# Library Coq.Strings.Ascii

Contributed by Laurent Théry (INRIA); Adapted to Coq V8 by the Coq Development Team
Require Import Bool BinPos BinNat Nnat.

# Definition of ascii characters

Definition of ascii character as a 8 bits constructor
Inductive ascii : Set := Ascii (_ _ _ _ _ _ _ _ : bool).
Delimit Scope char_scope with char.
Definition zero := Ascii false false false false false false false false.
Definition one := Ascii true false false false false false false false.
Definition shift (c : bool) (a : ascii) :=
match a with
| Ascii a1 a2 a3 a4 a5 a6 a7 a8 => Ascii c a1 a2 a3 a4 a5 a6 a7
end.
Definition of a decidable function that is effective
Definition ascii_dec : forall a b : ascii, { a = b } + { a <> b }.
Defined.

# Conversion between natural numbers modulo 256 and ascii characters

Auxillary 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).
The opposite functions
Local Open Scope list_scope.
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).
Proofs that we have indeed opposite function (below 256)
Theorem ascii_N_embedding :
forall a : ascii, ascii_of_N (N_of_ascii a) = a.
Theorem N_ascii_embedding :
forall n:N, (n < 256)%N -> N_of_ascii (ascii_of_N n) = n.
Theorem ascii_nat_embedding :
forall a : ascii, ascii_of_nat (nat_of_ascii a) = a.
Theorem nat_ascii_embedding :
forall n : nat, n < 256 -> nat_of_ascii (ascii_of_nat n) = n.

# Concrete syntax

Ascii characters can be represented in scope char_scope as follows:

- "c" represents itself if c is a character of code < 128,
- """" is an exception: it represents the ascii character 34 (double quote),
- "nnn" represents the ascii character of decimal code nnn.

For instance, both "065" and "A" denote the character `uppercase A', and both "034" and """" denote the character `double quote'.
Notice that the ascii characters of code >= 128 do not denote stand-alone utf8 characters so that only the notation "nnn" is available for them (unless your terminal is able to represent them, which is typically not the case in coqide).
Local Open Scope char_scope.
Example Space := " ".
Example DoubleQuote := """".
Example Beep := "007".