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(************************************************************************)
(*  v      *   The Coq Proof Assistant  /  The Coq Development Team     *)
(* <O___,, *   INRIA - CNRS - LIX - LRI - PPS - Copyright 1999-2018     *)
(*   \VV/  **************************************************************)
(*    //   *      This file is distributed under the terms of the       *)
(*         *       GNU Lesser General Public License Version 2.1        *)
(************************************************************************)

(** * Int63 numbers defines indeed a cyclic structure : Z/(2^31)Z *)

(**
Author: Arnaud Spiwack (+ Pierre Letouzey)
*)
Require Import CyclicAxioms.
Require Export ZArith.
Require Export Int63.
Import Zpow_facts.
Import Utf8.
Import Lia.

Local Open Scope int63_scope.
(** {2 Operators } **)

Definition Pdigits := Eval compute in P_of_succ_nat (size - 1).

Fixpoint positive_to_int_rec (n:nat) (p:positive) :=
  match n, p with
  | O, _ => (Npos p, 0)
  | S n, xH => (0%N, 1)
  | S n, xO p =>
    let (N,i) := positive_to_int_rec n p in
    (N, i << 1)
  | S n, xI p =>
    let (N,i) := positive_to_int_rec n p in
    (N, (i << 1) + 1)
  end.

Definition positive_to_int := positive_to_int_rec size.

Definition mulc_WW x y :=
  let (h, l) := mulc x y in
  if is_zero h then
    if is_zero l then W0
    else WW h l
  else WW h l.
Notation "n '*c' m" := (mulc_WW n m) (at level 40, no associativity) : int63_scope.

Definition pos_mod p x :=
  if p <= digits then
    let p := digits - p in
    (x << p) >> p
  else x.

Notation pos_mod_int := pos_mod.

Import ZnZ.

Instance int_ops : ZnZ.Ops int :=
{|
digits      := Pdigits; (* number of digits *)
zdigits     := Int63.digits; (* number of digits *)
to_Z        := Int63.to_Z; (* conversion to Z *)
of_pos      := positive_to_int; (* positive -> N*int63 :  p => N,i
                                      where p = N*2^31+phi i *)
head0       := Int63.head0;  (* number of head 0 *)
tail0       := Int63.tail0;  (* number of tail 0 *)
zero        := 0;
one         := 1;
minus_one   := Int63.max_int;
compare     := Int63.compare;
eq0         := Int63.is_zero;
opp_c       := Int63.oppc;
opp         := Int63.opp;
opp_carry   := Int63.oppcarry;
succ_c      := Int63.succc;
add_c       := Int63.addc;
add_carry_c := Int63.addcarryc;
succ        := Int63.succ;
add         := Int63.add;
add_carry   := Int63.addcarry;
pred_c      := Int63.predc;
sub_c       := Int63.subc;
sub_carry_c := Int63.subcarryc;
pred        := Int63.pred;
sub         := Int63.sub;
sub_carry   := Int63.subcarry;
mul_c       := mulc_WW;
mul         := Int63.mul;
square_c    := fun x => mulc_WW x x;
div21       := diveucl_21;
div_gt      := diveucl; (* this is supposed to be the special case of
                         division a/b where a > b *)
div         := diveucl;
modulo_gt   := Int63.mod;
modulo      := Int63.mod;
gcd_gt      := Int63.gcd;
gcd         := Int63.gcd;
add_mul_div := Int63.addmuldiv;
pos_mod     := pos_mod_int;
is_even     := Int63.is_even;
sqrt2       := Int63.sqrt2;
sqrt        := Int63.sqrt;
ZnZ.lor     := Int63.lor;
ZnZ.land    := Int63.land;
ZnZ.lxor    := Int63.lxor
|}.

Local Open Scope Z_scope.

Lemma is_zero_spec_aux : forall x : int, is_zero x = true -> [|x|] = 0%Z.
Proof.
intros x;rewrite is_zero_spec;intros H;rewrite H;trivial.
Qed.

Lemma positive_to_int_spec :
  forall p : positive,
    Zpos p =
      Z_of_N (fst (positive_to_int p)) * wB + to_Z (snd (positive_to_int p)).
Proof.
assert (H: (wB <= wB) -> forall p : positive,
  Zpos p = Z_of_N (fst (positive_to_int p)) * wB + [|snd (positive_to_int p)|] /\
  [|snd (positive_to_int p)|] < wB).
  2: intros p; case (H (Z.le_refl wB) p); auto.
unfold positive_to_int, wB at 1 3 4.
elim size.
intros _ p; simpl;
   rewrite to_Z_0, Pmult_1_r; split; auto with zarith; apply refl_equal.
intros n; rewrite inj_S; unfold Z.succ; rewrite Zpower_exp, Z.pow_1_r; auto with zarith.
intros IH Hle p.
assert (F1: 2 ^ Z_of_nat n <= wB); auto with zarith.
  assert (0 <= 2 ^ Z_of_nat n); auto with zarith.
case p; simpl.
intros p1.
generalize (IH F1 p1); case positive_to_int_rec; simpl.
intros n1 i (H1,H2).
rewrite Zpos_xI, H1.
replace [|i << 1  + 1|] with ([|i|] * 2 + 1).
split; auto with zarith; ring.
rewrite add_spec, lsl_spec, Zplus_mod_idemp_l, to_Z_1, Z.pow_1_r, Zmod_small; auto.
case (to_Z_bounded i); split; auto with zarith.
intros p1.
generalize (IH F1 p1); case positive_to_int_rec; simpl.
intros n1 i (H1,H2).
rewrite Zpos_xO, H1.
replace [|i << 1|] with ([|i|] * 2).
split; auto with zarith; ring.
rewrite lsl_spec, to_Z_1, Z.pow_1_r, Zmod_small; auto.
case (to_Z_bounded i); split; auto with zarith.
rewrite to_Z_1; assert (0 < 2^ Z_of_nat n); auto with zarith.
Qed.

Lemma mulc_WW_spec :
   forall x y,[|| x *c y ||] = [|x|] * [|y|].
Proof.
intros x y;unfold mulc_WW.
generalize (mulc_spec x y);destruct (mulc x y);simpl;intros Heq;rewrite Heq.
case_eq (is_zero i);intros;trivial.
apply is_zero_spec in H;rewrite H, to_Z_0.
case_eq (is_zero i0);intros;trivial.
apply is_zero_spec in H0;rewrite H0, to_Z_0, Zmult_comm;trivial.
Qed.

Lemma squarec_spec :
  forall x,
    [||x *c x||] = [|x|] * [|x|].
Proof (fun x => mulc_WW_spec x x).

Lemma diveucl_spec_aux : forall a b, 0 < [|b|] ->
  let (q,r) := diveucl a b in
  [|a|] = [|q|] * [|b|] + [|r|] /\
  0 <= [|r|] < [|b|].
Proof.
intros a b H;assert (W:= diveucl_spec a b).
assert ([|b|]>0) by (auto with zarith).
generalize (Z_div_mod [|a|] [|b|] H0).
destruct (diveucl a b);destruct (Z.div_eucl [|a|] [|b|]).
inversion W;rewrite Zmult_comm;trivial.
Qed.

Lemma shift_unshift_mod_2 : forall n p a, 0 <= p <= n ->
   ((a * 2 ^ (n - p)) mod (2^n) / 2 ^ (n - p)) mod (2^n) =
   a mod 2 ^ p.
Proof.
   intros n p a H.
   rewrite Zmod_small.
   - rewrite Zmod_eq by auto with zarith.
     unfold Zminus at 1.
     rewrite Zdiv.Z_div_plus_full_l by auto with zarith.
     replace (2 ^ n) with (2 ^ (n - p) * 2 ^ p) by (rewrite <- Zpower_exp; [ f_equal | | ]; lia).
     rewrite <- Zdiv_Zdiv, Z_div_mult by auto with zarith.
     rewrite (Zmult_comm (2^(n-p))), Zmult_assoc.
     rewrite Zopp_mult_distr_l.
     rewrite Z_div_mult by auto with zarith.
     symmetry; apply Zmod_eq; auto with zarith.
   - remember (a * 2 ^ (n - p)) as b.
     destruct (Z_mod_lt b (2^n)); auto with zarith.
     split.
     apply Z_div_pos; auto with zarith.
     apply Zdiv_lt_upper_bound; auto with zarith.
     apply Z.lt_le_trans with (2^n); auto with zarith.
     generalize (pow2_pos (n - p)); nia.
Qed.

Lemma div_le_0 : forall p x, 0 <= x -> 0 <= x / 2 ^ p.
Proof.
  intros p x Hle;destruct (Z_le_gt_dec 0 p).
  apply  Zdiv_le_lower_bound;auto with zarith.
  replace (2^p) with 0.
  destruct x;compute;intro;discriminate.
  destruct p;trivial;discriminate.
Qed.

Lemma div_lt : forall p x y, 0 <= x < y -> x / 2^p < y.
Proof.
  intros p x y H;destruct (Z_le_gt_dec 0 p).
  apply Zdiv_lt_upper_bound;auto with zarith.
  apply Z.lt_le_trans with y;auto with zarith.
  rewrite <- (Zmult_1_r y);apply Zmult_le_compat;auto with zarith.
  assert (0 < 2^p);auto with zarith.
  replace (2^p) with 0.
  destruct x;change (0<y);auto with zarith.
  destruct p;trivial;discriminate.
Qed.

Lemma P (A B C: Prop) :
  A → (B → C) → (A → B) → C.
Proof. tauto. Qed.

Lemma shift_unshift_mod_3:
  forall n p a : Z,
  0 <= p <= n ->
  (a * 2 ^ (n - p)) mod 2 ^ n / 2 ^ (n - p) = a mod 2 ^ p.
Proof.
intros;rewrite <- (shift_unshift_mod_2 n p a);[ | auto with zarith].
symmetry;apply Zmod_small.
generalize (a * 2 ^ (n - p));intros w.
generalize (2 ^ (n - p)) (pow2_pos (n - p)); intros x; apply P. lia. intros hx.
generalize (2 ^ n) (pow2_pos n); intros y; apply P. lia. intros hy.
elim_div. intros q r. apply P. lia.
elim_div. intros z t. refine (P _ _ _ _ _). lia.
intros [ ? [ ht | ] ]; [ | lia ]; subst w.
intros [ ? [ hr | ] ]; [ | lia ]; subst t.
nia.
Qed.

Lemma pos_mod_spec w p : φ(pos_mod p w) = φ(w) mod (2 ^ φ(p)).
Proof.
  simpl. unfold pos_mod_int.
  assert (W:=to_Z_bounded p);assert (W':=to_Z_bounded Int63.digits);assert (W'' := to_Z_bounded w).
  case lebP; intros hle.
  2: {
    symmetry; apply Zmod_small.
    assert (2 ^ [|Int63.digits|] < 2 ^ [|p|]); [ apply Zpower_lt_monotone; auto with zarith | ].
    change wB with (2 ^ [|Int63.digits|]) in *; auto with zarith. }
  rewrite <- (shift_unshift_mod_3 [|Int63.digits|] [|p|] [|w|]) by auto with zarith.
  replace ([|Int63.digits|] - [|p|]) with [|Int63.digits - p|] by (rewrite sub_spec, Zmod_small; auto with zarith).
  rewrite lsr_spec, lsl_spec; reflexivity.
Qed.

(** {2 Specification and proof} **)
Global Instance int_specs : ZnZ.Specs int_ops := {
    spec_to_Z   := to_Z_bounded;
    spec_of_pos := positive_to_int_spec;
    spec_zdigits := refl_equal _;
    spec_more_than_1_digit:= refl_equal _;
    spec_0 := to_Z_0;
    spec_1 := to_Z_1;
    spec_m1 := refl_equal _;
    spec_compare := compare_spec;
    spec_eq0 := is_zero_spec_aux;
    spec_opp_c := oppc_spec;
    spec_opp := opp_spec;
    spec_opp_carry := oppcarry_spec;
    spec_succ_c := succc_spec;
    spec_add_c := addc_spec;
    spec_add_carry_c := addcarryc_spec;
    spec_succ := succ_spec;
    spec_add := add_spec;
    spec_add_carry := addcarry_spec;
    spec_pred_c := predc_spec;
    spec_sub_c := subc_spec;
    spec_sub_carry_c := subcarryc_spec;
    spec_pred := pred_spec;
    spec_sub := sub_spec;
    spec_sub_carry := subcarry_spec;
    spec_mul_c := mulc_WW_spec;
    spec_mul := mul_spec;
    spec_square_c := squarec_spec;
    spec_div21 := diveucl_21_spec_aux;
    spec_div_gt := fun a b _ => diveucl_spec_aux a b;
    spec_div := diveucl_spec_aux;
    spec_modulo_gt := fun a b _ _ => mod_spec a b;
    spec_modulo := fun a b _ => mod_spec a b;
    spec_gcd_gt := fun a b _ => gcd_spec a b;
    spec_gcd := gcd_spec;
    spec_head00 := head00_spec;
    spec_head0 := head0_spec;
    spec_tail00 := tail00_spec;
    spec_tail0 := tail0_spec;
    spec_add_mul_div := addmuldiv_spec;
    spec_pos_mod := pos_mod_spec;
    spec_is_even := is_even_spec;
    spec_sqrt2 := sqrt2_spec;
    spec_sqrt := sqrt_spec;
    spec_land := land_spec';
    spec_lor := lor_spec';
    spec_lxor := lxor_spec' }.

Module Int63Cyclic <: CyclicType.
  Definition t := int.
  Definition ops := int_ops.
  Definition specs := int_specs.
End Int63Cyclic.

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