(** * Conversion between natural numbers modulo 256 and ascii characters *)

(** Auxiliary function that turns a positive into an ascii by
   looking at the last 8 bits, ie z mod 2^8 *)

Definition ascii_of_pos : positive -> ascii :=
 let loop := fix loop n p :=
   match n with
     | O => zero
     | S n' =>
       match p with
         | xH => one
         | xI p' => shift true (loop n' p')
         | xO p' => shift false (loop n' p')
       end
   end
 in loop 8.(** Conversion from [N] to [ascii] *)

Definition ascii_of_N (n : N) :=
  match n with
    | N0 => zero
    | Npos p => ascii_of_pos p
  end.(** Same for [nat] *)

Definition ascii_of_nat (a : nat) := ascii_of_N (N.of_nat a).Fixpoint N_of_digits (l:list bool) : N :=
 match l with
  | nil => 0
  | b :: l' => (if b then 1 else 0) + 2*(N_of_digits l')
 end%N.Definition N_of_ascii (a : ascii) : N :=
 let (a0,a1,a2,a3,a4,a5,a6,a7) := a in
 N_of_digits (a0::a1::a2::a3::a4::a5::a6::a7::nil).Definition nat_of_ascii (a : ascii) : nat := N.to_nat (N_of_ascii a).

Theorem nat_ascii_bounded : forall a : ascii, nat_of_ascii a < 256.