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(*         *   The Coq Proof Assistant / The Coq Development Team       *)
(*  v      *         Copyright INRIA, CNRS and contributors             *)
(* <O___,, * (see version control and CREDITS file for authors & dates) *)
(*   \VV/  **************************************************************)
(*    //   *    This file is distributed under the terms of the         *)
(*         *     GNU Lesser General Public License Version 2.1          *)
(*         *     (see LICENSE file for the text of the license)         *)
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Require Import Hexadecimal Ascii String.

(** * Conversion between hexadecimal numbers and Coq strings *)

(** Pretty straightforward, which is precisely the point of the
    [Hexadecimal.int] datatype. The only catch is [Hexadecimal.Nil] : we could
    choose to convert it as [""] or as ["0"]. In the first case, it is
    awkward to consider "" (or "-") as a number, while in the second case
    we don't have a perfect bijection. Since the second variant is implemented
    thanks to the first one, we provide both.

    Hexadecimal digits are lower case ('a'..'f'). We ignore upper case
    digits ('A'..'F') for the sake of simplicity. *)

Local Open Scope string_scope.

(** Parsing one char *)

Definition uint_of_char (a:ascii)(d:option uint) :=
  match d with
  | None => None
  | Some d =>
    match a with
    | "0" => Some (D0 d)
    | "1" => Some (D1 d)
    | "2" => Some (D2 d)
    | "3" => Some (D3 d)
    | "4" => Some (D4 d)
    | "5" => Some (D5 d)
    | "6" => Some (D6 d)
    | "7" => Some (D7 d)
    | "8" => Some (D8 d)
    | "9" => Some (D9 d)
    | "a" => Some (Da d)
    | "b" => Some (Db d)
    | "c" => Some (Dc d)
    | "d" => Some (Dd d)
    | "e" => Some (De d)
    | "f" => Some (Df d)
    | _ => None
    end
  end%char.

Lemma uint_of_char_spec c d d' :
  uint_of_char c (Some d) = Some d' ->
  (c = "0" /\ d' = D0 d \/
  c = "1" /\ d' = D1 d \/
  c = "2" /\ d' = D2 d \/
  c = "3" /\ d' = D3 d \/
  c = "4" /\ d' = D4 d \/
  c = "5" /\ d' = D5 d \/
  c = "6" /\ d' = D6 d \/
  c = "7" /\ d' = D7 d \/
  c = "8" /\ d' = D8 d \/
  c = "9" /\ d' = D9 d \/
  c = "a" /\ d' = Da d \/
  c = "b" /\ d' = Db d \/
  c = "c" /\ d' = Dc d \/
  c = "d" /\ d' = Dd d \/
  c = "e" /\ d' = De d \/
  c = "f" /\ d' = Df d)%char.
Proof.
  destruct c as [[|] [|] [|] [|] [|] [|] [|] [|]];
  intros [= <-]; intuition.
Qed.

(** Hexadecimal/String conversion where [Nil] is [""] *)

Module NilEmpty.

Fixpoint string_of_uint (d:uint) :=
  match d with
  | Nil => EmptyString
  | D0 d => String "0" (string_of_uint d)
  | D1 d => String "1" (string_of_uint d)
  | D2 d => String "2" (string_of_uint d)
  | D3 d => String "3" (string_of_uint d)
  | D4 d => String "4" (string_of_uint d)
  | D5 d => String "5" (string_of_uint d)
  | D6 d => String "6" (string_of_uint d)
  | D7 d => String "7" (string_of_uint d)
  | D8 d => String "8" (string_of_uint d)
  | D9 d => String "9" (string_of_uint d)
  | Da d => String "a" (string_of_uint d)
  | Db d => String "b" (string_of_uint d)
  | Dc d => String "c" (string_of_uint d)
  | Dd d => String "d" (string_of_uint d)
  | De d => String "e" (string_of_uint d)
  | Df d => String "f" (string_of_uint d)
  end.

Fixpoint uint_of_string s :=
  match s with
  | EmptyString => Some Nil
  | String a s => uint_of_char a (uint_of_string s)
  end.

Definition string_of_int (d:int) :=
  match d with
  | Pos d => string_of_uint d
  | Neg d => String "-" (string_of_uint d)
  end.

Definition int_of_string s :=
  match s with
  | EmptyString => Some (Pos Nil)
  | String a s' =>
    if Ascii.eqb a "-" then option_map Neg (uint_of_string s')
    else option_map Pos (uint_of_string s)
  end.

(* NB: For the moment whitespace between - and digits are not accepted.
   And in this variant [int_of_string "-" = Some (Neg Nil)].

Compute int_of_string "-123456890123456890123456890123456890".
Compute string_of_int (-123456890123456890123456890123456890).
*)

(** Corresponding proofs *)

Lemma usu d :
  uint_of_string (string_of_uint d) = Some d.
Proof.
  induction d; simpl; rewrite ?IHd; simpl; auto.
Qed.

Lemma sus s d :
  uint_of_string s = Some d -> string_of_uint d = s.
Proof.
  revert d.
  induction s; simpl.
  - now intros d [= <-].
  - intros d.
    destruct (uint_of_string s); [intros H | intros [=]].
    apply uint_of_char_spec in H.
    intuition subst; simpl; f_equal; auto.
Qed.

Lemma isi d : int_of_string (string_of_int d) = Some d.
Proof.
  destruct d; simpl.
  - unfold int_of_string.
    destruct (string_of_uint d) eqn:Hd.
    + now destruct d.
    + case Ascii.eqb_spec.
      * intros ->. now destruct d.
      * rewrite <- Hd, usu; auto.
  - rewrite usu; auto.
Qed.

Lemma sis s d :
  int_of_string s = Some d -> string_of_int d = s.
Proof.
  destruct s; [intros [= <-]| ]; simpl; trivial.
  case Ascii.eqb_spec.
  - intros ->. destruct (uint_of_string s) eqn:Hs; simpl; intros [= <-].
    simpl; f_equal. now apply sus.
  - destruct d; [ | now destruct uint_of_char].
    simpl string_of_int.
    intros. apply sus; simpl.
    destruct uint_of_char; simpl in *; congruence.
Qed.

End NilEmpty.

(** Hexadecimal/String conversions where [Nil] is ["0"] *)

Module NilZero.

Definition string_of_uint (d:uint) :=
  match d with
  | Nil => "0"
  | _ => NilEmpty.string_of_uint d
  end.

Definition uint_of_string s :=
  match s with
  | EmptyString => None
  | _ => NilEmpty.uint_of_string s
  end.

Definition string_of_int (d:int) :=
  match d with
  | Pos d => string_of_uint d
  | Neg d => String "-" (string_of_uint d)
  end.

Definition int_of_string s :=
  match s with
  | EmptyString => None
  | String a s' =>
    if Ascii.eqb a "-" then option_map Neg (uint_of_string s')
    else option_map Pos (uint_of_string s)
  end.

(** Corresponding proofs *)

Lemma uint_of_string_nonnil s : uint_of_string s <> Some Nil.
Proof.
  destruct s; simpl.
  - easy.
  - destruct (NilEmpty.uint_of_string s); [intros H | intros [=]].
    apply uint_of_char_spec in H.
    now intuition subst.
Qed.

Lemma sus s d :
  uint_of_string s = Some d -> string_of_uint d = s.
Proof.
  destruct s; [intros [=] | intros H].
  apply NilEmpty.sus in H. now destruct d.
Qed.

Lemma usu d :
  d<>Nil -> uint_of_string (string_of_uint d) = Some d.
Proof.
  destruct d; (now destruct 1) || (intros _; apply NilEmpty.usu).
Qed.

Lemma usu_nil :
  uint_of_string (string_of_uint Nil) = Some Hexadecimal.zero.
Proof.
  reflexivity.
Qed.

Lemma usu_gen d :
  uint_of_string (string_of_uint d) = Some d \/
  uint_of_string (string_of_uint d) = Some Hexadecimal.zero.
Proof.
  destruct d; (now right) || (left; now apply usu).
Qed.

Lemma isi d :
  d<>Pos Nil -> d<>Neg Nil ->
  int_of_string (string_of_int d) = Some d.
Proof.
  destruct d; simpl.
  - intros H _.
    unfold int_of_string.
    destruct (string_of_uint d) eqn:Hd.
    + now destruct d.
    + case Ascii.eqb_spec.
      * intros ->. now destruct d.
      * rewrite <- Hd, usu; auto. now intros ->.
  - intros _ H.
    rewrite usu; auto. now intros ->.
Qed.

Lemma isi_posnil :
  int_of_string (string_of_int (Pos Nil)) = Some (Pos Hexadecimal.zero).
Proof.
  reflexivity.
Qed.

(** Warning! (-0) won't parse (compatibility with the behavior of Z). *)

Lemma isi_negnil :
  int_of_string (string_of_int (Neg Nil)) = Some (Neg (D0 Nil)).
Proof.
  reflexivity.
Qed.

Lemma sis s d :
  int_of_string s = Some d -> string_of_int d = s.
Proof.
  destruct s; [intros [=]| ]; simpl.
  case Ascii.eqb_spec.
  - intros ->. destruct (uint_of_string s) eqn:Hs; simpl; intros [= <-].
    simpl; f_equal. now apply sus.
  - destruct d; [ | now destruct uint_of_char].
    simpl string_of_int.
    intros. apply sus; simpl.
    destruct uint_of_char; simpl in *; congruence.
Qed.

End NilZero.