Section mf.
Variable radix : Z.
Hypothesis radixMoreThanOne : (1 < radix)%Z.
Let radixMoreThanZERO := Zlt_1_O _ (Zlt_le_weak _ _ radixMoreThanOne).
Hint Resolve radixMoreThanZERO: zarith.
Let FtoRradix := FtoR radix.
Local Coercion FtoRradix : float >-> R.
Fixpoint maxDiv (v : Z) (p : nat) {struct p} : nat :=
match p with
| O => 0
| S p' =>
match ZdividesP v (Zpower_nat radix p) with
| left _ => p
| right _ => maxDiv v p'
end
end.
intros v p; elim p; simpl in |- *; auto.
intros n H'; case (ZdividesP v (radix * Zpower_nat radix n)); auto.
Qed.
intros v p; case p; simpl in |- *; auto.
intros H'; case H'.
apply Zdivides1.
intros n H'; case (ZdividesP v (radix * Zpower_nat radix n)); auto.
intros H'0; case H'; auto.
intros H'0; generalize (maxDivLess v n); auto with arith.
Qed.
intros v p; elim p.
unfold maxDiv in |- *; rewrite Zpower_nat_O; auto.
apply Zdivides1.
simpl in |- *.
intros n H'; case (ZdividesP v (radix * Zpower_nat radix n)); simpl in |- *;
auto with zarith.
Qed.
intros v p q; elim q.
simpl in |- *; case (ZdividesP v (radix * Zpower_nat radix p)); auto.
intros n H' H'0.
apply H'; auto; clear H'.
simpl in H'0; generalize H'0; clear H'0.
case (ZdividesP v (radix * (radix * Zpower_nat radix (n + p)))).
2: simpl in |- *; auto.
intros H' H'0; Contradict H'0; auto with zarith.
Qed.
intros v p q H' H'0.
apply maxDivSimplAux with (q := q - S p); auto.
replace (S (q - S p + p)) with q; auto with zarith.
Qed.
intros v p q H'; elim q.
simpl in |- *; auto.
intros n; simpl in |- *.
case (ZdividesP v (radix * Zpower_nat radix (n + p))); auto.
case (ZdividesP v (radix * (radix * Zpower_nat radix (n + p)))); auto.
intros H'0 H'1 H'2; Contradict H'2; auto with zarith.
case (ZdividesP v (radix * (radix * Zpower_nat radix (n + p)))); auto.
intros H'0 H'1 H'2; case H'1.
case H'0; intros z1 Hz1; exists (radix * z1)%Z;rewrite Hz1.
unfold Zpower_nat; simpl; ring.
Qed.
intros v p q H' H'0.
replace q with (S (q - S p + p)); auto with zarith.
apply maxDivSimplInvAux; auto.
Qed.
intros v p H'; split.
rewrite H'.
apply maxDivCorrect; auto.
red in |- *; intros H'0; generalize H'; clear H'.
simpl in |- *.
case (ZdividesP v (radix * Zpower_nat radix p)); simpl in |- *; auto.
intros H' H'1; Contradict H'1; auto with zarith.
Qed.
intros v H'.
apply maxDivUnique; auto.
apply maxDivSimpl with (q := digit radix v); auto.
apply maxDivLt; auto.
apply NotDividesDigit; auto.
Qed.
intros v p H' H'0; simpl in |- *.
case (ZdividesP v (radix * Zpower_nat radix p)); auto.
intros H'1; case H'0; simpl in |- *; auto.
intros H'1.
generalize H'; case p; simpl in |- *; auto.
intros n H'2; case (ZdividesP v (radix * Zpower_nat radix n)); auto.
intros H'3; case H'3; auto.
Qed.
intros v p H' H'0 H'1.
apply maxDivSimplInv; auto.
2: apply maxDivUniqueInverse; auto.
apply Zpower_nat_anti_monotone_lt with (n := radix); auto.
apply Z.le_lt_trans with (m := Z.abs v); auto.
rewrite <- (fun x => Z.abs_eq (Zpower_nat radix x)); auto with zarith;
apply ZDividesLe; auto.
apply digitMore; auto.
Qed.
intros v n H.
replace (digit radix v + n) with (digit radix (v * Zpower_nat radix n)); auto.
apply sym_equal.
apply maxDivUniqueInverseDigit; auto.
red in |- *; intros Z1; case (Zmult_integral _ _ Z1); intros Z2.
case H; auto.
absurd (0 < Zpower_nat radix n)%Z; auto with zarith.
rewrite Zpower_nat_is_exp.
repeat rewrite (fun x : Z => Zmult_comm x (Zpower_nat radix n)).
apply ZdividesMult; auto.
case (maxDivUniqueDigit v); auto.
replace (S (maxDiv v (digit radix v) + n)) with
(S (maxDiv v (digit radix v)) + n); auto.
rewrite Zpower_nat_is_exp.
repeat rewrite (fun x : Z => Zmult_comm x (Zpower_nat radix n)).
red in |- *; intros H'.
absurd (Zdivides v (Zpower_nat radix (S (maxDiv v (digit radix v))))).
case (maxDivUniqueDigit v); auto.
apply ZdividesDiv with (p := Zpower_nat radix n); auto with zarith.
apply digitAdd; auto with zarith.
Qed.
Definition LSB (x : float) :=
(Z_of_nat (maxDiv (Fnum x) (Fdigit radix x)) + Fexp x)%Z.
intros x n H'; unfold LSB, Fdigit in |- *; simpl in |- *.
rewrite digitAdd; auto with arith.
rewrite maxDivPlus; auto.
rewrite inj_plus; ring.
Qed.
intros x y H' H'0 H'1.
case (FshiftCorrectSym radix) with (2 := H'1); auto.
intros m1 H'2; elim H'2; intros m2 E; clear H'2.
rewrite (LSB_shift x m1); auto.
rewrite E; auto.
apply sym_equal; apply LSB_shift; auto.
apply (NisFzeroComp radix) with (x := x); auto.
Qed.
intros v p; elim p; simpl in |- *; auto.
intros n H; case (ZdividesP v (radix * Zpower_nat radix n));
case (ZdividesP (- v) (radix * Zpower_nat radix n)); auto.
intros Z1 Z2; case Z1.
case Z2; intros z1 Hz1; exists (- z1)%Z; rewrite Hz1; ring.
intros Z1 Z2; case Z2.
case Z1; intros z1 Hz1; exists (- z1)%Z.
rewrite <- (Z.opp_involutive v); rewrite Hz1; ring.
Qed.
intros x; unfold LSB in |- *; simpl in |- *.
rewrite Fdigit_opp; auto.
rewrite maxDiv_opp; auto.
Qed.
intros v p; elim p; simpl in |- *; auto.
intros n H; case (ZdividesP v (radix * Zpower_nat radix n));
case (ZdividesP (Z.abs v) (radix  * Zpower_nat radix n));
auto.
intros Z1 Z2; case Z1.
case Z2; intros z1 Hz1; exists (Z.abs z1); rewrite Hz1.
rewrite Zabs_Zmult; f_equal. apply Z.abs_eq. auto with zarith.
intros Z1 Z2; case Z2.
case Z1; intros z1 Hz1.
case (Zle_or_lt v 0); intros Z4.
exists (- z1)%Z; rewrite <- (Z.opp_involutive v);
rewrite <- (Zabs_eq_opp v); auto; rewrite Hz1; ring.
exists z1; rewrite <- (Z.abs_eq v); auto with zarith; rewrite Hz1; ring.
Qed.
intros x; unfold LSB in |- *; simpl in |- *.
rewrite Fdigit_abs; auto.
rewrite maxDiv_abs; auto.
Qed.
Definition MSB (x : float) := Z.pred (Z_of_nat (Fdigit radix x) + Fexp x).
intros; unfold MSB, Fshift, Fdigit in |- *; simpl in |- *.
rewrite digitAdd; auto with zarith.
rewrite inj_plus; unfold Z.pred in |- *; ring.
Qed.
intros x y H' H'0 H'1.
case (FshiftCorrectSym radix) with (2 := H'1); auto.
intros m1 H'2; elim H'2; intros m2 E; clear H'2.
rewrite (MSB_shift x m1); auto.
rewrite E; auto.
apply sym_equal; apply MSB_shift; auto.
apply (NisFzeroComp radix) with (x := x); auto.
Qed.
intros x; unfold MSB in |- *; simpl in |- *.
rewrite Fdigit_opp; auto.
Qed.
intros x; unfold MSB in |- *; simpl in |- *.
rewrite Fdigit_abs; auto.
Qed.
intros x H'; unfold LSB, MSB in |- *.
apply Zle_Zpred.
cut (maxDiv (Fnum x) (Fdigit radix x) < Fdigit radix x); auto with zarith.
apply maxDivLt; auto.
unfold Fdigit in |- *; apply NotDividesDigit; auto.
Qed.
intros x; unfold LSB in |- *.
auto with zarith.
Qed.
intros x; apply (oneExp_le radix); auto.
apply Fexp_le_LSB; auto.
Qed.
intros x H'; unfold MSB in |- *.
cut (Fdigit radix x <> 0%Z :>Z); unfold Z.pred in |- *;
auto with zarith.
unfold Fdigit in |- *.
red in |- *; intros H'0; absurd (digit radix (Fnum x) = 0); auto with zarith.
apply not_eq_sym; apply lt_O_neq; apply digitNotZero; auto.
Qed.
intros x H'; unfold MSB, FtoRradix, FtoR in |- *; simpl in |- *.
replace (Z.pred (Fdigit radix x + Fexp x)) with
(Z.pred (Fdigit radix x) + Fexp x)%Z; [ idtac | unfold Z.pred in |- *; ring ].
rewrite powerRZ_add; auto with real zarith.
rewrite Rmult_1_l.
repeat rewrite (fun r : R => Rmult_comm r (powerRZ radix (Fexp x))).
apply Rmult_le_compat_l; auto with real zarith.
rewrite <- inj_pred; auto with real zarith.
rewrite <- Zpower_nat_Z_powerRZ; auto.
apply Rle_IZR; auto.
unfold Fdigit in |- *; auto with arith.
apply digitLess; auto.
unfold Fdigit in |- *.
apply not_eq_sym; apply lt_O_neq; apply digitNotZero; auto.
Qed.
intros x.
rewrite (MSB_abs x).
unfold MSB, FtoRradix, FtoR in |- *.
rewrite <- Zsucc_pred; simpl in |- *.
rewrite powerRZ_add; auto with real zarith.
rewrite Rmult_1_l.
repeat rewrite (fun r : R => Rmult_comm r (powerRZ radix (Fexp x))).
apply Rmult_lt_compat_l; auto with real zarith.
rewrite <- Zpower_nat_Z_powerRZ; auto with arith.
apply Rlt_IZR.
unfold Fdigit in |- *; auto with arith.
unfold Fabs in |- *; simpl in |- *.
pattern (Z.abs (Fnum x)) at 1 in |- *; rewrite <- (Z.abs_eq (Z.abs (Fnum x)));
auto with zarith.
Qed.
intros x H'; apply Rle_trans with (FtoRradix (Float 1%nat (MSB x))).
apply (oneExp_le radix); auto.
apply LSB_le_MSB; auto.
apply MSB_le_abs; auto.
Qed.
intros x y H' H'0; unfold MSB in |- *.
rewrite <- H'0.
cut (Fdigit radix x <= Fdigit radix y)%Z;
[ unfold Z.pred in |- *; auto with zarith | idtac ].
unfold Fdigit in |- *; apply inj_le.
apply digit_monotone; auto.
apply le_IZR.
apply Rmult_le_reg_l with (r := powerRZ radix (Fexp x));
auto with real zarith.
repeat rewrite (Rmult_comm (powerRZ radix (Fexp x))); auto.
pattern (Fexp x) at 2 in |- *; rewrite H'0; auto.
Qed.
intros x y H' H'0 H'1; rewrite (MSB_abs x); rewrite (MSB_abs y).
case (Zle_or_lt (Fexp (Fabs x)) (Fexp (Fabs y))); simpl in |- *; intros Zle1.
rewrite
MSB_shift with (x := Fabs y) (n := Z.abs_nat (Fexp (Fabs y) - Fexp (Fabs x))).
apply MSB_monotoneAux; auto.
unfold FtoRradix in |- *; repeat rewrite Fabs_correct; auto with real arith.
rewrite FshiftCorrect; auto with real arith.
repeat rewrite Fabs_correct; auto with real arith.
repeat rewrite Rabs_Rabsolu; repeat rewrite <- Fabs_correct;
auto with real arith.
unfold Fshift in |- *; simpl in |- *.
rewrite inj_abs; [ ring | auto with zarith ].
apply Fabs_Fzero; auto.
rewrite
MSB_shift with (x := Fabs x) (n := Z.abs_nat (Fexp (Fabs x) - Fexp (Fabs y))).
apply MSB_monotoneAux; auto.
unfold FtoRradix in |- *; repeat rewrite Fabs_correct; auto with real arith.
rewrite FshiftCorrect; auto with real arith.
repeat rewrite Fabs_correct; auto with real arith.
repeat rewrite Rabs_Rabsolu; repeat rewrite <- Fabs_correct;
auto with real arith.
unfold Fshift in |- *; simpl in |- *.
rewrite inj_abs; [ ring | auto with zarith ].
apply Fabs_Fzero; auto.
Qed.
intros x y H' H'0; unfold MSB, Fmult, Fdigit in |- *; simpl in |- *.
replace
(Z.pred (digit radix (Fnum x) + Fexp x) +
Z.pred (digit radix (Fnum y) + Fexp y))%Z with
(Z.pred
(digit radix (Fnum x) + Z.pred (digit radix (Fnum y)) + (Fexp x + Fexp y)));
[ idtac | unfold Z.pred in |- *; ring ].
cut
(digit radix (Fnum x) + Z.pred (digit radix (Fnum y)) <=
digit radix (Fnum x * Fnum y))%Z;
[ unfold Z.pred in |- *; auto with zarith | idtac ].
rewrite <- inj_pred; auto with float zarith; try rewrite <- inj_plus.
apply inj_le.
rewrite <- digitAdd; auto with zarith.
apply digit_monotone; auto with zarith.
repeat rewrite Zabs_Zmult.
apply Zle_Zmult_comp_l; auto with zarith.
rewrite (fun x => Z.abs_eq (Zpower_nat radix x)); auto with zarith.
apply not_eq_sym; apply lt_O_neq; apply digitNotZero; auto.
Qed.
intros x y H' H'0.
rewrite <- oneZplus.
apply (oneExp_le radix); auto.
apply MSB_le_multAux; auto.
Qed.
intros x y H' H'0; unfold MSB, Fmult, Fdigit in |- *; simpl in |- *.
replace
(Z.succ
(Z.pred (digit radix (Fnum x) + Fexp x) +
Z.pred (digit radix (Fnum y) + Fexp y))) with
(Z.pred (digit radix (Fnum x) + digit radix (Fnum y) + (Fexp x + Fexp y)));
[ idtac | unfold Z.pred, Z.succ in |- *; ring ].
cut
(digit radix (Fnum x * Fnum y) <=
digit radix (Fnum x) + digit radix (Fnum y))%Z;
[ unfold Z.pred in |- *; auto with zarith | idtac ].
rewrite <- inj_plus.
apply inj_le; auto.
rewrite <- digitAdd; auto with arith.
apply digit_monotone; auto with arith.
repeat rewrite Zabs_Zmult.
apply Zle_Zmult_comp_l; auto with zarith.
rewrite (fun x => Z.abs_eq (Zpower_nat radix x)); auto with zarith.
Qed.

Theorem mult_le_MSB :
forall x y : float,
~ is_Fzero x ->
~ is_Fzero y ->
(Float 1%nat (MSB (Fmult x y)) <=
radix * Fmult (Float 1%nat (MSB x)) (Float 1%nat (MSB y)))%R.