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(************************************************************************)
(* v * The Coq Proof Assistant / The Coq Development Team *)
(* <O___,, * CNRS-Ecole Polytechnique-INRIA Futurs-Universite Paris Sud *)
(* \VV/ **************************************************************)
(* // * This file is distributed under the terms of the *)
(* * GNU Lesser General Public License Version 2.1 *)
(************************************************************************)
(*i $Id: Zcomplements.v 9245 2006-10-17 12:53:34Z notin $ i*)
Require Import ZArithRing.
Require Import ZArith_base.
Require Import Omega.
Require Import Wf_nat.
Open Local Scope Z_scope.
(**********************************************************************)
(** About parity *)
Lemma two_or_two_plus_one :
forall n:Z, {y : Z | n = 2 * y} + {y : Z | n = 2 * y + 1}.
Proof.
intro x; destruct x.
left; split with 0; reflexivity.
destruct p.
right; split with (Zpos p); reflexivity.
left; split with (Zpos p); reflexivity.
right; split with 0; reflexivity.
destruct p.
right; split with (Zneg (1 + p)).
rewrite BinInt.Zneg_xI.
rewrite BinInt.Zneg_plus_distr.
omega.
left; split with (Zneg p); reflexivity.
right; split with (-1); reflexivity.
Qed.
(**********************************************************************)
(** The biggest power of 2 that is stricly less than [a]
Easy to compute: replace all "1" of the binary representation by
"0", except the first "1" (or the first one :-) *)
Fixpoint floor_pos (a:positive) : positive :=
match a with
| xH => 1%positive
| xO a' => xO (floor_pos a')
| xI b' => xO (floor_pos b')
end.
Definition floor (a:positive) := Zpos (floor_pos a).
Lemma floor_gt0 : forall p:positive, floor p > 0.
Proof.
intro.
compute in |- *.
trivial.
Qed.
Lemma floor_ok : forall p:positive, floor p <= Zpos p < 2 * floor p.
Proof.
unfold floor in |- *.
intro a; induction a as [p| p| ].
simpl in |- *.
repeat rewrite BinInt.Zpos_xI.
rewrite (BinInt.Zpos_xO (xO (floor_pos p))).
rewrite (BinInt.Zpos_xO (floor_pos p)).
omega.
simpl in |- *.
repeat rewrite BinInt.Zpos_xI.
rewrite (BinInt.Zpos_xO (xO (floor_pos p))).
rewrite (BinInt.Zpos_xO (floor_pos p)).
rewrite (BinInt.Zpos_xO p).
omega.
simpl in |- *; omega.
Qed.
(**********************************************************************)
(** Two more induction principles over [Z]. *)
Theorem Z_lt_abs_rec :
forall P:Z -> Set,
(forall n:Z, (forall m:Z, Zabs m < Zabs n -> P m) -> P n) ->
forall n:Z, P n.
Proof.
intros P HP p.
set (Q := fun z => 0 <= z -> P z * P (- z)) in *.
cut (Q (Zabs p)); [ intros | apply (Z_lt_rec Q); auto with zarith ].
elim (Zabs_dec p); intro eq; rewrite eq; elim H; auto with zarith.
unfold Q in |- *; clear Q; intros.
apply pair; apply HP.
rewrite Zabs_eq; auto; intros.
elim (H (Zabs m)); intros; auto with zarith.
elim (Zabs_dec m); intro eq; rewrite eq; trivial.
rewrite Zabs_non_eq; auto with zarith.
rewrite Zopp_involutive; intros.
elim (H (Zabs m)); intros; auto with zarith.
elim (Zabs_dec m); intro eq; rewrite eq; trivial.
Qed.
Theorem Z_lt_abs_induction :
forall P:Z -> Prop,
(forall n:Z, (forall m:Z, Zabs m < Zabs n -> P m) -> P n) ->
forall n:Z, P n.
Proof.
intros P HP p.
set (Q := fun z => 0 <= z -> P z /\ P (- z)) in *.
cut (Q (Zabs p)); [ intros | apply (Z_lt_induction Q); auto with zarith ].
elim (Zabs_dec p); intro eq; rewrite eq; elim H; auto with zarith.
unfold Q in |- *; clear Q; intros.
split; apply HP.
rewrite Zabs_eq; auto; intros.
elim (H (Zabs m)); intros; auto with zarith.
elim (Zabs_dec m); intro eq; rewrite eq; trivial.
rewrite Zabs_non_eq; auto with zarith.
rewrite Zopp_involutive; intros.
elim (H (Zabs m)); intros; auto with zarith.
elim (Zabs_dec m); intro eq; rewrite eq; trivial.
Qed.
(** To do case analysis over the sign of [z] *)
Lemma Zcase_sign :
forall (n:Z) (P:Prop), (n = 0 -> P) -> (n > 0 -> P) -> (n < 0 -> P) -> P.
Proof.
intros x P Hzero Hpos Hneg.
induction x as [| p| p].
apply Hzero; trivial.
apply Hpos; apply Zorder.Zgt_pos_0.
apply Hneg; apply Zorder.Zlt_neg_0.
Qed.
Lemma sqr_pos : forall n:Z, n * n >= 0.
Proof.
intro x.
apply (Zcase_sign x (x * x >= 0)).
intros H; rewrite H; omega.
intros H; replace 0 with (0 * 0).
apply Zmult_ge_compat; omega.
omega.
intros H; replace 0 with (0 * 0).
replace (x * x) with (- x * - x).
apply Zmult_ge_compat; omega.
ring.
omega.
Qed.
(**********************************************************************)
(** A list length in Z, tail recursive. *)
Require Import List.
Fixpoint Zlength_aux (acc:Z) (A:Set) (l:list A) {struct l} : Z :=
match l with
| nil => acc
| _ :: l => Zlength_aux (Zsucc acc) A l
end.
Definition Zlength := Zlength_aux 0.
Implicit Arguments Zlength [A].
Section Zlength_properties.
Variable A : Set.
Implicit Type l : list A.
Lemma Zlength_correct : forall l, Zlength l = Z_of_nat (length l).
Proof.
assert (forall l (acc:Z), Zlength_aux acc A l = acc + Z_of_nat (length l)).
simple induction l.
simpl in |- *; auto with zarith.
intros; simpl (length (a :: l0)) in |- *; rewrite Znat.inj_S.
simpl in |- *; rewrite H; auto with zarith.
unfold Zlength in |- *; intros; rewrite H; auto.
Qed.
Lemma Zlength_nil : Zlength (A:=A) nil = 0.
Proof.
auto.
Qed.
Lemma Zlength_cons : forall (x:A) l, Zlength (x :: l) = Zsucc (Zlength l).
Proof.
intros; do 2 rewrite Zlength_correct.
simpl (length (x :: l)) in |- *; rewrite Znat.inj_S; auto.
Qed.
Lemma Zlength_nil_inv : forall l, Zlength l = 0 -> l = nil.
Proof.
intro l; rewrite Zlength_correct.
case l; auto.
intros x l'; simpl (length (x :: l')) in |- *.
rewrite Znat.inj_S.
intros; elimtype False; generalize (Zle_0_nat (length l')); omega.
Qed.
End Zlength_properties.
Implicit Arguments Zlength_correct [A].
Implicit Arguments Zlength_cons [A].
Implicit Arguments Zlength_nil_inv [A].
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