Commit 1e62a8e6 authored by Guillaume Melquiond's avatar Guillaume Melquiond
Browse files

Proved trunc_correct_format.

parent 89d7e497
......@@ -547,6 +547,52 @@ Definition truncate t :=
(Zdiv m p, (e + k)%Z, new_location p (Zmod m p) l)
else t.
Theorem truncate_correct_format :
forall m e,
m <> Z0 ->
let x := F2R (Float beta m e) in
generic_format beta fexp x ->
(e <= fexp (digits beta m + e))%Z ->
let '(m', e', l') := truncate (m, e, loc_Exact) in
x = F2R (Float beta m' e') /\ e' = canonic_exponent beta fexp x.
intros m e Hm x Fx He.
assert (Hc: canonic_exponent beta fexp x = fexp (digits beta m + e)).
unfold canonic_exponent, x.
now rewrite ln_beta_F2R_digits.
unfold truncate.
rewrite <- Hc.
set (k := (canonic_exponent beta fexp x - e)%Z).
case Zlt_bool_spec ; intros Hk.
(* *)
rewrite Fx at 1.
refine (let H := _ in conj _ H).
unfold k. ring.
rewrite <- H.
apply F2R_eq_compat.
replace (scaled_mantissa beta fexp x) with (Z2R (Zfloor (scaled_mantissa beta fexp x))).
rewrite Ztrunc_Z2R.
unfold scaled_mantissa.
rewrite <- H.
unfold x, F2R. simpl.
rewrite Rmult_assoc, <- bpow_plus.
ring_simplify (e + - (e + k))%Z.
clear -Hm Hk.
destruct k as [|k|k] ; try easy.
apply Zfloor_div.
intros H.
generalize (Zpower_pos_gt_0 beta k) (Zle_bool_imp_le _ _ (radix_prop beta)).
rewrite scaled_mantissa_generic with (1 := Fx).
now rewrite Zfloor_Z2R.
(* *)
apply refl_equal.
unfold k in Hk.
Theorem truncate_correct_partial :
forall x m e l,
(0 < x)%R ->
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