Module Zbits

Additional operations and proofs about binary integers, on top of the ZArith standard library.

Require Import Psatz Zquot.
Require Import Coqlib.

Modulo arithmetic

We define and state properties of equality and arithmetic modulo a positive integer.

Section EQ_MODULO.

Variable modul: Z.
Hypothesis modul_pos: modul > 0.

Definition eqmod (x y: Z) : Prop := exists k, x = k * modul + y.

Lemma eqmod_refl: forall x, eqmod x x.

Proof.

intros; red. exists 0. lia.
Qed.

Lemma eqmod_refl2: forall x y, x = y -> eqmod x y.

Proof.

intros. subst y. apply eqmod_refl.
Qed.

Lemma eqmod_sym: forall x y, eqmod x y -> eqmod y x.

Proof.

intros x y [k EQ]; red. exists (-k). subst x. ring.
Qed.

Lemma eqmod_trans: forall x y z, eqmod x y -> eqmod y z -> eqmod x z.

Proof.

intros x y z [k1 EQ1] [k2 EQ2]; red.
exists (k1 + k2). subst x; subst y. ring.
Qed.

Lemma eqmod_small_eq:
forall x y, eqmod x y -> 0 <= x < modul -> 0 <= y < modul -> x = y.

Proof.

  intros x y [k EQ] I1 I2.
  generalize (Zdiv_unique _ _ _ _ EQ I2). intro.
  rewrite (Z.div_small x modul I1) in H. subst k. lia.
Qed.

Lemma eqmod_mod_eq:
forall x y, eqmod x y -> x mod modul = y mod modul.

Proof.

intros x y [k EQ]. subst x.
rewrite Z.add_comm. apply Z_mod_plus. auto.
Qed.

Lemma eqmod_mod:
forall x, eqmod x (x mod modul).

Proof.

intros; red. exists (x / modul).
rewrite Z.mul_comm. apply Z_div_mod_eq. auto.
Qed.

Lemma eqmod_add:
forall a b c d, eqmod a b -> eqmod c d -> eqmod (a + c) (b + d).

Proof.

intros a b c d [k1 EQ1] [k2 EQ2]; red.
subst a; subst c. exists (k1 + k2). ring.
Qed.

Lemma eqmod_neg:
forall x y, eqmod x y -> eqmod (-x) (-y).

Proof.

intros x y [k EQ]; red. exists (-k). rewrite EQ. ring.
Qed.

Lemma eqmod_sub:
forall a b c d, eqmod a b -> eqmod c d -> eqmod (a - c) (b - d).

Proof.

intros a b c d [k1 EQ1] [k2 EQ2]; red.
subst a; subst c. exists (k1 - k2). ring.
Qed.

Lemma eqmod_mult:
forall a b c d, eqmod a c -> eqmod b d -> eqmod (a * b) (c * d).

Proof.

  intros a b c d [k1 EQ1] [k2 EQ2]; red.
  subst a; subst b.
  exists (k1 * k2 * modul + c * k2 + k1 * d).
  ring.
Qed.

End EQ_MODULO.

Lemma eqmod_divides:
forall n m x y, eqmod n x y -> Z.divide m n -> eqmod m x y.

Proof.

intros. destruct H as [k1 EQ1]. destruct H0 as [k2 EQ2].
exists (k1*k2). rewrite <- Z.mul_assoc. rewrite <- EQ2. auto.
Qed.

Fast normalization modulo 2^n

Fixpoint P_mod_two_p (p: positive) (n: nat) {struct n} : Z :=
  match n with
  | O => 0
  | S m =>
      match p with
      | xH => 1
      | xO q => Z.double (P_mod_two_p q m)
      | xI q => Z.succ_double (P_mod_two_p q m)
      end
  end.

Definition Z_mod_two_p (x: Z) (n: nat) : Z :=
  match x with
  | Z0 => 0
  | Zpos p => P_mod_two_p p n
  | Zneg p => let r := P_mod_two_p p n in if zeq r 0 then 0 else two_power_nat n - r
  end.

Lemma P_mod_two_p_range:
  forall n p, 0 <= P_mod_two_p p n < two_power_nat n.

Proof.

  induction n; simpl; intros.
  - rewrite two_power_nat_O. lia.
  - rewrite two_power_nat_S. destruct p.
    + generalize (IHn p). rewrite Z.succ_double_spec. lia.
    + generalize (IHn p). rewrite Z.double_spec. lia.
    + generalize (two_power_nat_pos n). lia.
Qed.

Lemma P_mod_two_p_eq:
forall n p, P_mod_two_p p n = (Zpos p) mod (two_power_nat n).

Proof.

  assert (forall n p, exists y, Zpos p = y * two_power_nat n + P_mod_two_p p n).
  {
    induction n; simpl; intros.
    - rewrite two_power_nat_O. exists (Zpos p). ring.
    - rewrite two_power_nat_S. destruct p.
      + destruct (IHn p) as [y EQ]. exists y.
        change (Zpos p~1) with (2 * Zpos p + 1). rewrite EQ.
        rewrite Z.succ_double_spec. ring.
      + destruct (IHn p) as [y EQ]. exists y.
        change (Zpos p~0) with (2 * Zpos p). rewrite EQ.
        rewrite (Z.double_spec (P_mod_two_p p n)). ring.
      + exists 0; lia.
  }
  intros.
  destruct (H n p) as [y EQ].
  symmetry. apply Zmod_unique with y. auto. apply P_mod_two_p_range.
Qed.

Lemma Z_mod_two_p_range:
forall n x, 0 <= Z_mod_two_p x n < two_power_nat n.

Proof.

  intros; unfold Z_mod_two_p. generalize (two_power_nat_pos n); intros.
  destruct x.
  - lia.
  - apply P_mod_two_p_range.
  - set (r := P_mod_two_p p n).
    assert (0 <= r < two_power_nat n) by apply P_mod_two_p_range.
    destruct (zeq r 0).
    + intuition auto with crelations zarith bool.
    + Psatz.lia.
Qed.

Lemma Z_mod_two_p_eq:
forall n x, Z_mod_two_p x n = x mod (two_power_nat n).

Proof.

  intros. unfold Z_mod_two_p. generalize (two_power_nat_pos n); intros.
  destruct x.
  - rewrite Zmod_0_l. auto.
  - apply P_mod_two_p_eq.
  - generalize (P_mod_two_p_range n p) (P_mod_two_p_eq n p). intros A B.
    exploit (Z_div_mod_eq (Zpos p) (two_power_nat n)); auto. intros C.
    set (q := Zpos p / two_power_nat n) in *.
    set (r := P_mod_two_p p n) in *.
    rewrite <- B in C.
    change (Z.neg p) with (- (Z.pos p)). destruct (zeq r 0).
    + symmetry. apply Zmod_unique with (-q). rewrite C; rewrite e. lia. lia.
    + symmetry. apply Zmod_unique with (-q - 1). rewrite C. lia. lia.
Qed.

Bit-level operations and properties

Shift x left by one and insert b as the low bit of the result.

Definition Zshiftin (b: bool) (x: Z) : Z :=
if b then Z.succ_double x else Z.double x.

Remark Ztestbit_0: forall n, Z.testbit 0 n = false.
Proof Z.testbit_0_l.

Remark Ztestbit_1: forall n, Z.testbit 1 n = zeq n 0.

Proof.

intros. destruct n; simpl; auto.
Qed.

Remark Ztestbit_m1: forall n, 0 <= n -> Z.testbit (-1) n = true.

Proof.

intros. destruct n; simpl; auto.
Qed.

Remark Zshiftin_spec:
forall b x, Zshiftin b x = 2 * x + (if b then 1 else 0).

Proof.

  unfold Zshiftin; intros. destruct b.
  - rewrite Z.succ_double_spec. lia.
  - rewrite Z.double_spec. lia.
Qed.

Remark Zshiftin_inj:
forall b1 x1 b2 x2,
Zshiftin b1 x1 = Zshiftin b2 x2 -> b1 = b2 /\ x1 = x2.

Proof.

  intros. rewrite !Zshiftin_spec in H.
  destruct b1; destruct b2.
  split; [auto|lia].
  extlia.
  extlia.
  split; [auto|lia].
Qed.

Remark Zdecomp:
forall x, x = Zshiftin (Z.odd x) (Z.div2 x).

Proof.

  intros. destruct x; simpl.
  - auto.
  - destruct p; auto.
  - destruct p; auto. simpl. rewrite Pos.pred_double_succ. auto.
Qed.

Remark Ztestbit_shiftin:
  forall b x n,
  0 <= n ->
  Z.testbit (Zshiftin b x) n = if zeq n 0 then b else Z.testbit x (Z.pred n).

Proof.

  intros. rewrite Zshiftin_spec. destruct (zeq n 0).
  - subst n. destruct b.
    + apply Z.testbit_odd_0.
    + rewrite Z.add_0_r. apply Z.testbit_even_0.
  - assert (0 <= Z.pred n) by lia.
    set (n' := Z.pred n) in *.
    replace n with (Z.succ n') by (unfold n'; lia).
    destruct b.
    + apply Z.testbit_odd_succ; auto.
    + rewrite Z.add_0_r. apply Z.testbit_even_succ; auto.
Qed.

Remark Ztestbit_shiftin_base:
forall b x, Z.testbit (Zshiftin b x) 0 = b.

Proof.

intros. rewrite Ztestbit_shiftin; reflexivity.
Qed.

Remark Ztestbit_shiftin_succ:
forall b x n, 0 <= n -> Z.testbit (Zshiftin b x) (Z.succ n) = Z.testbit x n.

Proof.

intros. rewrite Ztestbit_shiftin. rewrite zeq_false. rewrite Z.pred_succ. auto.
lia. lia.
Qed.

Lemma Zshiftin_ind:
  forall (P: Z -> Prop),
  P 0 ->
  (forall b x, 0 <= x -> P x -> P (Zshiftin b x)) ->
  forall x, 0 <= x -> P x.

Proof.

  intros. destruct x.
  - auto.
  - induction p.
    + change (P (Zshiftin true (Z.pos p))). auto.
    + change (P (Zshiftin false (Z.pos p))). auto.
    + change (P (Zshiftin true 0)). apply H0. lia. auto.
  - compute in H1. intuition congruence.
Qed.

Lemma Zshiftin_pos_ind:
  forall (P: Z -> Prop),
  P 1 ->
  (forall b x, 0 < x -> P x -> P (Zshiftin b x)) ->
  forall x, 0 < x -> P x.

Proof.

  intros. destruct x; simpl in H1; try discriminate.
  induction p.
    + change (P (Zshiftin true (Z.pos p))). auto.
    + change (P (Zshiftin false (Z.pos p))). auto.
    + auto.
Qed.

Bit-wise decomposition (Z.testbit)

Remark Ztestbit_eq:
forall n x, 0 <= n ->
Z.testbit x n = if zeq n 0 then Z.odd x else Z.testbit (Z.div2 x) (Z.pred n).

Proof.

intros. rewrite (Zdecomp x) at 1. apply Ztestbit_shiftin; auto.
Qed.

Remark Ztestbit_base:
forall x, Z.testbit x 0 = Z.odd x.

Proof.

intros. rewrite Ztestbit_eq; reflexivity.
Qed.

Remark Ztestbit_succ:
forall n x, 0 <= n -> Z.testbit x (Z.succ n) = Z.testbit (Z.div2 x) n.

Proof.

intros. rewrite Ztestbit_eq. rewrite zeq_false. rewrite Z.pred_succ. auto.
lia. lia.
Qed.

Lemma eqmod_same_bits:
  forall n x y,
  (forall i, 0 <= i < Z.of_nat n -> Z.testbit x i = Z.testbit y i) ->
  eqmod (two_power_nat n) x y.

Proof.

  induction n; intros.
  - change (two_power_nat 0) with 1. exists (x-y); ring.
  - rewrite two_power_nat_S.
    assert (eqmod (two_power_nat n) (Z.div2 x) (Z.div2 y)).
      apply IHn. intros. rewrite <- !Ztestbit_succ. apply H. rewrite Nat2Z.inj_succ; lia.
      lia. lia.
  destruct H0 as [k EQ].
  exists k. rewrite (Zdecomp x). rewrite (Zdecomp y).
  replace (Z.odd y) with (Z.odd x).
  rewrite EQ. rewrite !Zshiftin_spec. ring.
  exploit (H 0). rewrite Nat2Z.inj_succ; lia.
  rewrite !Ztestbit_base. auto.
Qed.

Lemma same_bits_eqmod:
  forall n x y i,
  eqmod (two_power_nat n) x y -> 0 <= i < Z.of_nat n ->
  Z.testbit x i = Z.testbit y i.

Proof.

  induction n; intros.
  - simpl in H0. extlia.
  - rewrite Nat2Z.inj_succ in H0. rewrite two_power_nat_S in H.
    rewrite !(Ztestbit_eq i); intuition.
    destruct H as [k EQ].
    assert (EQ': Zshiftin (Z.odd x) (Z.div2 x) =
                 Zshiftin (Z.odd y) (k * two_power_nat n + Z.div2 y)).
    {
      rewrite (Zdecomp x) in EQ. rewrite (Zdecomp y) in EQ.
      rewrite EQ. rewrite !Zshiftin_spec. ring.
    }
    exploit Zshiftin_inj; eauto. intros [A B].
    destruct (zeq i 0).
    + auto.
    + apply IHn. exists k; auto. lia.
Qed.

Lemma equal_same_bits:
  forall x y,
  (forall i, 0 <= i -> Z.testbit x i = Z.testbit y i) ->
  x = y.
Proof Z.bits_inj'.

Lemma Z_one_complement:
  forall i, 0 <= i ->
  forall x, Z.testbit (-x-1) i = negb (Z.testbit x i).

Proof.

  intros i0 POS0. pattern i0. apply Zlt_0_ind; auto.
  intros i IND POS x.
  rewrite (Zdecomp x). set (y := Z.div2 x).
  replace (- Zshiftin (Z.odd x) y - 1)
     with (Zshiftin (negb (Z.odd x)) (- y - 1)).
  rewrite !Ztestbit_shiftin; auto.
  destruct (zeq i 0). auto. apply IND. lia.
  rewrite !Zshiftin_spec. destruct (Z.odd x); simpl negb; ring.
Qed.

Lemma Ztestbit_above:
  forall n x i,
  0 <= x < two_power_nat n ->
  i >= Z.of_nat n ->
  Z.testbit x i = false.

Proof.

  induction n; intros.
  - change (two_power_nat 0) with 1 in H.
    replace x with 0 by lia.
    apply Z.testbit_0_l.
  - rewrite Nat2Z.inj_succ in H0. rewrite Ztestbit_eq. rewrite zeq_false.
    apply IHn. rewrite two_power_nat_S in H. rewrite (Zdecomp x) in H.
    rewrite Zshiftin_spec in H. destruct (Z.odd x); lia.
    lia. lia. lia.
Qed.

Lemma Ztestbit_above_neg:
  forall n x i,
  -two_power_nat n <= x < 0 ->
  i >= Z.of_nat n ->
  Z.testbit x i = true.

Proof.

  intros. set (y := -x-1).
  assert (Z.testbit y i = false).
    apply Ztestbit_above with n.
    unfold y; lia. auto.
  unfold y in H1. rewrite Z_one_complement in H1.
  change true with (negb false). rewrite <- H1. rewrite negb_involutive; auto.
  lia.
Qed.

Lemma Zsign_bit:
  forall n x,
  0 <= x < two_power_nat (S n) ->
  Z.testbit x (Z.of_nat n) = if zlt x (two_power_nat n) then false else true.

Proof.

  induction n; intros.
  - change (two_power_nat 1) with 2 in H.
    assert (x = 0 \/ x = 1) by lia.
    destruct H0; subst x; reflexivity.
  - rewrite Nat2Z.inj_succ. rewrite Ztestbit_eq. rewrite zeq_false. rewrite Z.pred_succ.
    rewrite IHn. rewrite two_power_nat_S.
    destruct (zlt (Z.div2 x) (two_power_nat n)); rewrite (Zdecomp x); rewrite Zshiftin_spec.
    rewrite zlt_true. auto. destruct (Z.odd x); lia.
    rewrite zlt_false. auto. destruct (Z.odd x); lia.
    rewrite (Zdecomp x) in H; rewrite Zshiftin_spec in H.
    rewrite two_power_nat_S in H. destruct (Z.odd x); lia.
    lia. lia.
Qed.

Lemma Ztestbit_le:
  forall x y,
  0 <= y ->
  (forall i, 0 <= i -> Z.testbit x i = true -> Z.testbit y i = true) ->
  x <= y.

Proof.

  intros x y0 POS0; revert x; pattern y0; apply Zshiftin_ind; auto; intros.
  - replace x with 0. lia. apply equal_same_bits; intros.
    rewrite Ztestbit_0. destruct (Z.testbit x i) as [] eqn:E; auto.
    exploit H; eauto. rewrite Ztestbit_0. auto.
  - assert (Z.div2 x0 <= x).
    { apply H0. intros. exploit (H1 (Z.succ i)).
        lia. rewrite Ztestbit_succ; auto. rewrite Ztestbit_shiftin_succ; auto.
    }
    rewrite (Zdecomp x0). rewrite !Zshiftin_spec.
    destruct (Z.odd x0) as [] eqn:E1; destruct b as [] eqn:E2; try lia.
    exploit (H1 0). lia. rewrite Ztestbit_base; auto.
    rewrite Ztestbit_shiftin_base. congruence.
Qed.

Lemma Ztestbit_mod_two_p:
  forall n x i,
  0 <= n -> 0 <= i ->
  Z.testbit (x mod (two_p n)) i = if zlt i n then Z.testbit x i else false.

Proof.

  intros n0 x i N0POS. revert x i; pattern n0; apply natlike_ind; auto.
  - intros. change (two_p 0) with 1. rewrite Zmod_1_r. rewrite Z.testbit_0_l.
    rewrite zlt_false; auto. lia.
  - intros. rewrite two_p_S; auto.
    replace (x0 mod (2 * two_p x))
       with (Zshiftin (Z.odd x0) (Z.div2 x0 mod two_p x)).
    rewrite Ztestbit_shiftin; auto. rewrite (Ztestbit_eq i x0); auto. destruct (zeq i 0).
    + rewrite zlt_true; auto. lia.
    + rewrite H0. destruct (zlt (Z.pred i) x).
      * rewrite zlt_true; auto. lia.
      * rewrite zlt_false; auto. lia.
      * lia.
    + rewrite (Zdecomp x0) at 3. set (x1 := Z.div2 x0). symmetry.
      apply Zmod_unique with (x1 / two_p x).
      rewrite !Zshiftin_spec. rewrite Z.add_assoc. f_equal.
      transitivity (2 * (two_p x * (x1 / two_p x) + x1 mod two_p x)).
      f_equal. apply Z_div_mod_eq. apply two_p_gt_ZERO; auto.
      ring.
      rewrite Zshiftin_spec. exploit (Z_mod_lt x1 (two_p x)). apply two_p_gt_ZERO; auto.
      destruct (Z.odd x0); lia.
Qed.

Corollary Ztestbit_two_p_m1:
forall n i, 0 <= n -> 0 <= i ->
Z.testbit (two_p n - 1) i = if zlt i n then true else false.

Proof.

  intros. replace (two_p n - 1) with ((-1) mod (two_p n)).
  rewrite Ztestbit_mod_two_p; auto. destruct (zlt i n); auto. apply Ztestbit_m1; auto.
  apply Zmod_unique with (-1). ring.
  exploit (two_p_gt_ZERO n). auto. lia.
Qed.

Corollary Ztestbit_neg_two_p:
forall n i, 0 <= n -> 0 <= i ->
Z.testbit (- (two_p n)) i = if zlt i n then false else true.

Proof.

  intros.
  replace (- two_p n) with (- (two_p n - 1) - 1) by lia.
  rewrite Z_one_complement by auto.
  rewrite Ztestbit_two_p_m1 by auto.
  destruct (zlt i n); auto.
Qed.

Lemma Z_add_is_or:
  forall i, 0 <= i ->
  forall x y,
  (forall j, 0 <= j <= i -> Z.testbit x j && Z.testbit y j = false) ->
  Z.testbit (x + y) i = Z.testbit x i || Z.testbit y i.

Proof.

  intros i0 POS0. pattern i0. apply Zlt_0_ind; auto.
  intros i IND POS x y EXCL.
  rewrite (Zdecomp x) in *. rewrite (Zdecomp y) in *.
  transitivity (Z.testbit (Zshiftin (Z.odd x || Z.odd y) (Z.div2 x + Z.div2 y)) i).
  - f_equal. rewrite !Zshiftin_spec.
    exploit (EXCL 0). lia. rewrite !Ztestbit_shiftin_base. intros.
Opaque Z.mul.
    destruct (Z.odd x); destruct (Z.odd y); simpl in *; discriminate || ring.
  - rewrite !Ztestbit_shiftin; auto.
    destruct (zeq i 0).
    + auto.
    + apply IND. lia. intros.
      exploit (EXCL (Z.succ j)). lia.
      rewrite !Ztestbit_shiftin_succ. auto.
      lia. lia.
Qed.

Zero and sign extensions

In pseudo-code:

    Fixpoint Zzero_ext (n: Z) (x: Z) : Z :=
      if zle n 0 then
        0
      else
        Zshiftin (Z.odd x) (Zzero_ext (Z.pred n) (Z.div2 x)).
    Fixpoint Zsign_ext (n: Z) (x: Z) : Z :=
      if zle n 1 then
        if Z.odd x then -1 else 0
      else
        Zshiftin (Z.odd x) (Zzero_ext (Z.pred n) (Z.div2 x)).

We encode this nat-like recursion using the Z.iter iteration function, in order to make the Zzero_ext and Zsign_ext functions efficiently executable within Coq.

Definition Zzero_ext (n: Z) (x: Z) : Z :=
  Z.iter n
    (fun rec x => Zshiftin (Z.odd x) (rec (Z.div2 x)))
    (fun x => 0)
    x.

Definition Zsign_ext (n: Z) (x: Z) : Z :=
  Z.iter (Z.pred n)
    (fun rec x => Zshiftin (Z.odd x) (rec (Z.div2 x)))
    (fun x => if Z.odd x && zlt 0 n then -1 else 0)
    x.

Lemma Ziter_base:
  forall (A: Type) n (f: A -> A) x, n <= 0 -> Z.iter n f x = x.

Proof.

intros. unfold Z.iter. destruct n; auto. compute in H. elim H; auto.
Qed.

Lemma Ziter_succ:
forall (A: Type) n (f: A -> A) x,
0 <= n -> Z.iter (Z.succ n) f x = f (Z.iter n f x).

Proof.

  intros. destruct n; simpl.
  - auto.
  - rewrite Pos.add_1_r. apply Pos.iter_succ.
  - compute in H. elim H; auto.
Qed.

Lemma Znatlike_ind:
  forall (P: Z -> Prop),
  (forall n, n <= 0 -> P n) ->
  (forall n, 0 <= n -> P n -> P (Z.succ n)) ->
  forall n, P n.

Proof.

  intros. destruct (zle 0 n).
  apply natlike_ind; auto. apply H; lia.
  apply H. lia.
Qed.

Lemma Zzero_ext_spec:
forall n x i, 0 <= i ->
Z.testbit (Zzero_ext n x) i = if zlt i n then Z.testbit x i else false.

Proof.

  unfold Zzero_ext. induction n using Znatlike_ind.
  - intros. rewrite Ziter_base; auto.
    rewrite zlt_false. rewrite Ztestbit_0; auto. lia.
  - intros. rewrite Ziter_succ; auto.
    rewrite Ztestbit_shiftin; auto.
    rewrite (Ztestbit_eq i x); auto.
    destruct (zeq i 0).
    + subst i. rewrite zlt_true; auto. lia.
    + rewrite IHn. destruct (zlt (Z.pred i) n).
      rewrite zlt_true; auto. lia.
      rewrite zlt_false; auto. lia.
      lia.
Qed.

Lemma Zsign_ext_spec:
forall n x i, 0 <= i ->
Z.testbit (Zsign_ext n x) i = Z.testbit x (if zlt i n then i else n - 1).

Proof.

  intros n0 x i I0. unfold Zsign_ext.
  unfold proj_sumbool; destruct (zlt 0 n0) as [N0|N0]; simpl.
- revert x i I0. pattern n0. apply Zlt_lower_bound_ind with (z := 1); [ | lia ].
  unfold Zsign_ext. intros.
  destruct (zeq x 1).
  + subst x; simpl.
    replace (if zlt i 1 then i else 0) with 0.
    rewrite Ztestbit_base.
    destruct (Z.odd x0); [ apply Ztestbit_m1; auto | apply Ztestbit_0 ].
    destruct (zlt i 1); lia.
  + set (x1 := Z.pred x). replace x1 with (Z.succ (Z.pred x1)) by lia.
    rewrite Ziter_succ by (unfold x1; lia). rewrite Ztestbit_shiftin by auto.
    destruct (zeq i 0).
    * subst i. rewrite zlt_true. rewrite Ztestbit_base; auto. lia.
    * rewrite H by (unfold x1; lia).
      unfold x1; destruct (zlt (Z.pred i) (Z.pred x)).
      ** rewrite zlt_true by lia.
         rewrite (Ztestbit_eq i x0) by lia.
         rewrite zeq_false by lia. auto.
      ** rewrite zlt_false by lia.
         rewrite (Ztestbit_eq (x - 1) x0) by lia.
         rewrite zeq_false by lia. auto.
- rewrite Ziter_base by lia. rewrite andb_false_r.
  rewrite Z.testbit_0_l, Z.testbit_neg_r. auto.
  destruct (zlt i n0); lia.
Qed.

Zzero_ext n x is x modulo 2^n

Lemma Zzero_ext_mod:
forall n x, 0 <= n -> Zzero_ext n x = x mod (two_p n).

Proof.

intros. apply equal_same_bits; intros.
rewrite Zzero_ext_spec, Ztestbit_mod_two_p by auto. auto.
Qed.

Zzero_ext n x is the unique integer congruent to x modulo 2^n in the range 0...2^n-1.

Lemma Zzero_ext_range:
forall n x, 0 <= n -> 0 <= Zzero_ext n x < two_p n.

Proof.

intros. rewrite Zzero_ext_mod; auto. apply Z_mod_lt. apply two_p_gt_ZERO. lia.
Qed.

Lemma eqmod_Zzero_ext:
forall n x, 0 <= n -> eqmod (two_p n) (Zzero_ext n x) x.

Proof.

intros. rewrite Zzero_ext_mod; auto. apply eqmod_sym. apply eqmod_mod.
apply two_p_gt_ZERO. lia.
Qed.

Relation between Zsign_ext n x and (Zzero_ext n x]

Lemma Zsign_ext_zero_ext:
forall n, 0 <= n -> forall x,
Zsign_ext n x = Zzero_ext n x - (if Z.testbit x (n - 1) then two_p n else 0).

Proof.

  intros. apply equal_same_bits; intros.
  rewrite Zsign_ext_spec by auto.
  destruct (Z.testbit x (n - 1)) eqn:SIGNBIT.
- set (n' := - two_p n).
  replace (Zzero_ext n x - two_p n) with (Zzero_ext n x + n') by (unfold n'; lia).
  rewrite Z_add_is_or; auto.
  rewrite Zzero_ext_spec by auto. unfold n'; rewrite Ztestbit_neg_two_p by lia.
  destruct (zlt i n). rewrite orb_false_r; auto. auto.
  intros. rewrite Zzero_ext_spec by lia. unfold n'; rewrite Ztestbit_neg_two_p by lia.
  destruct (zlt j n); auto using andb_false_r.
- replace (Zzero_ext n x - 0) with (Zzero_ext n x) by lia.
  rewrite Zzero_ext_spec by auto.
  destruct (zlt i n); auto.
Qed.

Zsign_ext n x is the unique integer congruent to x modulo 2^n in the range -2^(n-1)...2^(n-1) - 1.

Lemma Zsign_ext_range:
forall n x, 0 < n -> -two_p (n-1) <= Zsign_ext n x < two_p (n-1).

Proof.

  intros.
  assert (A: 0 <= Zzero_ext n x < two_p n) by (apply Zzero_ext_range; lia).
  assert (B: Z.testbit (Zzero_ext n x) (n - 1) =
             if zlt (Zzero_ext n x) (two_p (n - 1)) then false else true).
  { set (N := Z.to_nat (n - 1)).
    generalize (Zsign_bit N (Zzero_ext n x)).
    rewrite ! two_power_nat_two_p.
    rewrite inj_S. unfold N; rewrite Z2Nat.id by lia.
    intros X; apply X. replace (Z.succ (n - 1)) with n by lia. exact A.
  }
  assert (C: two_p n = 2 * two_p (n - 1)).
  { rewrite <- two_p_S by lia. f_equal; lia. }
  rewrite Zzero_ext_spec, zlt_true in B by lia.
  rewrite Zsign_ext_zero_ext by lia. rewrite B.
  destruct (zlt (Zzero_ext n x) (two_p (n - 1))); lia.
Qed.

Lemma eqmod_Zsign_ext:
forall n x, 0 <= n ->
eqmod (two_p n) (Zsign_ext n x) x.

Proof.

  intros. rewrite Zsign_ext_zero_ext by auto.
  apply eqmod_trans with (x - 0).
  apply eqmod_sub.
  apply eqmod_Zzero_ext; lia.
  exists (if Z.testbit x (n - 1) then 1 else 0). destruct (Z.testbit x (n - 1)); ring.
  apply eqmod_refl2; lia.
Qed.

Decomposition of a number as a sum of powers of two.

Fixpoint Z_one_bits (n: nat) (x: Z) (i: Z) {struct n}: list Z :=
  match n with
  | O => nil
  | S m =>
      if Z.odd x
      then i :: Z_one_bits m (Z.div2 x) (i+1)
      else Z_one_bits m (Z.div2 x) (i+1)
  end.

Fixpoint powerserie (l: list Z): Z :=
  match l with
  | nil => 0
  | x :: xs => two_p x + powerserie xs
  end.

Lemma Z_one_bits_powerserie:
  forall n x, 0 <= x < two_power_nat n -> x = powerserie (Z_one_bits n x 0).

Proof.

  assert (forall n x i,
    0 <= i ->
    0 <= x < two_power_nat n ->
    x * two_p i = powerserie (Z_one_bits n x i)).
  {
  induction n; intros.
  simpl. rewrite two_power_nat_O in H0.
  assert (x = 0) by lia. subst x. lia.
  rewrite two_power_nat_S in H0. simpl Z_one_bits.
  rewrite (Zdecomp x) in H0. rewrite Zshiftin_spec in H0.
  assert (EQ: Z.div2 x * two_p (i + 1) = powerserie (Z_one_bits n (Z.div2 x) (i + 1))).
    apply IHn. lia.
    destruct (Z.odd x); lia.
  rewrite two_p_is_exp in EQ. change (two_p 1) with 2 in EQ.
  rewrite (Zdecomp x) at 1. rewrite Zshiftin_spec.
  destruct (Z.odd x); simpl powerserie; rewrite <- EQ; ring.
  lia. lia.
  }
  intros. rewrite <- H. change (two_p 0) with 1. lia.
  lia. exact H0.
Qed.

Lemma Z_one_bits_range:
forall n x i, In i (Z_one_bits n x 0) -> 0 <= i < Z.of_nat n.

Proof.

  assert (forall n x i j,
    In j (Z_one_bits n x i) -> i <= j < i + Z.of_nat n).
  {
  induction n; simpl In.
  tauto.
  intros x i j. rewrite Nat2Z.inj_succ.
  assert (In j (Z_one_bits n (Z.div2 x) (i + 1)) -> i <= j < i + Z.succ (Z.of_nat n)).
    intros. exploit IHn; eauto. lia.
  destruct (Z.odd x); simpl.
  intros [A|B]. subst j. lia. auto.
  auto.
  }
  intros. generalize (H n x 0 i H0). lia.
Qed.

Remark Z_one_bits_zero:
forall n i, Z_one_bits n 0 i = nil.

Proof.

induction n; intros; simpl; auto.
Qed.

Remark Z_one_bits_two_p:
  forall n x i,
  0 <= x < Z.of_nat n ->
  Z_one_bits n (two_p x) i = (i + x) :: nil.

Proof.

  induction n; intros; simpl. simpl in H. extlia.
  rewrite Nat2Z.inj_succ in H.
  assert (x = 0 \/ 0 < x) by lia. destruct H0.
  subst x; simpl. decEq. lia. apply Z_one_bits_zero.
  assert (Z.odd (two_p x) = false /\ Z.div2 (two_p x) = two_p (x-1)).
    apply Zshiftin_inj. rewrite <- Zdecomp. rewrite !Zshiftin_spec.
    rewrite <- two_p_S. rewrite Z.add_0_r. f_equal; lia. lia.
  destruct H1 as [A B]; rewrite A; rewrite B.
  rewrite IHn. f_equal; lia. lia.
Qed.

Recognition of powers of two

Fixpoint P_is_power2 (p: positive) : bool :=
  match p with
  | xH => true
  | xO q => P_is_power2 q
  | xI q => false
  end.

Definition Z_is_power2 (x: Z) : option Z :=
  match x with
  | Z0 => None
  | Zpos p => if P_is_power2 p then Some (Z.log2 x) else None
  | Zneg _ => None
  end.

Remark P_is_power2_sound:
  forall p, P_is_power2 p = true -> Z.pos p = two_p (Z.log2 (Z.pos p)).

Proof.

  induction p; simpl P_is_power2; intros.
- discriminate.
- change (Z.pos p~0) with (2 * Z.pos p). apply IHp in H.
  rewrite Z.log2_double by extlia. rewrite two_p_S. congruence.
  apply Z.log2_nonneg.
- reflexivity.
Qed.

Lemma Z_is_power2_nonneg:
forall x i, Z_is_power2 x = Some i -> 0 <= i.

Proof.

  unfold Z_is_power2; intros. destruct x; try discriminate.
  destruct (P_is_power2 p) eqn:P; try discriminate.
  replace i with (Z.log2 (Z.pos p)) by congruence. apply Z.log2_nonneg.
Qed.

Lemma Z_is_power2_sound:
forall x i, Z_is_power2 x = Some i -> x = two_p i /\ i = Z.log2 x.

Proof.

  unfold Z_is_power2; intros. destruct x; try discriminate.
  destruct (P_is_power2 p) eqn:P; try discriminate.
  apply P_is_power2_sound in P. rewrite P; split; congruence.
Qed.

Corollary Z_is_power2_range:
forall n x i,
0 <= n -> 0 <= x < two_p n -> Z_is_power2 x = Some i -> 0 <= i < n.

Proof.

  intros.
  assert (x <> 0) by (red; intros; subst x; discriminate).
  apply Z_is_power2_sound in H1. destruct H1 as [P Q]. subst i.
  split. apply Z.log2_nonneg. apply Z.log2_lt_pow2. lia. rewrite <- two_p_equiv; tauto.
Qed.

Lemma Z_is_power2_complete:
forall i, 0 <= i -> Z_is_power2 (two_p i) = Some i.

Proof.

Opaque Z.log2.
  assert (A: forall x i, Z_is_power2 x = Some i -> Z_is_power2 (2 * x) = Some (Z.succ i)).
  { destruct x; simpl; intros; try discriminate.
    change (2 * Z.pos p) with (Z.pos (xO p)); simpl.
    destruct (P_is_power2 p); inv H. rewrite <- Z.log2_double by extlia. auto.
  }
  induction i using Znatlike_ind; intros.
- replace i with 0 by lia. reflexivity.
- rewrite two_p_S by lia. apply A. apply IHi; lia.
Qed.

Definition Z_is_power2m1 (x: Z) : option Z := Z_is_power2 (Z.succ x).

Lemma Z_is_power2m1_nonneg:
forall x i, Z_is_power2m1 x = Some i -> 0 <= i.

Proof.

unfold Z_is_power2m1; intros. eapply Z_is_power2_nonneg; eauto.
Qed.

Lemma Z_is_power2m1_sound:
forall x i, Z_is_power2m1 x = Some i -> x = two_p i - 1.

Proof.

unfold Z_is_power2m1; intros. apply Z_is_power2_sound in H. lia.
Qed.

Lemma Z_is_power2m1_complete:
forall i, 0 <= i -> Z_is_power2m1 (two_p i - 1) = Some i.

Proof.

intros. unfold Z_is_power2m1. replace (Z.succ (two_p i - 1)) with (two_p i) by lia.
apply Z_is_power2_complete; auto.
Qed.

Lemma Z_is_power2m1_range:
forall n x i,
0 <= n -> 0 <= x < two_p n -> Z_is_power2m1 x = Some i -> 0 <= i <= n.

Proof.

intros. destruct (zeq x (two_p n - 1)).
- subst x. rewrite Z_is_power2m1_complete in H1 by auto. inv H1; lia.
- unfold Z_is_power2m1 in H1. apply (Z_is_power2_range n (Z.succ x) i) in H1; lia.
Qed.

Relation between bitwise operations and multiplications / divisions by powers of 2

Left shifts and multiplications by powers of 2.

Lemma Zshiftl_mul_two_p:
forall x n, 0 <= n -> Z.shiftl x n = x * two_p n.

Proof.

  intros. destruct n; simpl.
  - lia.
  - pattern p. apply Pos.peano_ind.
    + change (two_power_pos 1) with 2. simpl. ring.
    + intros. rewrite Pos.iter_succ. rewrite H0.
      rewrite Pplus_one_succ_l. rewrite two_power_pos_is_exp.
      change (two_power_pos 1) with 2. ring.
  - compute in H. congruence.
Qed.

Right shifts and divisions by powers of 2.

Lemma Zshiftr_div_two_p:
forall x n, 0 <= n -> Z.shiftr x n = x / two_p n.

Proof.

  intros. destruct n; unfold Z.shiftr; simpl.
  - rewrite Zdiv_1_r. auto.
  - pattern p. apply Pos.peano_ind.
    + change (two_power_pos 1) with 2. simpl. apply Zdiv2_div.
    + intros. rewrite Pos.iter_succ. rewrite H0.
      rewrite Pplus_one_succ_l. rewrite two_power_pos_is_exp.
      change (two_power_pos 1) with 2.
      rewrite Zdiv2_div. rewrite Z.mul_comm. apply Zdiv_Zdiv.
      rewrite two_power_pos_nat. apply two_power_nat_pos. lia.
  - compute in H. congruence.
Qed.

Properties of shrx (signed division by a power of 2)

Lemma Zquot_Zdiv:
  forall x y,
  y > 0 ->
  Z.quot x y = if zlt x 0 then (x + y - 1) / y else x / y.

Proof.

  intros. destruct (zlt x 0).
  - symmetry. apply Zquot_unique_full with ((x + y - 1) mod y - (y - 1)).
     + red. right; split. lia.
       exploit (Z_mod_lt (x + y - 1) y); auto.
       rewrite Z.abs_eq. lia. lia.
     + transitivity ((y * ((x + y - 1) / y) + (x + y - 1) mod y) - (y-1)).
       rewrite <- Z_div_mod_eq. ring. auto. ring.
  - apply Zquot_Zdiv_pos; lia.
Qed.

Lemma Zdiv_shift:
forall x y, y > 0 ->
(x + (y - 1)) / y = x / y + if zeq (Z.modulo x y) 0 then 0 else 1.

Proof.

  intros. generalize (Z_div_mod_eq x y H). generalize (Z_mod_lt x y H).
  set (q := x / y). set (r := x mod y). intros.
  destruct (zeq r 0).
  apply Zdiv_unique with (y - 1). rewrite H1. rewrite e. ring. lia.
  apply Zdiv_unique with (r - 1). rewrite H1. ring. lia.
Qed.

Size of integers, in bits.

Definition Zsize (x: Z) : Z :=
  match x with
  | Zpos p => Zpos (Pos.size p)
  | _ => 0
  end.

Remark Zsize_pos: forall x, 0 <= Zsize x.

Proof.

destruct x; simpl. lia. compute; intuition congruence. lia.
Qed.

Remark Zsize_pos': forall x, 0 < x -> 0 < Zsize x.

Proof.

destruct x; simpl; intros; try discriminate. compute; auto.
Qed.

Lemma Zsize_shiftin:
forall b x, 0 < x -> Zsize (Zshiftin b x) = Z.succ (Zsize x).

Proof.

  intros. destruct x; compute in H; try discriminate.
  destruct b.
  change (Zshiftin true (Zpos p)) with (Zpos (p~1)).
  simpl. f_equal. rewrite Pos.add_1_r; auto.
  change (Zshiftin false (Zpos p)) with (Zpos (p~0)).
  simpl. f_equal. rewrite Pos.add_1_r; auto.
Qed.

Lemma Ztestbit_size_1:
forall x, 0 < x -> Z.testbit x (Z.pred (Zsize x)) = true.

Proof.

  intros x0 POS0; pattern x0; apply Zshiftin_pos_ind; auto.
  intros. rewrite Zsize_shiftin; auto.
  replace (Z.pred (Z.succ (Zsize x))) with (Z.succ (Z.pred (Zsize x))) by lia.
  rewrite Ztestbit_shiftin_succ. auto. generalize (Zsize_pos' x H); lia.
Qed.

Lemma Ztestbit_size_2:
forall x, 0 <= x -> forall i, i >= Zsize x -> Z.testbit x i = false.

Proof.

  intros x0 POS0. destruct (zeq x0 0).
  - subst x0; intros. apply Ztestbit_0.
  - pattern x0; apply Zshiftin_pos_ind.
    + simpl. intros. change 1 with (Zshiftin true 0). rewrite Ztestbit_shiftin.
      rewrite zeq_false. apply Ztestbit_0. lia. lia.
    + intros. rewrite Zsize_shiftin in H1; auto.
      generalize (Zsize_pos' _ H); intros.
      rewrite Ztestbit_shiftin. rewrite zeq_false. apply H0. lia.
      lia. lia.
    + lia.
Qed.

Lemma Zsize_interval_1:
forall x, 0 <= x -> 0 <= x < two_p (Zsize x).

Proof.

  intros.
  assert (x = x mod (two_p (Zsize x))).
    apply equal_same_bits; intros.
    rewrite Ztestbit_mod_two_p; auto.
    destruct (zlt i (Zsize x)). auto. apply Ztestbit_size_2; auto.
    apply Zsize_pos; auto.
  rewrite H0 at 1. rewrite H0 at 3. apply Z_mod_lt. apply two_p_gt_ZERO. apply Zsize_pos; auto.
Qed.

Lemma Zsize_interval_2:
forall x n, 0 <= n -> 0 <= x < two_p n -> n >= Zsize x.

Proof.

  intros. set (N := Z.to_nat n).
  assert (Z.of_nat N = n) by (apply Z2Nat.id; auto).
  rewrite <- H1 in H0. rewrite <- two_power_nat_two_p in H0.
  destruct (zeq x 0).
  subst x; simpl; lia.
  destruct (zlt n (Zsize x)); auto.
  exploit (Ztestbit_above N x (Z.pred (Zsize x))). auto. lia.
  rewrite Ztestbit_size_1. congruence. lia.
Qed.

Lemma Zsize_monotone:
forall x y, 0 <= x <= y -> Zsize x <= Zsize y.

Proof.

  intros. apply Z.ge_le. apply Zsize_interval_2. apply Zsize_pos.
  exploit (Zsize_interval_1 y). lia.
  lia.
Qed.

Bit insertion, bit extraction

Extract and optionally sign-extend bits from...from+len-1 of x

Definition Zextract_u (x: Z) (from: Z) (len: Z) : Z :=
  Zzero_ext len (Z.shiftr x from).

Definition Zextract_s (x: Z) (from: Z) (len: Z) : Z :=
  Zsign_ext len (Z.shiftr x from).

Lemma Zextract_u_spec:
  forall x from len i,
  0 <= from -> 0 <= len -> 0 <= i ->
  Z.testbit (Zextract_u x from len) i =
  if zlt i len then Z.testbit x (from + i) else false.

Proof.

unfold Zextract_u; intros. rewrite Zzero_ext_spec, Z.shiftr_spec by auto.
rewrite Z.add_comm. auto.
Qed.

Lemma Zextract_s_spec:
  forall x from len i,
  0 <= from -> 0 < len -> 0 <= i ->
  Z.testbit (Zextract_s x from len) i =
  Z.testbit x (from + (if zlt i len then i else len - 1)).

Proof.

  unfold Zextract_s; intros. rewrite Zsign_ext_spec by auto. rewrite Z.shiftr_spec.
  rewrite Z.add_comm. auto.
  destruct (zlt i len); lia.
Qed.

Insert bits 0...len-1 of y into bits to...to+len-1 of x

Definition Zinsert (x y: Z) (to: Z) (len: Z) : Z :=
  let mask := Z.shiftl (two_p len - 1) to in
  Z.lor (Z.land (Z.shiftl y to) mask) (Z.ldiff x mask).

Lemma Zinsert_spec:
  forall x y to len i,
  0 <= to -> 0 <= len -> 0 <= i ->
  Z.testbit (Zinsert x y to len) i =
    if zle to i && zlt i (to + len)
    then Z.testbit y (i - to)
    else Z.testbit x i.

Proof.

  unfold Zinsert; intros. set (mask := two_p len - 1).
  assert (M: forall j, 0 <= j -> Z.testbit mask j = if zlt j len then true else false).
  { intros; apply Ztestbit_two_p_m1; auto. }
  rewrite Z.lor_spec, Z.land_spec, Z.ldiff_spec by auto.
  destruct (zle to i).
- rewrite ! Z.shiftl_spec by auto. rewrite ! M by lia.
  unfold proj_sumbool; destruct (zlt (i - to) len); simpl;
  rewrite andb_true_r, andb_false_r.
+ rewrite zlt_true by lia. apply orb_false_r.
+ rewrite zlt_false by lia; auto.
- rewrite ! Z.shiftl_spec_low by lia. simpl. apply andb_true_r.
Qed.