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(************************************************************************)
(* * The Coq Proof Assistant / The Coq Development Team *)
(* v * INRIA, CNRS and contributors - Copyright 1999-2018 *)
(* <O___,, * (see CREDITS file for the list of authors) *)
(* \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) *)
(************************************************************************)
Require Import Sumbool.
Require Import BinInt.
Require Import Zorder.
Require Import Zcompare.
Local Open Scope Z_scope.
(* begin hide *)
(* Trivial, to deprecate? *)
Lemma Dcompare_inf : forall r:comparison, {r = Eq} + {r = Lt} + {r = Gt}.
Proof.
induction r; auto.
Defined.
(* end hide *)
Lemma Zcompare_rect (P:Type) (n m:Z) :
((n ?= m) = Eq -> P) -> ((n ?= m) = Lt -> P) -> ((n ?= m) = Gt -> P) -> P.
Proof.
intros H1 H2 H3.
destruct (n ?= m); auto.
Defined.
Lemma Zcompare_rec (P:Set) (n m:Z) :
((n ?= m) = Eq -> P) -> ((n ?= m) = Lt -> P) -> ((n ?= m) = Gt -> P) -> P.
Proof. apply Zcompare_rect. Defined.
Notation Z_eq_dec := Z.eq_dec (compat "8.6").
Section decidability.
Variables x y : Z.
(** * Decidability of order on binary integers *)
Definition Z_lt_dec : {x < y} + {~ x < y}.
Proof.
unfold Z.lt; case Z.compare; (now left) || (now right).
Defined.
Definition Z_le_dec : {x <= y} + {~ x <= y}.
Proof.
unfold Z.le; case Z.compare; (now left) || (right; tauto).
Defined.
Definition Z_gt_dec : {x > y} + {~ x > y}.
Proof.
unfold Z.gt; case Z.compare; (now left) || (now right).
Defined.
Definition Z_ge_dec : {x >= y} + {~ x >= y}.
Proof.
unfold Z.ge; case Z.compare; (now left) || (right; tauto).
Defined.
Definition Z_lt_ge_dec : {x < y} + {x >= y}.
Proof.
exact Z_lt_dec.
Defined.
Lemma Z_lt_le_dec : {x < y} + {y <= x}.
Proof.
elim Z_lt_ge_dec.
* now left.
* right; now apply Z.ge_le.
Defined.
Definition Z_le_gt_dec : {x <= y} + {x > y}.
Proof.
elim Z_le_dec; auto with arith.
intro. right. Z.swap_greater. now apply Z.nle_gt.
Defined.
Definition Z_gt_le_dec : {x > y} + {x <= y}.
Proof.
exact Z_gt_dec.
Defined.
Definition Z_ge_lt_dec : {x >= y} + {x < y}.
Proof.
elim Z_ge_dec; auto with arith.
intro. right. Z.swap_greater. now apply Z.lt_nge.
Defined.
Definition Z_le_lt_eq_dec : x <= y -> {x < y} + {x = y}.
Proof.
intro H.
apply Zcompare_rec with (n := x) (m := y).
intro. right. elim (Z.compare_eq_iff x y); auto with arith.
intro. left. elim (Z.compare_eq_iff x y); auto with arith.
intro H1. absurd (x > y); auto with arith.
Defined.
End decidability.
(** * Cotransitivity of order on binary integers *)
Lemma Zlt_cotrans : forall n m:Z, n < m -> forall p:Z, {n < p} + {p < m}.
Proof.
intros x y H z.
case (Z_lt_ge_dec x z).
intro.
left.
assumption.
intro.
right.
apply Z.le_lt_trans with (m := x).
apply Z.ge_le.
assumption.
assumption.
Defined.
Lemma Zlt_cotrans_pos : forall n m:Z, 0 < n + m -> {0 < n} + {0 < m}.
Proof.
intros x y H.
case (Zlt_cotrans 0 (x + y) H x).
- now left.
- right.
apply Z.add_lt_mono_l with (p := x).
now rewrite Z.add_0_r.
Defined.
Lemma Zlt_cotrans_neg : forall n m:Z, n + m < 0 -> {n < 0} + {m < 0}.
Proof.
intros x y H; case (Zlt_cotrans (x + y) 0 H x); intro Hxy;
[ right; apply Z.add_lt_mono_l with (p := x); rewrite Z.add_0_r | left ];
assumption.
Defined.
Lemma not_Zeq_inf : forall n m:Z, n <> m -> {n < m} + {m < n}.
Proof.
intros x y H.
case Z_lt_ge_dec with x y.
intro.
left.
assumption.
intro H0.
generalize (Z.ge_le _ _ H0).
intro.
case (Z_le_lt_eq_dec _ _ H1).
intro.
right.
assumption.
intro.
apply False_rec.
apply H.
symmetry .
assumption.
Defined.
Lemma Z_dec : forall n m:Z, {n < m} + {n > m} + {n = m}.
Proof.
intros x y.
case (Z_lt_ge_dec x y).
intro H.
left.
left.
assumption.
intro H.
generalize (Z.ge_le _ _ H).
intro H0.
case (Z_le_lt_eq_dec y x H0).
intro H1.
left.
right.
apply Z.lt_gt.
assumption.
intro.
right.
symmetry .
assumption.
Defined.
Lemma Z_dec' : forall n m:Z, {n < m} + {m < n} + {n = m}.
Proof.
intros x y.
case (Z.eq_dec x y); intro H;
[ right; assumption | left; apply (not_Zeq_inf _ _ H) ].
Defined.
(* begin hide *)
(* To deprecate ? *)
Corollary Z_zerop : forall x:Z, {x = 0} + {x <> 0}.
Proof.
exact (fun x:Z => Z.eq_dec x 0).
Defined.
Corollary Z_notzerop : forall (x:Z), {x <> 0} + {x = 0}.
Proof (fun x => sumbool_not _ _ (Z_zerop x)).
Corollary Z_noteq_dec : forall (x y:Z), {x <> y} + {x = y}.
Proof (fun x y => sumbool_not _ _ (Z.eq_dec x y)).
(* end hide *)
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