(* Copyright 2012-2015 by Adam Petcher.				* * Use of this source code is governed by the license described	* * in the LICENSE file at the root of the source tree.		*)
(* This file contains the definition of the base 2 logarithm function (rounded down) over natural numbers. *)
(* Some of the definitions and theory in this file were added to Coq 8.4, so we should refactor this at some point. *)
Set Implicit Arguments.
(* log2 is like log_inf except it returns nat, so I don't have to reason about an unnecessary conversion. This function is only used to implement lognat below.*)
Fixpoint log2(n : positive) : nat :=
match n with
| xH => 0
| xO n' | xI n' => (S (log2 n'))
end.
intros. auto.
Qed.
intros. omega.
Qed.
induction a;
simpl in *; try apply le_s; auto.
Qed.
induction a;
simpl in *; try apply le_s; auto.
Qed.
Definition nat_to_pos (n : nat) : (n <> 0) -> positive :=
fun _ => (P_of_succ_nat (pred n)).
intros. omega.
Qed.
intros. omega.
Qed.
intros.
destruct a.
congruence.
unfold nat_to_pos in *.
simpl in *.
intros. auto.
Qed.
intros. omega.
Qed.
intros. omega.
Qed.
induction a; intros; simpl in *.
congruence.
destruct (eq_nat_dec a 0).
subst.
unfold nat_to_pos.
auto.
assert (a + S (a + 0) <> 0) by omega.
specialize (factor_s_conv H). intros.
unfold nat_to_pos in *.
rewrite (H0 pf2). clear H0.  clear pf2.
assert (a + S (a + 0) = S (a + (a + 0))) by omega.
rewrite H0.
assert (a + (a + 0) <> 0) by omega.
specialize (factor_s_conv H1). intros.
unfold nat_to_pos in H2.
rewrite <- H0 in H2.
rewrite <- H0.
rewrite (H2 H). clear H. clear H2.
rewrite (IHa n). clear IHa.
simpl.
destruct a. intros. destruct n. auto.
auto.
omega.
Qed.
Definition lognat(n : nat) : (n <> 0) -> nat :=
fun pf => (log2 (nat_to_pos pf)).
intros.
unfold lognat in *.
rewrite (factor_double_conv pf1 pf2).
eapply log2_prod_sum.
Qed.
induction a.
intros.
destruct pf1. auto.
intros.
unfold lognat in *.
rewrite (factor_s_conv pf1).
apply log2_monotonic_b.
Qed.
(* for good measure, we will define exponentiation for nat and prove that lognat is correct *)
Fixpoint expnat(a e : nat) : nat :=
match e with
| 0 => 1
| (S e') => a * (expnat a e')
end.
intros.
destruct a. omega.
destruct b. omega.
simpl.
remember (b + a * (S b)) as c.
omega.
Qed.
induction e.
simpl. auto.
simpl.
intros.
eapply mult_ne; eauto.
Qed.
omega.
Qed.
intros.
apply (expnat_nz e nz_2).
Qed.
induction e1.
intros.
simpl.
specialize (expnat_2_nz e2). intros.
omega.
induction e2;
intros.
omega.
simpl.
specialize (IHe1 e2).
omega.
Qed.
induction b.
intros. simpl in *. auto.
intros.
specialize (lognat_prod_sum (expnat_2_nz b)). intros.
specialize (IHb (expnat_2_nz b)).
rewrite IHb in H.
symmetry in H.
simpl in *.
apply H.
Qed.
intros. omega.
Qed.
induction a.
intros.
destruct pf1. auto.
intros.
unfold lognat in *.
rewrite (factor_s_conv pf1).
apply log2_monotonic_b.
Qed.
intuition.
Qed.
induction c.
intros.
assert (a = b) by intuition.
subst. auto.
intros.
assert (a >= (S b)) by omega.
assert (c = a - (S b)) by omega.
eapply ge_trans.
apply (IHc a (S b) pfa (s_nat_nz pfb)).
apply H1.
apply H2.
apply lognat_ge_monotonic_b.
Qed.
intros.
remember (a - b) as c.
apply (@lognat_ge_monotonic_h c); auto.
Qed.
intros.
remember (b - a) as c.
apply (@lognat_ge_monotonic_h c); auto.
Qed.
unfold not in *.
intros.
apply H0.
apply H.
apply H1.
Qed.
intuition.
Qed.
intros.
specialize (lognat_ge_monotonic pfa pfb).
intros.
specialize (mt H0).
intros.
apply ge_not. omega.
Qed.
intros.
simpl.
rewrite fold_add.
assert (expnat 2 (lognat pf) <> 0). apply expnat_nz. auto.
assert (2 * (expnat 2 (lognat pf)) <> 0). omega.
eapply (lognat_ge_monotonic_mt pf H0).
rewrite <- (lognat_prod_sum H).
rewrite lognat_correct_eq.
omega.
Qed.
intros.
unfold lognat in *.
rewrite (factor_s_conv pf1 pf2).
assert (a <> 0). omega.
rewrite (factor_double_conv H pf1).
simpl.
auto.
Qed.
intros. omega.
Qed.
intros.
rewrite <- (lognat_prod_sum pf3).
rewrite <- (lognat_prod_sum pf4).
omega.
Qed.
induction a.
exists 0. simpl.  auto.
elim IHa. clear IHa.
intros.
destruct H.
subst. simpl.
exists x.
right. auto.
subst. simpl.
exists (S x).
left.
simpl.
omega.
Qed.
induction b.
intros. simpl in *. omega.
intros.
elim (nat_comp a). intros.
destruct H0.
subst.
destruct (eq_nat_dec x 0).
subst. omega.
rewrite <- (lognat_prod_sum n pf) in H.
inversion H. clear H.
rewrite H1.
apply IHb in H1.
simpl in *. omega.
subst.
destruct (eq_nat_dec x 0).
subst. unfold lognat in H. simpl in *. omega.
assert ((2 * x) <> 0). omega.
rewrite (lognat_odd x H0 pf) in H.
rewrite <- (lognat_prod_sum n H0) in H.
inversion H.
rewrite H2.
apply IHb in H2.
simpl.
omega.
Qed.

Theorem lognat_correct : forall(a : nat)(pf : a <> 0),
(expnat 2 (lognat pf)) <= a < (expnat 2 (S (lognat pf))).