Require Import Arith Lia.

From Undecidability.Shared.Libs.DLW.Utils
  Require Import utils_tac utils_nat gcd crt sums.

From Undecidability.Shared.Libs.DLW.Vec Require Import pos vec.

From Undecidability.MuRec Require Import recalg recomp prim_min ra_utils.

Set Implicit Arguments.

Set Default Proof Using "Type".

Local Notation "'⟦' f '⟧'" := (@ra_rel _ f) (at level 0).
Local Notation power := (mscal mult 1).

Opaque ra_cst_n ra_iter_n ra_iter ra_prim_min ra_prim_max.
Opaque ra_plus ra_pred ra_minus ra_mult ra_exp ra_div ra_rem.
Opaque ra_ite ra_eq.
Opaque ra_div2 ra_mod2 ra_pow2 ra_notdiv_pow2.


Section ra_recomp.



  Definition ra_decomp_l : recalg 1.
  Proof.
    apply ra_comp with 2; [ | fill_vec ].
    + apply ra_prim_min.
      ra root ra_notdiv_pow2.
      * ra root ra_succ.
        ra arg pos0.
      * ra arg pos1.
    + ra root ra_succ.
      ra arg pos0.
    + ra root ra_succ.
      ra arg pos0.
  Defined.

  Fact ra_decomp_l_prim_rec : prim_rec ra_decomp_l.
  Proof.
    ra prim rec.
    apply ra_prim_min_prim_rec.
    ra prim rec.
  Qed.

  Fact ra_decomp_l_val_0 n : ⟦ra_decomp_l⟧ (n##vec_nil) (prim_min (fun i => notdiv_pow2 (S i) (S n)) (S n)).
  Proof.
    exists (S n##S n##vec_nil); split.
    + apply ra_prim_min_val with (f := fun v i => notdiv_pow2 (S i) (vec_head v)).
      intros i v; vec split v with j; vec nil v.
      exists (S i##j##vec_nil); split; auto.
      pos split; simpl; auto.
      exists (i##vec_nil); split; simpl; auto.
      pos split; simpl; auto.
    + pos split; simpl; auto;
        exists (n##vec_nil); split; simpl; auto;
        pos split; simpl; auto.
  Qed.

  Fact ra_decomp_l_rel n : { a | ⟦ra_decomp_l⟧ (n##vec_nil) a
                              /\ divides (power a 2) (S n)
                              /\ ~ divides (power (S a) 2) (S n) }.
  Proof.
    exists (prim_min (fun i => notdiv_pow2 (S i) (S n)) (S n)); split.
    1: { apply ra_decomp_l_val_0. }
    generalize (prim_min_spec (fun i => notdiv_pow2 (S i) (S n)) (S n));
      intros (H1 & H2).
    1: { apply notdiv_pow2_spec_2; intros C.
         apply divides_le in C; try discriminate.
         generalize (@power_ge_n (S (S n)) 2); intros; lia. }
    apply notdiv_pow2_spec_2 in H1; split; auto.
    revert H2; generalize (prim_min (fun i : nat => notdiv_pow2 (S i) (S n)) (S n)); intros [ | k ] Hk.
    + rewrite power_0; apply divides_1.
    + apply notdiv_pow2_spec_1, Hk; lia.
  Qed.

  Hint Resolve ra_decomp_l_prim_rec : core.
  Opaque ra_decomp_l.

  Definition ra_decomp_r : recalg 1.
  Proof.
    ra root ra_div.
    + ra root ra_succ.
      ra arg pos0.
    + ra root ra_pow2.
      ra root ra_succ.
      ra root ra_decomp_l.
      ra arg pos0.
  Defined.

  Fact ra_decomp_r_prim_rec : prim_rec ra_decomp_r.
  Proof. ra prim rec. Qed.

  Fact ra_decomp_r_rel n : { b | ⟦ra_decomp_r⟧ (n##vec_nil) b
                              /\ exists a, ⟦ra_decomp_l⟧ (n##vec_nil) a
                                        /\ S n = power a 2 * (2*b+1) }.
  Proof.
    destruct (ra_decomp_l_rel n) as (a & H1 & H2 & H3).
    exists (div (S n) (power (S a) 2)); split.
    + exists (S n##power (S a) 2##vec_nil); split.
      { apply ra_div_val; generalize (@power_ge_1 (S a) 2); intros; lia. }
      pos split; simpl; auto.
      * exists (n##vec_nil); split; simpl; auto.
        pos split; simpl; auto.
      * exists (S a##vec_nil); split; auto.
        pos split; simpl; auto.
        exists (a##vec_nil); split; simpl; auto.
        pos split; simpl; auto.
        exists (n##vec_nil); split; auto.
        pos split; simpl; auto.
    + exists a; split; auto.
      generalize (div_rem_spec1 (S n) (power a 2)).
      apply divides_rem_eq in H2; rewrite H2, Nat.add_0_r.
      rewrite power_S in *.
      clear H1; revert H2 H3.
      generalize (S n) (power a 2) (power2_gt_0 a); clear n a; intros n p H1 H2 H3 H4.
      rewrite H4 at 1; rewrite mult_comm; f_equal.
      rewrite divides_rem_eq in H3.
      rewrite (mult_comm _ p), div_mult; try lia.
      rewrite mult_comm, rem_mult in H3; try lia.
      rewrite (@div_rem_spec1 (div n p) 2) at 1.
      rewrite H2 in H3; simpl in H3.
      generalize (@div_rem_spec2 (div n p) 2); intros H5.
      rewrite mult_comm in H3.
      destruct (rem (div n p) 2) as [ | [ | ] ]; lia.
  Qed.

  Fact ra_decomp_lr_val n : ⟦ra_decomp_l⟧ (n##vec_nil) (decomp_l n) /\ ⟦ra_decomp_r⟧ (n##vec_nil) (decomp_r n).
  Proof.
    destruct (ra_decomp_r_rel n) as (b & H1 & a & H2 & H3).
    rewrite (decomp_lr_spec n) in H3.
    apply decomp_uniq in H3; destruct H3; subst a b; auto.
  Qed.

  Fact ra_decomp_l_val n : ⟦ra_decomp_l⟧ (n##vec_nil) (decomp_l n).
  Proof. apply ra_decomp_lr_val. Qed.

  Fact ra_decomp_r_val n : ⟦ra_decomp_r⟧ (n##vec_nil) (decomp_r n).
  Proof. apply ra_decomp_lr_val. Qed.

  Definition ra_recomp : recalg 2.
  Proof.
    ra root ra_pred.
    ra root ra_mult.
    + ra root ra_pow2.
      ra arg pos0.
    + ra root ra_succ.
      ra root ra_mult.
      * ra cst 2.
      * ra arg pos1.
  Defined.

  Fact ra_recomp_prim_rec : prim_rec ra_recomp.
  Proof. ra prim rec. Qed.

  Fact ra_recomp_val a b : ⟦ra_recomp⟧ (a##b##vec_nil) (recomp a b).
  Proof.
    unfold recomp.
    rewrite plus_comm.
    exists (power a 2 * (S (2*b)) ## vec_nil); split; auto.
    pos split; simpl.
    exists (power a 2##S (2*b)##vec_nil); split; auto.
    pos split; simpl; auto.
    + exists (a##vec_nil); split; auto.
      pos split; simpl; auto.
    + exists (2*b##vec_nil); split; simpl; auto.
      pos split; simpl; auto.
      exists (2##b##vec_nil); split; auto.
      { apply ra_mult_val. }
      pos split; simpl; auto.
  Qed.

End ra_recomp.

#[export] Hint Resolve ra_decomp_l_prim_rec ra_decomp_r_prim_rec ra_recomp_prim_rec
             ra_decomp_l_val ra_decomp_r_val ra_recomp_val : core.
Opaque ra_decomp_l ra_decomp_r ra_recomp.

Section ra_inject_project.


  Fixpoint ra_inject n : recalg n.
  Proof.
    destruct n as [ | n ].
    + ra cst 0.
    + ra root ra_recomp.
      * ra arg pos0.
      * apply ra_comp with (1 := ra_inject n).
        apply vec_set_pos.
        intros p; apply ra_proj, pos_nxt, p.
  Defined.

  Fact ra_inject_prim_rec n : prim_rec (ra_inject n).
  Proof.
    induction n as [ | n IHn ]; simpl; split; auto.
    + pos split.
    + pos split; ra prim rec.
      intro; vec pos simpl.
  Qed.

  Fact ra_inject_val n v : ⟦ra_inject n⟧ v (inject v).
  Proof.
    induction n as [ | n IHn ]; simpl.
    + vec nil v; simpl; auto.
    + vec split v with x.
      exists (x##inject v##vec_nil); split.
      * apply ra_recomp_val.
      * pos split; simpl; auto.
        exists v; split; auto.
        intro; vec pos simpl.
  Qed.

  Fixpoint ra_project n (p : pos n) : recalg 1 :=
    match p with
      | pos_fst => ra_decomp_l
      | pos_nxt p => ra_comp (ra_project p) (ra_decomp_r##vec_nil)
    end.

  Fact ra_project_prim_rec n p : prim_rec (@ra_project n p).
  Proof.
    induction p as [ | n p IHp ]; simpl; auto; split; auto.
    pos split; auto.
  Qed.

  Fact ra_project_val n p x : ⟦@ra_project n p⟧ (x##vec_nil) (vec_pos (@project n x) p).
  Proof.
    revert x; induction p as [ | n p IHp ]; intros x; simpl; auto.
    exists (decomp_r x##vec_nil); split.
    + apply IHp.
    + pos split; simpl; auto.
  Qed.

  Fact ra_project_rel n p v x : ⟦@ra_project n p⟧ v x <-> x = vec_pos (@project n (vec_head v)) p.
  Proof.
    vec split v with a; vec nil v.
    split.
    + intros H; apply ra_rel_fun with (1 := H), ra_project_val.
    + intros; subst; apply ra_project_val.
  Qed.

  Definition ra_vec_project n : vec (recalg 1) n.
  Proof. apply vec_set_pos, ra_project. Defined.

End ra_inject_project.


#[export] Hint Resolve ra_project_prim_rec ra_inject_prim_rec
             ra_project_val ra_inject_val : core.