gistfile1.txt

let scan_pkt buf =
  let open Usane in
  let len = Cstruct.len buf in
  let partial () =
    if len < (1024 * 64) then
      None
    else
      invalid_arg "Buffer is too big"
  in
  let wrap () =
    (* Using pad_len as int32 saves us a lot of conversions. *)
    let pkt_len = get_pkt_hdr_pkt_len buf in
    let pad_len = Int32.of_int (get_pkt_hdr_pad_len buf) in
    if pkt_len = Int32.zero || Uint32.(pkt_len >= max_pkt_len) then
      invalid_arg "Bad pkt_len"
    else if Uint32.(pad_len >= pkt_len) then
      invalid_arg "Bad pad_len";
    let buf = Cstruct.shift buf sizeof_pkt_hdr in
    let len = Cstruct.len buf in
    (* This is a partial packet, hold onto t *)
    if Uint32.(pkt_len > (of_int len)) then
      partial ()
    else
      let payload_len, u1 = Uint32.(sub pkt_len pad_len) in
      let payload_len, u2 = Uint32.pred payload_len in
      if u1 || u2 then
        invalid_arg "Bad payload_len";
      Some (Cstruct.set_len buf (Int32.to_int payload_len))
  in
  if len < 2 then
    ok None
  else
    trap_exn wrap ()