Library stdpp.finite

From stdpp Require Export countable vector.
From stdpp Require Import options.

Class Finite A `{EqDecision A} := {
  enum : list A;
  NoDup_enum : NoDup enum;
  elem_of_enum x : x enum
Global Hint Mode Finite ! - : typeclass_instances.
Global Arguments enum : clear implicits.
Global Arguments enum _ {_ _} : assert.
Global Arguments NoDup_enum : clear implicits.
Global Arguments NoDup_enum _ {_ _} : assert.
Definition card A `{Finite A} := length (enum A).

Program Definition finite_countable `{Finite A} : Countable A := {|
  encode := λ x,
    Pos.of_nat $ S $ default 0 $ fst <$> list_find (x =.) (enum A);
  decode := λ p, enum A !! pred (Pos.to_nat p)
Next Obligation.
  intros ?? [xs Hxs HA] x; unfold encode, decode; simpl.
  destruct (list_find_elem_of (x =.) xs x) as [[i y] Hi]; auto.
  rewrite by done; simpl; rewrite Hi; simpl.
  destruct (list_find_Some (x =.) xs i y); naive_solver.
Global Hint Immediate finite_countable : typeclass_instances.

Definition find `{Finite A} (P : A Prop) `{ x, Decision (P x)} : option A :=
  list_find P (enum A) ≫= decode_nat fst.

Lemma encode_lt_card `{finA: Finite A} (x : A) : encode_nat x < card A.
  destruct finA as [xs Hxs HA]; unfold encode_nat, encode, card; simpl.
  rewrite by done; simpl.
  destruct (list_find _ xs) as [[i y]|] eqn:HE; simpl.
  - apply list_find_Some in HE as (?&?&?); eauto using lookup_lt_Some.
  - destruct xs; simpl; [|lia].
    exfalso; eapply not_elem_of_nil, (HA x).
Lemma encode_decode A `{finA: Finite A} i :
  i < card A x : A, decode_nat i = Some x encode_nat x = i.
  destruct finA as [xs Hxs HA].
  unfold encode_nat, decode_nat, encode, decode, card; simpl.
  intros Hi. apply lookup_lt_is_Some in Hi. destruct Hi as [x Hx].
   x. rewrite ! by done; simpl.
  destruct (list_find_elem_of (x =.) xs x) as [[j y] Hj]; auto.
  split; [done|]; rewrite Hj; simpl.
  apply list_find_Some in Hj as (?&->&?). eauto using NoDup_lookup.
Lemma find_Some `{finA: Finite A} (P : A Prop) `{ x, Decision (P x)} (x : A) :
  find P = Some x P x.
  destruct finA as [xs Hxs HA]; unfold find, decode_nat, decode; simpl.
  intros Hx. destruct (list_find _ _) as [[i y]|] eqn:Hi; simplify_eq/=.
  rewrite ! in Hx by done.
  destruct (list_find_Some P xs i y); naive_solver.
Lemma find_is_Some `{finA: Finite A} (P : A Prop) `{ x, Decision (P x)} (x : A) :
  P x y, find P = Some y P y.
  destruct finA as [xs Hxs HA]; unfold find, decode; simpl.
  intros Hx. destruct (list_find_elem_of P xs x) as [[i y] Hi]; auto.
  rewrite Hi; unfold decode_nat, decode; simpl. rewrite ! by done.
  simpl. apply list_find_Some in Hi; naive_solver.

Definition encode_fin `{Finite A} (x : A) : fin (card A) :=
  Fin.of_nat_lt (encode_lt_card x).
Program Definition decode_fin `{Finite A} (i : fin (card A)) : A :=
  match Some_dec (decode_nat i) return _ with
  | inleft (x _) ⇒ x | inright __
Next Obligation.
  intros A ?? i ?; exfalso.
  destruct (encode_decode A i); naive_solver auto using fin_to_nat_lt.
Lemma decode_encode_fin `{Finite A} (x : A) : decode_fin (encode_fin x) = x.
  unfold decode_fin, encode_fin. destruct (Some_dec _) as [[x' Hx]|Hx].
  { by rewrite fin_to_nat_to_fin, decode_encode_nat in Hx; simplify_eq. }
  exfalso; by rewritefin_to_nat_to_fin, decode_encode_nat in Hx.

Lemma fin_choice {n} {B : fin n Type} (P : i, B i Prop) :
  ( i, y, P i y) f, i, P i (f i).
  induction n as [|n IH]; intros Hex.
  { (fin_0_inv _); intros i; inv_fin i. }
  destruct (IH _ _ (λ i, Hex (FS i))) as [f Hf], (Hex 0%fin) as [y Hy].
   (fin_S_inv _ y f); intros i; by inv_fin i.
Lemma finite_choice `{Finite A} {B : A Type} (P : x, B x Prop) :
  ( x, y, P x y) f, x, P x (f x).
  intros Hex. destruct (fin_choice _ (λ i, Hex (decode_fin i))) as [f ?].
   (λ x, eq_rect _ _ (f(encode_fin x)) _ (decode_encode_fin x)); intros x.
  destruct (decode_encode_fin x); simpl; auto.

Lemma card_0_inv P `{finA: Finite A} : card A = 0 A P.
  intros ? x. destruct finA as [[|??] ??]; simplify_eq.
  by destruct (not_elem_of_nil x).
Lemma finite_inhabited A `{finA: Finite A} : 0 < card A Inhabited A.
  unfold card; intros. destruct finA as [[|x ?] ??]; simpl in *; [exfalso;lia|].
  constructor; exact x.
Lemma finite_inj_submseteq `{finA: Finite A} `{finB: Finite B} (f: A B)
  `{!Inj (=) (=) f} : f <$> enum A ⊆+ enum B.
  intros. destruct finA, finB. apply NoDup_submseteq; auto using NoDup_fmap_2.
Lemma finite_inj_Permutation `{Finite A} `{Finite B} (f : A B)
  `{!Inj (=) (=) f} : card A = card B f <$> enum A ≡ₚ enum B.
  intros. apply submseteq_Permutation_length_eq.
  - by rewrite fmap_length.
  - by apply finite_inj_submseteq.
Lemma finite_inj_surj `{Finite A} `{Finite B} (f : A B)
  `{!Inj (=) (=) f} : card A = card B Surj (=) f.
  intros HAB y. destruct (elem_of_list_fmap_2 f (enum A) y) as (x&?&?); eauto.
  rewrite finite_inj_Permutation; auto using elem_of_enum.

Lemma finite_surj A `{Finite A} B `{Finite B} :
  0 < card A card B g : B A, Surj (=) g.
  intros [??]. destruct (finite_inhabited A) as [x']; auto with lia.
   (λ y : B, default x' (decode_nat (encode_nat y))).
  intros x. destruct (encode_decode B (encode_nat x)) as (y&Hy1&Hy2).
  { pose proof (encode_lt_card x); lia. }
   y. by rewrite Hy2, decode_encode_nat.
Lemma finite_inj A `{Finite A} B `{Finite B} :
  card A card B f : A B, Inj (=) (=) f.
  - intros. destruct (decide (card A = 0)) as [HA|?].
    { (card_0_inv B HA). intros y. apply (card_0_inv _ HA y). }
    destruct (finite_surj A B) as (g&?); auto with lia.
    destruct (surj_cancel g) as (f&?). f. apply cancel_inj.
  - intros [f ?]. unfold card. rewrite <-(fmap_length f).
    by apply submseteq_length, (finite_inj_submseteq f).
Lemma finite_bijective A `{Finite A} B `{Finite B} :
  card A = card B f : A B, Inj (=) (=) f Surj (=) f.
  - intros; destruct (proj1 (finite_inj A B)) as [f ?]; auto with lia.
     f; split; [done|]. by apply finite_inj_surj.
  - intros (f&?&?). apply (anti_symm (≤)); apply finite_inj.
    + by f.
    + destruct (surj_cancel f) as (g&?). g. apply cancel_inj.
Lemma inj_card `{Finite A} `{Finite B} (f : A B)
  `{!Inj (=) (=) f} : card A card B.
Proof. apply finite_inj. eauto. Qed.
Lemma surj_card `{Finite A} `{Finite B} (f : A B)
  `{!Surj (=) f} : card B card A.
  destruct (surj_cancel f) as (g&?).
  apply inj_card with g, cancel_inj.
Lemma bijective_card `{Finite A} `{Finite B} (f : A B)
  `{!Inj (=) (=) f} `{!Surj (=) f} : card A = card B.
Proof. apply finite_bijective. eauto. Qed.

Decidability of quantification over finite types
Section forall_exists.
  Context `{Finite A} (P : A Prop).

  Lemma Forall_finite : Forall P (enum A) ( x, P x).
  Proof. rewrite Forall_forall. intuition auto using elem_of_enum. Qed.
  Lemma Exists_finite : Exists P (enum A) ( x, P x).
  Proof. rewrite Exists_exists. naive_solver eauto using elem_of_enum. Qed.

  Context `{ x, Decision (P x)}.

  Global Instance forall_dec: Decision ( x, P x).
  Proof using Type×.
   refine (cast_if (decide (Forall P (enum A))));
    abstract by rewrite <-Forall_finite.
  Global Instance exists_dec: Decision ( x, P x).
  Proof using Type×.
   refine (cast_if (decide (Exists P (enum A))));
    abstract by rewrite <-Exists_finite.
End forall_exists.

Section enc_finite.
  Context `{EqDecision A}.
  Context (to_nat : A nat) (of_nat : nat A) (c : nat).
  Context (of_to_nat : x, of_nat (to_nat x) = x).
  Context (to_nat_c : x, to_nat x < c).
  Context (to_of_nat : i, i < c to_nat (of_nat i) = i).

  Local Program Instance enc_finite : Finite A := {| enum := of_nat <$> seq 0 c |}.
  Next Obligation.
    apply NoDup_alt. intros i j x. rewrite !list_lookup_fmap. intros Hi Hj.
    destruct (seq _ _ !! i) as [i'|] eqn:Hi',
      (seq _ _ !! j) as [j'|] eqn:Hj'; simplify_eq/=.
    apply lookup_seq in Hi' as [-> ?]. apply lookup_seq in Hj' as [-> ?].
    rewrite <-(to_of_nat i), <-(to_of_nat j) by done. by f_equal.
  Next Obligation.
    intros x. rewrite elem_of_list_fmap. (to_nat x).
    split; auto. by apply elem_of_list_lookup_2 with (to_nat x), lookup_seq.
  Lemma enc_finite_card : card A = c.
  Proof. unfold card. simpl. by rewrite fmap_length, seq_length. Qed.
End enc_finite.

Section surjective_finite.
  Context `{Finite A, EqDecision B} (f : A B).
  Context `{!Surj (=) f}.

  Program Definition surjective_finite: Finite B :=
    {| enum := remove_dups (f <$> enum A) |}.
  Next Obligation. apply NoDup_remove_dups. Qed.
  Next Obligation.
    intros y. rewrite elem_of_remove_dups, elem_of_list_fmap.
    destruct (surj f y). eauto using elem_of_enum.
End surjective_finite.

Section bijective_finite.
  Context `{Finite A, EqDecision B} (f : A B) (g : B A).
  Context `{!Inj (=) (=) f, !Cancel (=) f g}.

  Definition bijective_finite : Finite B :=
    let _ := cancel_surj (f:=f) (g:=g) in
    surjective_finite f.
End bijective_finite.

Global Program Instance option_finite `{Finite A} : Finite (option A) :=
  {| enum := None :: (Some <$> enum A) |}.
Next Obligation.
  - rewrite elem_of_list_fmap. by intros (?&?&?).
  - apply (NoDup_fmap_2 _); auto using NoDup_enum.
Next Obligation.
  intros ??? [x|]; [right|left]; auto.
  apply elem_of_list_fmap. eauto using elem_of_enum.
Lemma option_cardinality `{Finite A} : card (option A) = S (card A).
Proof. unfold card. simpl. by rewrite fmap_length. Qed.

Global Program Instance Empty_set_finite : Finite Empty_set := {| enum := [] |}.
Next Obligation. by apply NoDup_nil. Qed.
Next Obligation. intros []. Qed.
Lemma Empty_set_card : card Empty_set = 0.
Proof. done. Qed.

Global Program Instance unit_finite : Finite () := {| enum := [tt] |}.
Next Obligation. apply NoDup_singleton. Qed.
Next Obligation. intros []. by apply elem_of_list_singleton. Qed.
Lemma unit_card : card unit = 1.
Proof. done. Qed.

Global Program Instance bool_finite : Finite bool := {| enum := [true; false] |}.
Next Obligation.
  constructor; [ by rewrite elem_of_list_singleton | apply NoDup_singleton ].
Next Obligation. intros [|]; [ left | right; left ]. Qed.
Lemma bool_card : card bool = 2.
Proof. done. Qed.

Global Program Instance sum_finite `{Finite A, Finite B} : Finite (A + B)%type :=
  {| enum := (inl <$> enum A) ++ (inr <$> enum B) |}.
Next Obligation.
  intros. apply NoDup_app; split_and?.
  - apply (NoDup_fmap_2 _). by apply NoDup_enum.
  - intro. rewrite !elem_of_list_fmap. intros (?&?&?) (?&?&?); congruence.
  - apply (NoDup_fmap_2 _). by apply NoDup_enum.
Next Obligation.
  intros ?????? [x|y]; rewrite elem_of_app, !elem_of_list_fmap;
    [left|right]; (eexists; split; [done|apply elem_of_enum]).
Lemma sum_card `{Finite A, Finite B} : card (A + B) = card A + card B.
Proof. unfold card. simpl. by rewrite app_length, !fmap_length. Qed.

Global Program Instance prod_finite `{Finite A, Finite B} : Finite (A × B)%type :=
  {| enum := foldr (λ x, (pair x <$> enum B ++.)) [] (enum A) |}.
Next Obligation.
  intros A ?????. induction (NoDup_enum A) as [|x xs Hx Hxs IH]; simpl.
  { constructor. }
  apply NoDup_app; split_and?.
  - by apply (NoDup_fmap_2 _), NoDup_enum.
  - intros [? y]. rewrite elem_of_list_fmap. intros (?&?&?); simplify_eq.
    clear IH. induction Hxs as [|x' xs ?? IH]; simpl.
    { rewrite elem_of_nil. tauto. }
    rewrite elem_of_app, elem_of_list_fmap.
    intros [(?&?&?)|?]; simplify_eq.
    + destruct Hx. by left.
    + destruct IH; [ | by auto ]. by intro; destruct Hx; right.
  - done.
Next Obligation.
  intros ?????? [x y]. induction (elem_of_enum x); simpl.
  - rewrite elem_of_app, !elem_of_list_fmap. eauto using elem_of_enum.
  - rewrite elem_of_app; eauto.
Lemma prod_card `{Finite A} `{Finite B} : card (A × B) = card A × card B.
  unfold card; simpl. induction (enum A); simpl; auto.
  rewrite app_length, fmap_length. auto.

Definition list_enum {A} (l : list A) : n, list { l : list A | length l = n } :=
  fix go n :=
  match n with
  | 0 ⇒ [[]eq_refl]
  | S nfoldr (λ x, (sig_map (x ::.) (λ _ H, f_equal S H) <$> (go n) ++.)) [] l

Global Program Instance list_finite `{Finite A} n : Finite { l : list A | length l = n } :=
  {| enum := list_enum (enum A) n |}.
Next Obligation.
  intros A ?? n. induction n as [|n IH]; simpl; [apply NoDup_singleton |].
  revert IH. generalize (list_enum (enum A) n). intros l Hl.
  induction (NoDup_enum A) as [|x xs Hx Hxs IH]; simpl; auto; [constructor |].
  apply NoDup_app; split_and?.
  - by apply (NoDup_fmap_2 _).
  - intros [k1 Hk1]. clear Hxs IH. rewrite elem_of_list_fmap.
    intros ([k2 Hk2]&?&?) Hxk2; simplify_eq/=. destruct Hx. revert Hxk2.
    induction xs as [|x' xs IH]; simpl in *; [by rewrite elem_of_nil |].
    rewrite elem_of_app, elem_of_list_fmap, elem_of_cons.
    intros [([??]&?&?)|?]; simplify_eq/=; auto.
  - apply IH.
Next Obligation.
  intros A ?? n [l Hl]. revert l Hl.
  induction n as [|n IH]; intros [|x l] Hl; simpl; simplify_eq.
  { apply elem_of_list_singleton. by apply (sig_eq_pi _). }
  revert IH. generalize (list_enum (enum A) n). intros k Hk.
  induction (elem_of_enum x) as [x xs|x xs]; simpl in ×.
  - rewrite elem_of_app, elem_of_list_fmap. left. injection Hl. intros Hl'.
    eexists (lHl'). split; [|done]. by apply (sig_eq_pi _).
  - rewrite elem_of_app. eauto.

Lemma list_card `{Finite A} n : card { l : list A | length l = n } = card A ^ n.
  unfold card; simpl. induction n as [|n IH]; simpl; auto.
  rewrite <-IH. clear IH. generalize (list_enum (enum A) n).
  induction (enum A) as [|x xs IH]; intros l; simpl; auto.
  by rewrite app_length, fmap_length, IH.

Fixpoint fin_enum (n : nat) : list (fin n) :=
  match n with 0 ⇒ [] | S n ⇒ 0%fin :: (FS <$> fin_enum n) end.
Global Program Instance fin_finite n : Finite (fin n) := {| enum := fin_enum n |}.
Next Obligation.
  intros n. induction n; simpl; constructor.
  - rewrite elem_of_list_fmap. by intros (?&?&?).
  - by apply (NoDup_fmap _).
Next Obligation.
  intros n i. induction i as [|n i IH]; simpl;
    rewrite elem_of_cons, ?elem_of_list_fmap; eauto.
Lemma fin_card n : card (fin n) = n.
Proof. unfold card; simpl. induction n; simpl; rewrite ?fmap_length; auto. Qed.

Lemma finite_sig_dec `{!EqDecision A} (P : A Prop) `{Finite (sig P)} x :
  Decision (P x).
  assert {xs : list A | x, P x x xs} as [xs ?].
  { clear x. (proj1_sig <$> enum _). intros x. split; intros Hx.
    - apply elem_of_list_fmap_1_alt with (x Hx); [apply elem_of_enum|]; done.
    - apply elem_of_list_fmap in Hx as [[x' Hx'] [-> _]]; done. }
  destruct (decide (x xs)); [left | right]; naive_solver.
Section sig_finite.
  Context {A} (P : A Prop) `{ x, Decision (P x)}.

  Fixpoint list_filter_sig (l : list A) : list (sig P) :=
    match l with
    | [][]
    | x :: lmatch decide (P x) with
              | left Hx H :: list_filter_sig l
              | _list_filter_sig l
  Lemma list_filter_sig_filter (l : list A) :
    proj1_sig <$> list_filter_sig l = filter P l.
    induction l as [| a l IH]; [done |].
    simpl; rewrite filter_cons.
    destruct (decide _); csimpl; by rewrite IH.

  Context `{Finite A} `{ x, ProofIrrel (P x)}.

  Global Program Instance sig_finite : Finite (sig P) :=
    {| enum := list_filter_sig (enum A) |}.
  Next Obligation.
    eapply NoDup_fmap_1. rewrite list_filter_sig_filter.
    apply NoDup_filter, NoDup_enum.
  Next Obligation.
    intros p. apply (elem_of_list_fmap_2_inj proj1_sig).
    rewrite list_filter_sig_filter, elem_of_list_filter.
    split; [by destruct p | apply elem_of_enum].
  Lemma sig_card : card (sig P) = length (filter P (enum A)).
  Proof. by rewrite <-list_filter_sig_filter, fmap_length. Qed.
End sig_finite.

Lemma finite_pigeonhole `{Finite A} `{Finite B} (f : A B) :
  card B < card A x1 x2, x1 x2 f x1 = f x2.
  intros. apply dec_stable; intros Heq.
  cut (Inj eq eq f); [intros ?%inj_card; lia|].
  intros x1 x2 ?. apply dec_stable. naive_solver.

Lemma nat_pigeonhole (f : nat nat) (n1 n2 : nat) :
  n2 < n1
  ( i, i < n1 f i < n2)
   i1 i2, i1 < i2 < n1 f i1 = f i2.
  intros Hn Hf. pose (f' (i : fin n1) := nat_to_fin (Hf _ (fin_to_nat_lt i))).
  destruct (finite_pigeonhole f') as (i1&i2&Hi&Hf'); [by rewrite !fin_card|].
  apply (not_inj (f:=fin_to_nat)) in Hi. apply (f_equal fin_to_nat) in Hf'.
  unfold f' in Hf'. rewrite !fin_to_nat_to_fin in Hf'.
  pose proof (fin_to_nat_lt i1); pose proof (fin_to_nat_lt i2).
  destruct (decide (i1 < i2)); [ i1, i2| i2, i1]; lia.

Lemma list_pigeonhole {A} (l1 l2 : list A) :
  l1 l2
  length l2 < length l1
   i1 i2 x, i1 < i2 l1 !! i1 = Some x l1 !! i2 = Some x.
  intros Hl Hlen.
  assert ( i : fin (length l1), (j : fin (length l2)) x,
    l1 !! (fin_to_nat i) = Some x
    l2 !! (fin_to_nat j) = Some x) as [f Hf]%fin_choice.
  { intros i. destruct (lookup_lt_is_Some_2 l1 i)
      as [x Hix]; [apply fin_to_nat_lt|].
    assert (x l2) as [j Hjx]%elem_of_list_lookup_1
      by (by eapply Hl, elem_of_list_lookup_2).
     (nat_to_fin (lookup_lt_Some _ _ _ Hjx)), x.
    by rewrite fin_to_nat_to_fin. }
  destruct (finite_pigeonhole f) as (i1&i2&Hi&Hf'); [by rewrite !fin_card|].
  destruct (Hf i1) as (x1&?&?), (Hf i2) as (x2&?&?).
  assert (x1 = x2) asby congruence.
  apply (not_inj (f:=fin_to_nat)) in Hi. apply (f_equal fin_to_nat) in Hf'.
  destruct (decide (i1 < i2)); [ i1, i2| i2, i1]; eauto with lia.