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Module mathcomp.experimental_reals.discrete

From HB Require Import structures.
From mathcomp Require Import all_ssreflect all_algebra.
From mathcomp.classical Require Import boolp.
From mathcomp Require Import xfinmap reals.
From Coq Require Setoid.

Set Implicit Arguments.
Unset Strict Implicit.
Unset Printing Implicit Defensive.

Import GRing.Theory Num.Theory.

Local Open Scope ring_scope.
Local Open Scope real_scope.

Section ProofIrrelevantChoice.

Context {T : choiceType}.

Lemma existsTP (P : T -> Prop) : { x : T | P x } + (forall x, ~ P x).

Proof.

case: (boolP `[<exists x : T, P x>]) => [/exists_asboolP | /asboolPn] h.
  by case/cid: h => w Pw; left; exists w; apply/asboolP.
by right=> x Px; apply/h; exists x.
Qed.

End ProofIrrelevantChoice.

Section PredSubtype.
Section Def.
Variable T : Type.
Variable E : pred T.

Record pred_sub : Type :=
  PSubSub { rsval :> T; rsvalP : rsval \in E }.

HB.instance Definition _ := [isSub for rsval].
End Def.

HB.instance Definition _ (T : eqType) (E : pred T) :=
  [Equality of pred_sub E by <:].

HB.instance Definition _ (T : choiceType) (E : pred T) :=
  [Choice of pred_sub E by <:].

HB.instance Definition _ (T : countType) (E : pred T) :=
  [Countable of pred_sub E by <:].
End PredSubtype.

Notation "[ 'psub' E ]" := (@pred_sub _ E)
  (format "[ 'psub' E ]").

Section PIncl.
Variables (T : Type) (E F : pred T) (le : {subset E <= F}).

Definition pincl (x : [psub E]) : [psub F] :=
  PSubSub (le (valP x)).
End PIncl.

Section Countable.
Variable (T : Type) (E : pred T).

Variant countable : Type :=
  Countable
    (rpickle : [psub E] -> nat)
    (runpickle : nat -> option [psub E])
    of pcancel rpickle runpickle.

Definition rpickle (c : countable) :=
  let: Countable p _ _ := c in p.

Definition runpickle (c : countable) :=
  let: Countable _ p _ := c in p.

Lemma rpickleK c: pcancel (rpickle c) (runpickle c).

Proof.

by case: c. Qed.

End Countable.

Section CountableTheory.
Lemma countable_countable (T : countType) (E : pred T) : countable E.

Proof.

by exists choice.pickle choice.unpickle; apply/choice.pickleK. Qed.

Section CanCountable.
Variables (T : Type) (U : countType) (E : pred T).
Variables (f : [psub E] -> U) (g : U -> [psub E]).

Lemma can_countable : cancel f g -> countable E.

Proof.

pose p := choice.pickle \o f; pose u n := omap g (choice.unpickle n).
move=> can_fg; apply (@Countable _ E p u) => x.
by rewrite {}/u {}/p /= choice.pickleK /= can_fg.
Qed.

End CanCountable.

Section CountType.
Variables (T : eqType) (E : pred T) (c : countable E).

Definition countable_countMixin := Countable.copy [psub E]
  (pcan_type (rpickleK c)).
Definition countable_choiceMixin := Choice.copy [psub E]
  (pcan_type (rpickleK c)).
End CountType.
End CountableTheory.

Section Finite.
Variables (T : eqType).

CoInductive finite (E : pred T) : Type :=
| Finite s of uniq s & {subset E <= s}.
End Finite.

Section FiniteTheory.
Context {T : choiceType}.


Lemma finiteP (E : pred T) : (exists s : seq T, {subset E <= s}) -> finite E.

Proof.

case/cid=> s sEs; exists (undup s); first by rewrite undup_uniq.
by move=> x; rewrite mem_undup; exact: sEs.
Qed.

Lemma finiteNP (E : pred T): (forall s : seq T, ~ {subset E <= s}) ->
  forall n, exists s : seq T, [/\ size s = n, uniq s & {subset s <= E}].

Proof.

move=> finN; elim=> [|n [s] [<- uq_s sE]]; first by exists [::].
have [x sxN xE]: exists2 x, x \notin s & x \in E.
  apply: contra_notP (finN (filter (mem E) s)) => /forall2NP finE x Ex.
  move/or_asboolP: (finE x).
  by rewrite !asbool_neg !asboolb negbK Ex mem_filter orbF [(mem E) x]Ex.
exists (x :: s) => /=; rewrite sxN; split=> // y.
by rewrite in_cons => /orP[/eqP->//|/sE].
Qed.

End FiniteTheory.

Section FiniteCountable.
Variables (T : eqType) (E : pred T).

Lemma finite_countable : finite E -> countable E.

Proof.

case=> s uqs Es; pose t := pmap (fun x => (insub x : option [psub E])) s.
pose f x := index x t; pose g i := nth None [seq Some x | x <- t] i.
apply (@Countable _ E f g) => x; rewrite {}/f {}/g /=.
have x_in_t: x \in t; first case: x => x h.
  by rewrite {}/t mem_pmap_sub /= Es.
by rewrite (nth_map x) ?index_mem ?nth_index.
Qed.

End FiniteCountable.

Section CountSub.
Variables (T : eqType) (E F : pred T).

Lemma countable_sub: {subset E <= F} -> countable F -> countable E.

Proof.

move=> le_EF [f g fgK]; pose f' (x : [psub E]) := f (pincl le_EF x).
pose g' x := obind (insub (sT := [psub E])) (omap val (g x)).
by exists f' g' => x; rewrite /f' /g' fgK /= valK.
Qed.

End CountSub.

Section CountableUnion.
Variables (T : eqType) (E : nat -> pred T).

Hypothesis cE : forall i, countable (E i).

Lemma cunion_countable : countable [pred x | `[< exists i, x \in E i >]].

Proof.

pose Ci i : countType := HB.pack [psub (E i)] (countable_countMixin (cE i)).
pose S := { i : nat & Ci i }; set F := [pred x | _].
have H: forall (x : [psub F]), exists i : nat, val x \in E i.
  by case=> x /= /asboolP[i] Eix; exists i.
have G: forall (x : S), val (tagged x) \in F.
  by case=> i [x /= Eix]; apply/asboolP; exists i.
pose f (x : [psub F]) : S := Tagged (fun i => [psub E i])
  (PSubSub (xchooseP (H x))).
pose g (x : S) := PSubSub (G x).
by have /can_countable: cancel f g by case=> x hx; apply/val_inj.
Qed.

End CountableUnion.