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(* -*- coding: utf-8 -*- *)
(************************************************************************)
(* v * The Coq Proof Assistant / The Coq Development Team *)
(* <O___,, * INRIA - CNRS - LIX - LRI - PPS - Copyright 1999-2014 *)
(* \VV/ **************************************************************)
(* // * This file is distributed under the terms of the *)
(* * GNU Lesser General Public License Version 2.1 *)
(************************************************************************)
(** Contributed by Laurent Théry (INRIA);
Adapted to Coq V8 by the Coq Development Team *)
Require Import Bool BinPos BinNat Nnat.
Declare ML Module "ascii_syntax_plugin".
(** * Definition of ascii characters *)
(** Definition of ascii character as a 8 bits constructor *)
Inductive ascii : Set := Ascii (_ _ _ _ _ _ _ _ : bool).
Delimit Scope char_scope with char.
Bind Scope char_scope with ascii.
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}.
decide equality; apply bool_dec.
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.
Proof.
destruct a as [[|][|][|][|][|][|][|][|]]; vm_compute; reflexivity.
Qed.
Theorem N_ascii_embedding :
forall n:N, (n < 256)%N -> N_of_ascii (ascii_of_N n) = n.
Proof.
destruct n.
reflexivity.
do 8 (destruct p; [ | | intros; vm_compute; reflexivity ]);
intro H; vm_compute in H; destruct p; discriminate.
Qed.
Theorem ascii_nat_embedding :
forall a : ascii, ascii_of_nat (nat_of_ascii a) = a.
Proof.
destruct a as [[|][|][|][|][|][|][|][|]]; compute; reflexivity.
Qed.
Theorem nat_ascii_embedding :
forall n : nat, n < 256 -> nat_of_ascii (ascii_of_nat n) = n.
Proof.
intros. unfold nat_of_ascii, ascii_of_nat.
rewrite N_ascii_embedding.
apply Nat2N.id.
unfold N.lt.
change 256%N with (N.of_nat 256).
rewrite <- Nat2N.inj_compare.
rewrite <- Compare_dec.nat_compare_lt. auto.
Qed.
(** * 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".
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