off_guest_status = 0x020
let off_host_cmd = 0x024
let off_guest_cmd_ack = 0x028
let off_health_string = 0x100
let health_string_len = 256

(* uint64 big-endian accessors *)
let heartbeat buf = Bytes.get_int64_be buf off_heartbeat
let set_heartbeat buf v = Bytes.set_int64_be buf off_heartbeat v
let heartbeat_ack buf = Bytes.get_int64_be buf off_heartbeat_ack
let set_heartbeat_ack buf v = Bytes.set_int64_be buf off_heartbeat_ack v

(* uint32 big-endian accessors *)
let time_version buf = Int32.to_int (Bytes.get_int32_be buf off_time_version)

let set_time_version buf v =
  Bytes.set_int32_be buf off_time_version (Int32.of_int v)

let time_seconds buf = Bytes.get_int64_be buf off_time_seconds
let set_time_seconds buf v = Bytes.set_int64_be buf off_time_seconds v
let time_nanos buf = Int32.to_int (Bytes.get_int32_be buf off_time_nanos)

let set_time_nanos buf v =
  Bytes.set_int32_be buf off_time_nanos (Int32.of_int v)

let guest_status buf = Int32.to_int (Bytes.get_int32_be buf off_guest_status)

let set_guest_status buf v =
  Bytes.set_int32_be buf off_guest_status (Int32.of_int v)

let host_cmd buf = Int32.to_int (Bytes.get_int32_be buf off_host_cmd)
let set_host_cmd buf v = Bytes.set_int32_be buf off_host_cmd (Int32.of_int v)
let guest_cmd_ack buf = Int32.to_int (Bytes.get_int32_be buf off_guest_cmd_ack)

let set_guest_cmd_ack buf v =
  Bytes.set_int32_be buf off_guest_cmd_ack (Int32.of_int v)

let health_string buf =
  let s = Bytes.sub_string buf off_health_string health_string_len in
  (* Find null terminator *)
  match String.index_opt s '\x00' with
  | Some i -> String.sub s 0 i
  | None -> s

let set_health_string buf s =
  let len = min health_string_len (String.length s) in
  Bytes.blit_string s 0 buf off_health_string len;
  (* Null-terminate *)
  if len < health_string_len then
    Bytes.fill buf (off_health_string + len) (health_string_len - len) '\x00'

(* Mission time with seqlock *)
type mission_time = { seconds : int64; nanos : int }

let write_mission_time buf t =
  let v = time_version buf + 1 in
  set_time_version buf v;
  (* odd = write in progress *)
  set_time_seconds buf t.seconds;
  set_time_nanos buf t.nanos;
  set_time_version buf (v + 1)
(* even = write complete *)

let read_mission_time buf =
  let rec loop () =
    let v1 = time_version buf in
    if v1 land 1 <> 0 then loop () (* spin-wait: host is mid-write *)
    else
      let seconds = time_seconds buf in
      let nanos = time_nanos buf in
      let v2 = time_version buf in
      if v1 <> v2 then loop () (* torn read, retry *) else { seconds; nanos }
  in
  loop ()

(* Command word bits *)
let cmd_shutdown = 1 lsl 0
let cmd_param_reload = 1 lsl 1
let cmd_dp_ack = 1 lsl 2