let shift_char c k =
  let n = Char.code c in
  if n >= 97 && n <= 122 then
    Char.chr (((n - 97 + k) mod 26 + 26) mod 26 + 97)
  else c

let encode s k =
  String.init (String.length s) (fun i -> shift_char s.[i] k)
;;

(* ROT13 round-trip: encode (encode "hello" 13) 13 = "hello".
   Sum the codes of two chars to give a deterministic integer check. *)
let r = encode (encode "hello" 13) 13 in
Char.code r.[0] + Char.code r.[4]