Library stdpp.infinite

From stdpp Require Export list.
From stdpp Require Import relations pretty.
From stdpp Require Import options.

Set Default Proof Using "Type*".

Generic constructions

If A is infinite, and there is an injection from A to B, then B is also infinite. Note that due to constructivity we need a rather strong notion of being injective, we also need a partial function B option A back.
Program Definition inj_infinite `{Infinite A} {B}
    (f : A B) (g : B option A) (Hgf : x, g (f x) = Some x) :
  Infinite B := {| infinite_fresh := f fresh omap g |}.
Next Obligation.
  intros A ? B f g Hfg xs Hxs; simpl in ×.
  apply (infinite_is_fresh (omap g xs)), elem_of_list_omap; eauto.
Next Obligation. solve_proper. Qed.

If there is an injective function f : nat B, then B is infinite. This construction works as follows: to obtain an element not in xs, we return the first element f 0, f 1, f 2, ... that is not in xs.
This construction has a nice computational behavior to e.g. find fresh strings. Given some prefix s, we use f n := if n is S n' then s +:+ pretty n else s. The construction then finds the first string starting with s followed by a number that's not in the input list. For example, given ["H", "H1", "H4"] and s := "H", it would find "H2".
Section search_infinite.
  Context {B} (f : nat B).

  Let R (xs : list B) (n1 n2 : nat) :=
    n2 < n1 (f (n1 - 1)) xs.
  Lemma search_infinite_step xs n : f n xs R xs (1 + n) n.
  Proof. split; [lia|]. replace (1 + n - 1) with n by lia; eauto. Qed.

  Context `{!Inj (=) (=) f, !EqDecision B}.

  Lemma search_infinite_R_wf xs : well_founded (R xs).
    revert xs. assert (help : xs n n',
      Acc (R (filter (.≠ f n') xs)) n n' < n Acc (R xs) n).
    { induction 1 as [n _ IH]. constructor; intros n2 [??]. apply IH; [|lia].
      split; [done|]. apply elem_of_list_filter; naive_solver lia. }
    intros xs. induction (well_founded_ltof _ length xs) as [xs _ IH].
    intros n1; constructor; intros n2 [Hn Hs].
    apply help with (n2 - 1); [|lia]. apply IH. eapply filter_length_lt; eauto.

  Definition search_infinite_go (xs : list B) (n : nat)
      (go : n', R xs n' n B) : B :=
    let x := f n in
    match decide (x xs) with
    | left Hxgo (S n) (search_infinite_step xs n Hx)
    | right _x

  Program Definition search_infinite : Infinite B := {|
    infinite_fresh xs :=
      Fix_F _ (search_infinite_go xs) (wf_guard 32 (search_infinite_R_wf xs) 0)
  Next Obligation.
    intros xs. unfold fresh.
    generalize 0 (wf_guard 32 (search_infinite_R_wf xs) 0). revert xs.
    fix FIX 3; intros xs n [?]; simpl; unfold search_infinite_go at 1; simpl.
    destruct (decide _); auto.
  Next Obligation.
    intros xs1 xs2 Hxs. unfold fresh.
    generalize (wf_guard 32 (search_infinite_R_wf xs1) 0).
    generalize (wf_guard 32 (search_infinite_R_wf xs2) 0). generalize 0.
    fix FIX 2. intros n [acc1] [acc2]; simpl; unfold search_infinite_go.
    destruct (decide ( _ xs1)) as [H1|H1], (decide (_ xs2)) as [H2|H2]; auto.
    - destruct H2. by rewrite <-Hxs.
    - destruct H1. by rewrite Hxs.
End search_infinite.

To select a fresh number from a given list x₀ ... x, a possible approach is to compute (S x₀) `max` ... `max` (S xₙ) `max` 0. For non-empty lists of non-negative numbers this is equal to taking the maximal element x and adding one.
The construction below generalizes this construction to any type A, function f : A A A. and initial value a. The fresh element is computed as x₀ `f` ... `f` x `f` a. For numbers, the prototypical instance is f x y := S x `max` y and a:=0, which gives the behavior described before. Note that this gives a (i.e. 0 for numbers) for the empty list.
Section max_infinite.
  Context {A} (f : A A A) (a : A) (lt : relation A).
  Context (HR : x, ¬lt x x).
  Context (HR1 : x y, lt x (f x y)).
  Context (HR2 : x x' y, lt x x' lt x (f y x')).
  Context (Hf : x x' y, f x (f x' y) = f x' (f x y)).

  Program Definition max_infinite: Infinite A := {|
    infinite_fresh := foldr f a
  Next Obligation.
    cut ( xs x, x xs lt x (foldr f a xs)).
    { intros help xs []%help%HR. }
    induction 1; simpl; auto.
  Next Obligation. by apply (foldr_permutation_proper _ _ _). Qed.
End max_infinite.

Instances for number types
Global Program Instance nat_infinite : Infinite nat :=
  max_infinite (Nat.max S) 0 (<) _ _ _ _.
Solve Obligations with (intros; simpl; lia).

Global Program Instance N_infinite : Infinite N :=
  max_infinite (N.max N.succ) 0%N _ _ _ _.
Solve Obligations with (intros; simpl; lia).

Global Program Instance positive_infinite : Infinite positive :=
  max_infinite (Pos.max Pos.succ) 1%positive _ _ _ _.
Solve Obligations with (intros; simpl; lia).

Global Program Instance Z_infinite: Infinite Z :=
  max_infinite (Z.max Z.succ) 0%Z _ _ _ _.
Solve Obligations with (intros; simpl; lia).

Instances for option, sum, products
Global Instance option_infinite `{Infinite A} : Infinite (option A) :=
  inj_infinite Some id (λ _, eq_refl).

Global Instance sum_infinite_l `{Infinite A} {B} : Infinite (A + B) :=
  inj_infinite inl (maybe inl) (λ _, eq_refl).
Global Instance sum_infinite_r {A} `{Infinite B} : Infinite (A + B) :=
  inj_infinite inr (maybe inr) (λ _, eq_refl).

Global Instance prod_infinite_l `{Infinite A, Inhabited B} : Infinite (A × B) :=
  inj_infinite (., inhabitant) (Some fst) (λ _, eq_refl).
Global Instance prod_infinite_r `{Inhabited A, Infinite B} : Infinite (A × B) :=
  inj_infinite (inhabitant ,.) (Some snd) (λ _, eq_refl).

Instance for lists
Global Program Instance list_infinite `{Inhabited A} : Infinite (list A) := {|
  infinite_fresh xxs := replicate (fresh (length <$> xxs)) inhabitant
Next Obligation.
  intros A ? xs ?. destruct (infinite_is_fresh (length <$> xs)).
  apply elem_of_list_fmap. eexists; split; [|done].
  unfold fresh. by rewrite replicate_length.
Next Obligation. unfold fresh. by intros A ? xs1 xs2 →. Qed.

Instance for strings
Global Program Instance string_infinite : Infinite string :=
  search_infinite pretty.