From 0f2b6099073ef9494f09cfd68edfd64508561b62 Mon Sep 17 00:00:00 2001
From: Pierre Letouzey <pierre.letouzey@inria.fr>
Date: Tue, 7 Mar 2017 12:31:29 +0100
Subject: Decimal goodies : conversion to/from Coq strings

 Just because it's fun and easy. Not used by the Numeral Notation command.
---
 theories/Numbers/DecimalString.v | 263 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 263 insertions(+)
 create mode 100644 theories/Numbers/DecimalString.v

(limited to 'theories')

diff --git a/theories/Numbers/DecimalString.v b/theories/Numbers/DecimalString.v
new file mode 100644
index 000000000..7e937f481
--- /dev/null
+++ b/theories/Numbers/DecimalString.v
@@ -0,0 +1,263 @@
+(************************************************************************)
+(*  v      *   The Coq Proof Assistant  /  The Coq Development Team     *)
+(* <O___,, *   INRIA - CNRS - LIX - LRI - PPS - Copyright 1999-2016     *)
+(*   \VV/  **************************************************************)
+(*    //   *      This file is distributed under the terms of the       *)
+(*         *       GNU Lesser General Public License Version 2.1        *)
+(************************************************************************)
+
+Require Import Decimal Ascii String.
+
+(** * Conversion between decimal numbers and Coq strings *)
+
+(** Pretty straightforward, which is precisely the point of the
+    [Decimal.int] datatype. The only catch is [Decimal.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. *)
+
+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)
+   | _ => 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)%char.
+Proof.
+ destruct c as [[|] [|] [|] [|] [|] [|] [|] [|]];
+ intros [= <-]; intuition.
+Qed.
+
+(** Decimal/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)
+ 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_dec 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.
+   + destruct ascii_dec; subst.
+     * 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.
+ destruct ascii_dec; subst; simpl.
+ - 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.
+
+(** Decimal/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_dec 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 Decimal.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 Decimal.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.
+   + destruct ascii_dec; subst.
+     * 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 Decimal.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.
+ destruct ascii_dec; subst; simpl.
+ - 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.
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