Fcore_generic_fmt.v 43 KB
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(**
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This file is part of the Flocq formalization of floating-point
arithmetic in Coq: http://flocq.gforge.inria.fr/

Copyright (C) 2010 Sylvie Boldo
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#<br />#
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Copyright (C) 2010 Guillaume Melquiond

This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 3 of the License, or (at your option) any later version.

This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
COPYING file for more details.
*)

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(** * What is a real number belonging to a format, and many properties. *)
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Require Import Fcore_Raux.
Require Import Fcore_defs.
Require Import Fcore_rnd.
Require Import Fcore_float_prop.
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Section Generic.
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Variable beta : radix.

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Notation bpow e := (bpow beta e).
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Section Format.

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Variable fexp : Z -> Z.

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(** To be a good fexp *)
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Class Valid_exp :=
  valid_exp :
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  forall k : Z,
  ( (fexp k < k)%Z -> (fexp (k + 1) <= k)%Z ) /\
  ( (k <= fexp k)%Z ->
    (fexp (fexp k + 1) <= fexp k)%Z /\
    forall l : Z, (l <= fexp k)%Z -> fexp l = fexp k ).

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Context { valid_exp_ : Valid_exp }.
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Definition canonic_exponent x :=
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  fexp (ln_beta beta x).
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Definition canonic (f : float beta) :=
  Fexp f = canonic_exponent (F2R f).
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Definition scaled_mantissa x :=
  (x * bpow (- canonic_exponent x))%R.

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Definition generic_format (x : R) :=
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  x = F2R (Float beta (Ztrunc (scaled_mantissa x)) (canonic_exponent x)).
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(** Basic facts *)
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Theorem generic_format_0 :
  generic_format 0.
Proof.
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unfold generic_format, scaled_mantissa.
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rewrite Rmult_0_l.
change (Ztrunc 0) with (Ztrunc (Z2R 0)).
now rewrite Ztrunc_Z2R, F2R_0.
Qed.

Theorem canonic_exponent_opp :
  forall x,
  canonic_exponent (-x) = canonic_exponent x.
Proof.
intros x.
unfold canonic_exponent.
now rewrite ln_beta_opp.
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Qed.

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Theorem canonic_exponent_abs :
  forall x,
  canonic_exponent (Rabs x) = canonic_exponent x.
Proof.
intros x.
unfold canonic_exponent.
now rewrite ln_beta_abs.
Qed.

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Theorem generic_format_bpow :
  forall e, (fexp (e + 1) <= e)%Z ->
  generic_format (bpow e).
Proof.
intros e H.
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unfold generic_format, scaled_mantissa, canonic_exponent.
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rewrite ln_beta_bpow.
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rewrite <- bpow_plus.
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rewrite <- (Z2R_Zpower beta (e + - fexp (e + 1))).
rewrite Ztrunc_Z2R.
rewrite <- F2R_bpow.
rewrite F2R_change_exp with (1 := H).
now rewrite Zmult_1_l.
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now apply Zle_minus_le_0.
Qed.

Theorem generic_format_bpow' :
  forall e, (fexp e <= e)%Z ->
  generic_format (bpow e).
Proof.
intros e He.
apply generic_format_bpow.
destruct (Zle_lt_or_eq _ _ He).
now apply valid_exp.
rewrite <- H.
apply valid_exp_.
rewrite H.
apply Zle_refl.
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Qed.

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Theorem generic_format_F2R :
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  forall m e,
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  ( m <> 0 -> canonic_exponent (F2R (Float beta m e)) <= e )%Z ->
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  generic_format (F2R (Float beta m e)).
Proof.
intros m e.
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destruct (Z_eq_dec m 0) as [Zm|Zm].
intros _.
rewrite Zm, F2R_0.
apply generic_format_0.
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unfold generic_format, scaled_mantissa.
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set (e' := canonic_exponent (F2R (Float beta m e))).
intros He.
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specialize (He Zm).
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unfold F2R at 3. simpl.
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rewrite  F2R_change_exp with (1 := He).
apply F2R_eq_compat.
rewrite Rmult_assoc, <- bpow_plus, <- Z2R_Zpower, <- Z2R_mult.
now rewrite Ztrunc_Z2R.
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now apply Zle_left.
Qed.

Theorem canonic_opp :
  forall m e,
  canonic (Float beta m e) ->
  canonic (Float beta (-m) e).
Proof.
intros m e H.
unfold canonic.
now rewrite <- opp_F2R, canonic_exponent_opp.
Qed.

Theorem canonic_unicity :
  forall f1 f2,
  canonic f1 ->
  canonic f2 ->
  F2R f1 = F2R f2 ->
  f1 = f2.
Proof.
intros (m1, e1) (m2, e2).
unfold canonic. simpl.
intros H1 H2 H.
rewrite H in H1.
rewrite <- H2 in H1. clear H2.
rewrite H1 in H |- *.
apply (f_equal (fun m => Float beta m e2)).
apply F2R_eq_reg with (1 := H).
Qed.

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Theorem scaled_mantissa_generic :
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  forall x,
  generic_format x ->
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  scaled_mantissa x = Z2R (Ztrunc (scaled_mantissa x)).
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Proof.
intros x Hx.
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unfold scaled_mantissa.
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pattern x at 1 3 ; rewrite Hx.
unfold F2R. simpl.
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rewrite Rmult_assoc, <- bpow_plus, Zplus_opp_r, Rmult_1_r.
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now rewrite Ztrunc_Z2R.
Qed.

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Theorem scaled_mantissa_bpow :
  forall x,
  (scaled_mantissa x * bpow (canonic_exponent x))%R = x.
Proof.
intros x.
unfold scaled_mantissa.
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rewrite Rmult_assoc, <- bpow_plus, Zplus_opp_l.
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apply Rmult_1_r.
Qed.

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Theorem scaled_mantissa_0 :
  scaled_mantissa 0 = R0.
Proof.
apply Rmult_0_l.
Qed.

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Theorem scaled_mantissa_opp :
  forall x,
  scaled_mantissa (-x) = (-scaled_mantissa x)%R.
Proof.
intros x.
unfold scaled_mantissa.
rewrite canonic_exponent_opp.
now rewrite Ropp_mult_distr_l_reverse.
Qed.

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Theorem scaled_mantissa_abs :
  forall x,
  scaled_mantissa (Rabs x) = Rabs (scaled_mantissa x).
Proof.
intros x.
unfold scaled_mantissa.
rewrite canonic_exponent_abs, Rabs_mult.
apply f_equal.
apply sym_eq.
apply Rabs_pos_eq.
apply bpow_ge_0.
Qed.

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Theorem generic_format_opp :
  forall x, generic_format x -> generic_format (-x).
Proof.
intros x Hx.
unfold generic_format.
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rewrite scaled_mantissa_opp, canonic_exponent_opp.
rewrite Ztrunc_opp.
rewrite <- opp_F2R.
now apply f_equal.
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Qed.

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Theorem generic_format_abs :
  forall x, generic_format x -> generic_format (Rabs x).
Proof.
intros x Hx.
unfold generic_format.
rewrite scaled_mantissa_abs, canonic_exponent_abs.
rewrite Ztrunc_abs.
rewrite <- abs_F2R.
now apply f_equal.
Qed.

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Theorem generic_format_abs_inv :
  forall x, generic_format (Rabs x) -> generic_format x.
Proof.
intros x.
unfold generic_format, Rabs.
case Rcase_abs ; intros _.
rewrite scaled_mantissa_opp, canonic_exponent_opp, Ztrunc_opp.
intros H.
rewrite <- (Ropp_involutive x) at 1.
rewrite H, <- opp_F2R.
apply Ropp_involutive.
easy.
Qed.

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Theorem canonic_exponent_fexp :
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  forall x ex,
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  (bpow (ex - 1) <= Rabs x < bpow ex)%R ->
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  canonic_exponent x = fexp ex.
Proof.
intros x ex Hx.
unfold canonic_exponent.
now rewrite ln_beta_unique with (1 := Hx).
Qed.

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Theorem canonic_exponent_fexp_pos :
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  forall x ex,
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  (bpow (ex - 1) <= x < bpow ex)%R ->
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  canonic_exponent x = fexp ex.
Proof.
intros x ex Hx.
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apply canonic_exponent_fexp.
rewrite Rabs_pos_eq.
exact Hx.
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apply Rle_trans with (2 := proj1 Hx).
apply bpow_ge_0.
Qed.

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(** Properties when the real number is "small" (kind of subnormal) *)
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Theorem mantissa_small_pos :
  forall x ex,
  (bpow (ex - 1) <= x < bpow ex)%R ->
  (ex <= fexp ex)%Z ->
  (0 < x * bpow (- fexp ex) < 1)%R.
Proof.
intros x ex Hx He.
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split.
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apply Rmult_lt_0_compat.
apply Rlt_le_trans with (2 := proj1 Hx).
apply bpow_gt_0.
apply bpow_gt_0.
apply Rmult_lt_reg_r with (bpow (fexp ex)).
apply bpow_gt_0.
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rewrite Rmult_assoc, <- bpow_plus, Zplus_opp_l.
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rewrite Rmult_1_r, Rmult_1_l.
apply Rlt_le_trans with (1 := proj2 Hx).
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now apply bpow_le.
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Qed.

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Theorem scaled_mantissa_small :
  forall x ex,
  (Rabs x < bpow ex)%R ->
  (ex <= fexp ex)%Z ->
  (Rabs (scaled_mantissa x) < 1)%R.
Proof.
intros x ex Ex He.
destruct (Req_dec x 0) as [Zx|Zx].
rewrite Zx, scaled_mantissa_0, Rabs_R0.
now apply (Z2R_lt 0 1).
rewrite <- scaled_mantissa_abs.
unfold scaled_mantissa.
rewrite canonic_exponent_abs.
unfold canonic_exponent.
destruct (ln_beta beta x) as (ex', Ex').
simpl.
specialize (Ex' Zx).
apply (mantissa_small_pos _ _ Ex').
assert (ex' <= fexp ex)%Z.
apply Zle_trans with (2 := He).
apply bpow_lt_bpow with beta.
now apply Rle_lt_trans with (2 := Ex).
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now rewrite (proj2 (proj2 (valid_exp _) He)).
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Qed.

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Theorem mantissa_DN_small_pos :
  forall x ex,
  (bpow (ex - 1) <= x < bpow ex)%R ->
  (ex <= fexp ex)%Z ->
  Zfloor (x * bpow (- fexp ex)) = Z0.
Proof.
intros x ex Hx He.
apply Zfloor_imp. simpl.
assert (H := mantissa_small_pos x ex Hx He).
split ; try apply Rlt_le ; apply H.
Qed.

Theorem mantissa_UP_small_pos :
  forall x ex,
  (bpow (ex - 1) <= x < bpow ex)%R ->
  (ex <= fexp ex)%Z ->
  Zceil (x * bpow (- fexp ex)) = 1%Z.
Proof.
intros x ex Hx He.
apply Zceil_imp. simpl.
assert (H := mantissa_small_pos x ex Hx He).
split ; try apply Rlt_le ; apply H.
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Qed.

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(** Generic facts about any format *)
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Theorem generic_format_discrete :
  forall x m,
  let e := canonic_exponent x in
  (F2R (Float beta m e) < x < F2R (Float beta (m + 1) e))%R ->
  ~ generic_format x.
Proof.
intros x m e (Hx,Hx2) Hf.
apply Rlt_not_le with (1 := Hx2). clear Hx2.
rewrite Hf.
fold e.
apply F2R_le_compat.
apply Zlt_le_succ.
apply lt_Z2R.
rewrite <- scaled_mantissa_generic with (1 := Hf).
apply Rmult_lt_reg_r with (bpow e).
apply bpow_gt_0.
now rewrite scaled_mantissa_bpow.
Qed.

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Theorem generic_format_canonic :
  forall f, canonic f ->
  generic_format (F2R f).
Proof.
intros (m, e) Hf.
unfold canonic in Hf. simpl in Hf.
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unfold generic_format, scaled_mantissa.
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rewrite <- Hf.
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apply F2R_eq_compat.
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unfold F2R. simpl.
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rewrite Rmult_assoc, <- bpow_plus, Zplus_opp_r, Rmult_1_r.
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now rewrite Ztrunc_Z2R.
Qed.

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Theorem generic_format_ge_bpow :
  forall emin,
  ( forall e, (emin <= fexp e)%Z ) ->
  forall x,
  (0 < x)%R ->
  generic_format x ->
  (bpow emin <= x)%R.
Proof.
intros emin Emin x Hx Fx.
rewrite Fx.
apply Rle_trans with (bpow (fexp (ln_beta beta x))).
now apply bpow_le.
apply bpow_le_F2R.
apply F2R_gt_0_reg with beta (canonic_exponent x).
now rewrite <- Fx.
Qed.

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Theorem canonic_exp_ge:
  forall prec,
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  (forall e, (e - fexp e <= prec)%Z) ->
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  (* OK with FLX, FLT and FTZ *)
  forall x, generic_format x ->
  (Rabs x < bpow (prec + canonic_exponent x))%R.
intros prec Hp x Hx.
case (Req_dec x 0); intros Hxz.
rewrite Hxz, Rabs_R0.
apply bpow_gt_0.
unfold canonic_exponent.
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destruct (ln_beta beta x) as (ex,Ex) ; simpl.
specialize (Ex Hxz).
apply Rlt_le_trans with (1 := proj2 Ex).
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apply bpow_le.
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specialize (Hp ex).
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omega.
Qed.

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Theorem generic_format_bpow_inv :
  forall e,
    generic_format (bpow e) ->
   (fexp e <= e)%Z.
Proof.
intros e He.
apply Znot_gt_le; intros He2.
assert (e+1 <= fexp (e+1))%Z.
replace (fexp (e+1)) with (fexp e).
omega.
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destruct (valid_exp e) as (Y1,Y2).
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apply sym_eq; apply Y2; omega.
absurd (bpow e=0)%R.
apply sym_not_eq; apply Rlt_not_eq.
apply bpow_gt_0.
rewrite He.
replace (Ztrunc (scaled_mantissa (bpow e))) with 0%Z.
apply F2R_0.
apply sym_eq.
rewrite Ztrunc_floor.
unfold scaled_mantissa, canonic_exponent.
apply mantissa_DN_small_pos; trivial.
rewrite ln_beta_bpow.
split.
apply Req_le.
apply f_equal.
ring.
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apply bpow_lt.
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omega.
now rewrite ln_beta_bpow.
unfold scaled_mantissa.
apply Rmult_le_pos; apply bpow_ge_0.
Qed.

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Section Fcore_generic_round_pos.
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(** * Rounding functions: R -> Z *)
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Variable rnd : R -> Z.

Class Valid_rnd := {
  Zrnd_monotone : forall x y, (x <= y)%R -> (rnd x <= rnd y)%Z ;
  Zrnd_Z2R : forall n, rnd (Z2R n) = n
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}.

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Context { valid_rnd : Valid_rnd }.
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Theorem Zrnd_DN_or_UP :
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  forall x, rnd x = Zfloor x \/ rnd x = Zceil x.
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Proof.
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intros x.
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destruct (Zle_or_lt (rnd x) (Zfloor x)) as [Hx|Hx].
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left.
apply Zle_antisym with (1 := Hx).
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rewrite <- (Zrnd_Z2R (Zfloor x)).
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apply Zrnd_monotone.
apply Zfloor_lb.
right.
apply Zle_antisym.
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rewrite <- (Zrnd_Z2R (Zceil x)).
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apply Zrnd_monotone.
apply Zceil_ub.
rewrite Zceil_floor_neq.
omega.
intros H.
rewrite <- H in Hx.
rewrite Zfloor_Z2R, Zrnd_Z2R in Hx.
apply Zlt_irrefl with (1 := Hx).
Qed.

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(** * the most useful one: R -> F *)
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Definition round x :=
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  F2R (Float beta (rnd (scaled_mantissa x)) (canonic_exponent x)).
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Theorem round_monotone_pos :
  forall x y, (0 < x)%R -> (x <= y)%R -> (round x <= round y)%R.
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Proof.
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intros x y Hx Hxy.
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unfold round, scaled_mantissa, canonic_exponent.
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destruct (ln_beta beta x) as (ex, Hex). simpl.
destruct (ln_beta beta y) as (ey, Hey). simpl.
specialize (Hex (Rgt_not_eq _ _ Hx)).
specialize (Hey (Rgt_not_eq _ _ (Rlt_le_trans _ _ _ Hx Hxy))).
rewrite Rabs_pos_eq in Hex.
2: now apply Rlt_le.
rewrite Rabs_pos_eq in Hey.
2: apply Rle_trans with (2:=Hxy); now apply Rlt_le.
assert (He: (ex <= ey)%Z).
cut (ex - 1 < ey)%Z. omega.
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apply (lt_bpow beta).
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apply Rle_lt_trans with (1 := proj1 Hex).
apply Rle_lt_trans with (1 := Hxy).
apply Hey.
destruct (Zle_or_lt ey (fexp ey)) as [Hy1|Hy1].
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rewrite (proj2 (proj2 (valid_exp ey) Hy1) ex).
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apply F2R_le_compat.
apply Zrnd_monotone.
apply Rmult_le_compat_r.
apply bpow_ge_0.
exact Hxy.
now apply Zle_trans with ey.
destruct (Zle_lt_or_eq _ _ He) as [He'|He'].
destruct (Zle_or_lt ey (fexp ex)) as [Hx2|Hx2].
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rewrite (proj2 (proj2 (valid_exp ex) (Zle_trans _ _ _ He Hx2)) ey Hx2).
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apply F2R_le_compat.
apply Zrnd_monotone.
apply Rmult_le_compat_r.
apply bpow_ge_0.
exact Hxy.
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apply Rle_trans with (F2R (Float beta (rnd (bpow (ey - 1) * bpow (- fexp ey))) (fexp ey))).
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rewrite <- bpow_plus.
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rewrite <- (Z2R_Zpower beta (ey - 1 + -fexp ey)). 2: omega.
rewrite Zrnd_Z2R.
destruct (Zle_or_lt ex (fexp ex)) as [Hx1|Hx1].
apply Rle_trans with (F2R (Float beta 1 (fexp ex))).
apply F2R_le_compat.
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rewrite <- (Zrnd_Z2R 1).
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apply Zrnd_monotone.
apply Rlt_le.
exact (proj2 (mantissa_small_pos _ _ Hex Hx1)).
unfold F2R. simpl.
rewrite Z2R_Zpower. 2: omega.
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rewrite <- bpow_plus, Rmult_1_l.
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apply bpow_le.
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omega.
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apply Rle_trans with (F2R (Float beta (rnd (bpow ex * bpow (- fexp ex))) (fexp ex))).
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apply F2R_le_compat.
apply Zrnd_monotone.
apply Rmult_le_compat_r.
apply bpow_ge_0.
apply Rlt_le.
apply Hex.
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rewrite <- bpow_plus.
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rewrite <- Z2R_Zpower. 2: omega.
rewrite Zrnd_Z2R.
unfold F2R. simpl.
rewrite 2!Z2R_Zpower ; try omega.
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rewrite <- 2!bpow_plus.
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apply bpow_le.
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omega.
apply F2R_le_compat.
apply Zrnd_monotone.
apply Rmult_le_compat_r.
apply bpow_ge_0.
apply Hey.
rewrite He'.
apply F2R_le_compat.
apply Zrnd_monotone.
apply Rmult_le_compat_r.
apply bpow_ge_0.
exact Hxy.
Qed.

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Theorem round_generic :
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  forall x,
  generic_format x ->
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  round x = x.
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Proof.
intros x Hx.
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unfold round.
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rewrite scaled_mantissa_generic with (1 := Hx).
rewrite Zrnd_Z2R.
now apply sym_eq.
Qed.

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Theorem round_0 :
  round 0 = R0.
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Proof.
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unfold round, scaled_mantissa.
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rewrite Rmult_0_l.
fold (Z2R 0).
rewrite Zrnd_Z2R.
apply F2R_0.
Qed.

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Theorem round_bounded_large_pos :
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  forall x ex,
  (fexp ex < ex)%Z ->
  (bpow (ex - 1) <= x < bpow ex)%R ->
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  (bpow (ex - 1) <= round x <= bpow ex)%R.
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Proof.
intros x ex He Hx.
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unfold round, scaled_mantissa.
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rewrite (canonic_exponent_fexp_pos _ _ Hx).
unfold F2R. simpl.
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destruct (Zrnd_DN_or_UP (x * bpow (- fexp ex))) as [Hr|Hr] ; rewrite Hr.
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(* DN *)
split.
replace (ex - 1)%Z with (ex - 1 + - fexp ex + fexp ex)%Z by ring.
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rewrite bpow_plus.
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apply Rmult_le_compat_r.
apply bpow_ge_0.
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assert (Hf: Z2R (Zpower beta (ex - 1 - fexp ex)) = bpow (ex - 1 + - fexp ex)).
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apply Z2R_Zpower.
omega.
rewrite <- Hf.
apply Z2R_le.
apply Zfloor_lub.
rewrite Hf.
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rewrite bpow_plus.
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apply Rmult_le_compat_r.
apply bpow_ge_0.
apply Hx.
apply Rle_trans with (2 := Rlt_le _ _ (proj2 Hx)).
apply Rmult_le_reg_r with (bpow (- fexp ex)).
apply bpow_gt_0.
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rewrite Rmult_assoc, <- bpow_plus, Zplus_opp_r, Rmult_1_r.
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apply Zfloor_lb.
(* UP *)
split.
apply Rle_trans with (1 := proj1 Hx).
apply Rmult_le_reg_r with (bpow (- fexp ex)).
apply bpow_gt_0.
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rewrite Rmult_assoc, <- bpow_plus, Zplus_opp_r, Rmult_1_r.
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apply Zceil_ub.
pattern ex at 3 ; replace ex with (ex - fexp ex + fexp ex)%Z by ring.
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rewrite bpow_plus.
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apply Rmult_le_compat_r.
apply bpow_ge_0.
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assert (Hf: Z2R (Zpower beta (ex - fexp ex)) = bpow (ex - fexp ex)).
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apply Z2R_Zpower.
omega.
rewrite <- Hf.
apply Z2R_le.
apply Zceil_glb.
rewrite Hf.
unfold Zminus.
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rewrite bpow_plus.
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apply Rmult_le_compat_r.
apply bpow_ge_0.
apply Rlt_le.
apply Hx.
Qed.

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Theorem round_bounded_small_pos :
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  forall x ex,
  (ex <= fexp ex)%Z ->
  (bpow (ex - 1) <= x < bpow ex)%R ->
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  round x = R0 \/ round x = bpow (fexp ex).
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Proof.
intros x ex He Hx.
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unfold round, scaled_mantissa.
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rewrite (canonic_exponent_fexp_pos _ _ Hx).
unfold F2R. simpl.
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destruct (Zrnd_DN_or_UP (x * bpow (-fexp ex))) as [Hr|Hr] ; rewrite Hr.
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(* DN *)
left.
apply Rmult_eq_0_compat_r.
apply (@f_equal _ _ Z2R _ Z0).
apply Zfloor_imp.
refine (let H := _ in conj (Rlt_le _ _ (proj1 H)) (proj2 H)).
now apply mantissa_small_pos.
(* UP *)
right.
pattern (bpow (fexp ex)) at 2 ; rewrite <- Rmult_1_l.
apply (f_equal (fun m => (m * bpow (fexp ex))%R)).
apply (@f_equal _ _ Z2R _ 1%Z).
apply Zceil_imp.
refine (let H := _ in conj (proj1 H) (Rlt_le _ _ (proj2 H))).
now apply mantissa_small_pos.
Qed.

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Theorem generic_format_round_pos :
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  forall x,
  (0 < x)%R ->
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  generic_format (round x).
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Proof.
intros x Hx0.
destruct (ln_beta beta x) as (ex, Hex).
specialize (Hex (Rgt_not_eq _ _ Hx0)).
rewrite Rabs_pos_eq in Hex. 2: now apply Rlt_le.
destruct (Zle_or_lt ex (fexp ex)) as [He|He].
(* small *)
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destruct (round_bounded_small_pos _ _ He Hex) as [Hr|Hr] ; rewrite Hr.
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apply generic_format_0.
apply generic_format_bpow.
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now apply valid_exp.
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(* large *)
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generalize (round_bounded_large_pos _ _ He Hex).
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intros (Hr1, Hr2).
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destruct (Rle_or_lt (bpow ex) (round x)) as [Hr|Hr].
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rewrite <- (Rle_antisym _ _ Hr Hr2).
apply generic_format_bpow.
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now apply valid_exp.
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assert (Hr' := conj Hr1 Hr).
unfold generic_format, scaled_mantissa.
rewrite (canonic_exponent_fexp_pos _ _ Hr').
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unfold round, scaled_mantissa.
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rewrite (canonic_exponent_fexp_pos _ _ Hex).
unfold F2R at 3. simpl.
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rewrite Rmult_assoc, <- bpow_plus, Zplus_opp_r, Rmult_1_r.
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now rewrite Ztrunc_Z2R.
Qed.

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End Fcore_generic_round_pos.
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Theorem round_ext :
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  forall rnd1 rnd2,
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  ( forall x, rnd1 x = rnd2 x ) ->
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  forall x,
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  round rnd1 x = round rnd2 x.
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Proof.
intros rnd1 rnd2 Hext x.
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unfold round.
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now rewrite Hext.
Qed.

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Section Zround_opp.
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Variable rnd : R -> Z.
Context { valid_rnd : Valid_rnd rnd }.
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Definition Zrnd_opp x := Zopp (rnd (-x)).
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Global Instance valid_rnd_opp : Valid_rnd Zrnd_opp.
Proof with auto with typeclass_instances.
split.
(* *)
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intros x y Hxy.
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unfold Zrnd_opp.
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apply Zopp_le_cancel.
rewrite 2!Zopp_involutive.
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apply Zrnd_monotone...
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now apply Ropp_le_contravar.
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(* *)
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intros n.
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unfold Zrnd_opp.
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rewrite <- Z2R_opp, Zrnd_Z2R...
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apply Zopp_involutive.
Qed.

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Theorem round_opp :
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  forall x,
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  round rnd (- x) = Ropp (round Zrnd_opp x).
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Proof.
intros x.
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unfold round.
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rewrite opp_F2R, canonic_exponent_opp, scaled_mantissa_opp.
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apply F2R_eq_compat.
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apply sym_eq.
exact (Zopp_involutive _).
Qed.

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End Zround_opp.
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(** IEEE-754 roundings: up, down and to zero *)
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Global Instance valid_rnd_DN : Valid_rnd Zfloor.
Proof.
split.
apply Zfloor_le.
apply Zfloor_Z2R.
Qed.

Global Instance valid_rnd_UP : Valid_rnd Zceil.
Proof.
split.
apply Zceil_le.
apply Zceil_Z2R.
Qed.

Global Instance valid_rnd_ZR : Valid_rnd Ztrunc.
Proof.
split.
apply Ztrunc_le.
apply Ztrunc_Z2R.
Qed.

Section monotone.

Variable rnd : R -> Z.
Context { valid_rnd : Valid_rnd rnd }.
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Theorem round_DN_or_UP :
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  forall x,
  round rnd x = round Zfloor x \/ round rnd x = round Zceil x.
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Proof.
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intros x.
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unfold round.
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destruct (Zrnd_DN_or_UP rnd (scaled_mantissa x)) as [Hx|Hx].
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left. now rewrite Hx.
right. now rewrite Hx.
Qed.

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Theorem round_monotone :
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  forall x y, (x <= y)%R -> (round rnd x <= round rnd y)%R.
Proof with auto with typeclass_instances.
intros x y Hxy.
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destruct (total_order_T x 0) as [[Hx|Hx]|Hx].
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3: now apply round_monotone_pos.
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(* x < 0 *)
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unfold round.
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destruct (Rlt_or_le y 0) as [Hy|Hy].
(* . y < 0 *)
rewrite <- (Ropp_involutive x), <- (Ropp_involutive y).
rewrite (scaled_mantissa_opp (-x)), (scaled_mantissa_opp (-y)).
rewrite (canonic_exponent_opp (-x)), (canonic_exponent_opp (-y)).
apply Ropp_le_cancel.
rewrite 2!opp_F2R.
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apply (round_monotone_pos (Zrnd_opp rnd) (-y) (-x)).
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rewrite <- Ropp_0.
now apply Ropp_lt_contravar.
now apply Ropp_le_contravar.
(* . 0 <= y *)
apply Rle_trans with R0.
apply F2R_le_0_compat. simpl.
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rewrite <- (Zrnd_Z2R rnd 0).
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apply Zrnd_monotone...
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simpl.
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rewrite <- (Rmult_0_l (bpow (- fexp (ln_beta beta x)))).
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apply Rmult_le_compat_r.
apply bpow_ge_0.
now apply Rlt_le.
apply F2R_ge_0_compat. simpl.
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rewrite <- (Zrnd_Z2R rnd 0).
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apply Zrnd_monotone...
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apply Rmult_le_pos.
exact Hy.
apply bpow_ge_0.
(* x = 0 *)
rewrite Hx.
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rewrite round_0...
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apply F2R_ge_0_compat.
simpl.
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rewrite <- (Zrnd_Z2R rnd 0).
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apply Zrnd_monotone...
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apply Rmult_le_pos.
now rewrite <- Hx.
apply bpow_ge_0.
Qed.

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Theorem round_monotone_l :
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  forall x y, generic_format x -> (x <= y)%R -> (x <= round rnd y)%R.
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Proof.
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intros x y Hx Hxy.
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rewrite <- (round_generic rnd x Hx).
now apply round_monotone.
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Qed.
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Theorem round_monotone_r :
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  forall x y, generic_format y -> (x <= y)%R -> (round rnd x <= y)%R.
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Proof.
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intros x y Hy Hxy.
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rewrite <- (round_generic rnd y Hy).
now apply round_monotone.
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Qed.
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End monotone.

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Theorem round_abs_abs :
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  forall P : R -> R -> Prop,
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  ( forall rnd (Hr : Valid_rnd rnd) x, P x (round rnd x) ) ->
  forall rnd {Hr : Valid_rnd rnd} x, P (Rabs x) (Rabs (round rnd x)).
Proof with auto with typeclass_instances.
intros P HP rnd Hr x.
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destruct (Rle_or_lt 0 x) as [Hx|Hx].
(* . *)
rewrite 2!Rabs_pos_eq.
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now apply HP.
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rewrite <- (round_0 rnd).
now apply round_monotone.
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exact Hx.
(* . *)
rewrite (Rabs_left _ Hx).
rewrite Rabs_left1.
pattern x at 2 ; rewrite <- Ropp_involutive.
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rewrite round_opp.
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rewrite Ropp_involutive.
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apply HP...
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rewrite <- (round_0 rnd).
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apply round_monotone...
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now apply Rlt_le.
Qed.

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Section monotone_abs.

Variable rnd : R -> Z.
Context { valid_rnd : Valid_rnd rnd }.

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Theorem round_monotone_abs_l :
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  forall x y, generic_format x -> (x <= Rabs y)%R -> (x <= Rabs (round rnd y))%R.
Proof with auto with typeclass_instances.
intros x y.
apply round_abs_abs...
clear rnd valid_rnd y.
intros rnd' Hrnd y Hy.
apply round_monotone_l...
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Qed.

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Theorem round_monotone_abs_r :
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  forall x y, generic_format y -> (Rabs x <= y)%R -> (Rabs (round rnd x) <= y)%R.
Proof with auto with typeclass_instances.
intros x y.
apply round_abs_abs...
clear rnd valid_rnd x.
intros rnd' Hrnd x Hx.
apply round_monotone_r...
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Qed.

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End monotone_abs.

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Theorem round_DN_opp :
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  forall x,
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  round Zfloor (-x) = (- round Zceil x)%R.
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Proof.
intros x.
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unfold round.
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rewrite scaled_mantissa_opp.
rewrite opp_F2R.
unfold Zceil.
rewrite Zopp_involutive.
now rewrite canonic_exponent_opp.
Qed.

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Theorem round_UP_opp :
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  forall x,
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  round Zceil (-x) = (- round Zfloor x)%R.
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Proof.
intros x.
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unfold round.
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rewrite scaled_mantissa_opp.
rewrite opp_F2R.
unfold Zceil.
rewrite Ropp_involutive.
now rewrite canonic_exponent_opp.
Qed.

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Theorem generic_format_round :
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  forall rnd { Hr : Valid_rnd rnd } x,
  generic_format (round rnd x).
Proof with auto with typeclass_instances.
intros rnd Zrnd x.
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destruct (total_order_T x 0) as [[Hx|Hx]|Hx].
rewrite <- (Ropp_involutive x).
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destruct (round_DN_or_UP rnd (- - x)) as [Hr|Hr] ; rewrite Hr.
rewrite round_DN_opp.
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apply generic_format_opp.
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apply generic_format_round_pos...
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now apply Ropp_0_gt_lt_contravar.
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rewrite round_UP_opp.
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apply generic_format_opp.
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apply generic_format_round_pos...
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now apply Ropp_0_gt_lt_contravar.
rewrite Hx.
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rewrite round_0...
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apply generic_format_0.
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now apply generic_format_round_pos.
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Qed.

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Theorem round_DN_pt :
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  forall x,
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  Rnd_DN_pt generic_format x (round Zfloor x).
Proof with auto with typeclass_instances.
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intros x.
split.
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apply generic_format_round...
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split.
pattern x at 2 ; rewrite <- scaled_mantissa_bpow.
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unfold round, F2R. simpl.
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apply Rmult_le_compat_r.
apply bpow_ge_0.
apply Zfloor_lb.
intros g Hg Hgx.
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apply round_monotone_l...
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Qed.

Theorem generic_format_satisfies_any :
  satisfies_any generic_format.
Proof.
split.
(* symmetric set *)
exact generic_format_0.
exact generic_format_opp.
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(* round down *)
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intros x.
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eexists.
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apply round_DN_pt.
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Qed.

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Theorem round_UP_pt :
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  forall x,
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  Rnd_UP_pt generic_format x (round Zceil x).
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Proof.
intros x.
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rewrite <- (Ropp_involutive x).
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rewrite round_UP_opp.
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apply Rnd_DN_UP_pt_sym.
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apply generic_format_opp.
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apply round_DN_pt.
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