Section pred.
Variable b : Fbound.
Variable radix : Z.
Variable precision : nat.
Let FtoRradix := FtoR radix.
Local Coercion FtoRradix : float >-> R.
Hypothesis radixMoreThanOne : (1 < radix)%Z.
Hypothesis precisionNotZero : precision <> 0.
Hypothesis pGivesBound : Zpos (vNum b) = Zpower_nat radix precision.
Definition FPred (x : float) :=
match Z_eq_bool (Fnum x) (- pPred (vNum b)) with
| true => Float (- nNormMin radix precision) (Z.succ (Fexp x))
| false =>
match Z_eq_bool (Fnum x) (nNormMin radix precision) with
| true =>
match Z_eq_bool (Fexp x) (- dExp b) with
| true => Float (Z.pred (Fnum x)) (Fexp x)
| false => Float (pPred (vNum b)) (Z.pred (Fexp x))
end
| false => Float (Z.pred (Fnum x)) (Fexp x)
end
end.
intros x H'; unfold FPred in |- *.
generalize (Z_eq_bool_correct (Fnum x) (- pPred (vNum b)));
case (Z_eq_bool (Fnum x) (- pPred (vNum b))); auto.
intros H'0; Contradict H'0; auto.
Qed.
intros x H' H'0; unfold FPred in |- *.
generalize (Z_eq_bool_correct (Fnum x) (- pPred (vNum b)));
case (Z_eq_bool (Fnum x) (- pPred (vNum b))); auto.
intros H'1; absurd (0%nat < Fnum x)%Z; auto with zarith arith.
apply Zle_not_lt; rewrite H'1; replace (Z_of_nat 0) with (- (0))%Z;
[ apply Zle_Zopp | simpl in |- *; auto ].
unfold pPred in |- *; apply Zle_Zpred; red in |- *; simpl in |- *; auto.
rewrite H'.
apply nNormPos; auto with zarith.
intros H'1;
generalize (Z_eq_bool_correct (Fnum x) (nNormMin radix precision));
case (Z_eq_bool (Fnum x) (nNormMin radix precision)).
intros H'2; generalize (Z_eq_bool_correct (Fexp x) (- dExp b));
case (Z_eq_bool (Fexp x) (- dExp b)); auto.
intros H'3; Contradict H'0; auto.
intros H'2; Contradict H'2; auto.
Qed.
unfold FPred in |- *; simpl in |- *.
generalize (Z_eq_bool_correct (nNormMin radix precision) (- pPred (vNum b)));
case (Z_eq_bool (nNormMin radix precision) (- pPred (vNum b)));
auto.
intros H'0; absurd (0 < pPred (vNum b))%Z; auto with zarith arith.
rewrite <- (Z.opp_involutive (pPred (vNum b))); rewrite <- H'0.
apply Zle_not_lt; replace 0%Z with (- (0))%Z;
[ apply Zle_Zopp | simpl in |- *; auto ].
apply Zlt_le_weak; apply nNormPos; auto with float zarith.
unfold pPred in |- *; apply Zlt_succ_pred; simpl in |- *;
auto with float zarith.
simpl in |- *; apply vNumbMoreThanOne with (3 := pGivesBound); auto.
intros H';
generalize
(Z_eq_bool_correct (nNormMin radix precision) (nNormMin radix precision));
case (Z_eq_bool (nNormMin radix precision) (nNormMin radix precision)).
intros H'0; generalize (Z_eq_bool_correct (- dExp b) (- dExp b));
case (Z_eq_bool (- dExp b) (- dExp b)); auto.
intros H'1; Contradict H'1; auto.
intros H'1; Contradict H'1; auto.
Qed.
intros x H' H'0; unfold FPred in |- *.
generalize (Z_eq_bool_correct (Fnum x) (- pPred (vNum b)));
case (Z_eq_bool (Fnum x) (- pPred (vNum b))); auto.
intros H'1; Contradict H'; auto.
intros H'1;
generalize (Z_eq_bool_correct (Fnum x) (nNormMin radix precision));
case (Z_eq_bool (Fnum x) (nNormMin radix precision));
auto.
intros H'2; Contradict H'0; auto.
Qed.
intros x.
generalize (Z_eq_bool_correct (Fnum x) (- pPred (vNum b)));
case (Z_eq_bool (Fnum x) (- pPred (vNum b))); intros H'1.
rewrite FPredSimpl1; auto; rewrite FSuccSimpl1; auto.
unfold Fopp in |- *; simpl in |- *; rewrite H'1; auto with zarith.
generalize (Z_eq_bool_correct (Fnum x) (nNormMin radix precision));
case (Z_eq_bool (Fnum x) (nNormMin radix precision));
intros H'2.
generalize (Z_eq_bool_correct (Fexp x) (- dExp b));
case (Z_eq_bool (Fexp x) (- dExp b)); intros H'3.
replace x with (Float (Fnum x) (Fexp x)).
rewrite H'2; rewrite H'3; rewrite FPredSimpl3; unfold Fopp in |- *;
simpl in |- *; rewrite FSuccSimpl3; simpl in |- *;
auto.
rewrite <- Zopp_Zpred_Zs; rewrite Z.opp_involutive; auto.
case x; simpl in |- *; auto.
rewrite FPredSimpl2; auto; rewrite FSuccSimpl2; unfold Fopp in |- *;
simpl in |- *; try rewrite Z.opp_involutive;
auto.
rewrite H'2; auto.
rewrite FPredSimpl4; auto; rewrite FSuccSimpl4; auto.
unfold Fopp in |- *; simpl in |- *; rewrite <- Zopp_Zpred_Zs;
rewrite Z.opp_involutive; auto.
unfold Fopp in |- *; simpl in |- *; Contradict H'1; rewrite <- H'1;
rewrite Z.opp_involutive; auto.
unfold Fopp in |- *; simpl in |- *; Contradict H'2; auto with zarith.
Qed.
intros x H'; rewrite (FPredFopFSucc x).
pattern x at 1 in |- *; rewrite <- (Fopp_Fopp x).
rewrite <- Fopp_Fminus_dist.
rewrite Fopp_Fminus.
unfold FtoRradix in |- *; rewrite FSuccDiff1; auto.
replace (Fnum (Fopp x)) with (- Fnum x)%Z.
Contradict H'; rewrite <- (Z.opp_involutive (Fnum x)); rewrite H';
auto with zarith.
case x; simpl in |- *; auto.
Qed.

Theorem FPredDiff2 :
forall x : float,
Fnum x = nNormMin radix precision ->
Fexp x = (- dExp b)%Z -> Fminus radix x (FPred x) = Float 1%nat (Fexp x) :>R.