Library prosa.classic.util.pick

From mathcomp Require Import ssreflect ssrbool ssrfun eqtype ssrnat seq fintype.

(* In this file, we define functions for picking numbers in an interval 0, n). *)

Auxiliary Functions

Definition default0 {n} (x: option 'I_n) : nat := if x is Some y then y else 0.

Definition arg_pred_nat n (P: pred 'I_n) ord :=
  [pred i | P i & [ j: 'I_n, P j ==> ord i j]].

Definition pred_min_nat n (P: pred 'I_n) := arg_pred_nat n P leq.
Definition pred_max_nat n (P: pred 'I_n) := arg_pred_nat n P (fun x ygeq x y).
Definition to_pred_ord n (P: pred nat) := (fun x:'I_nP (nat_of_ord x)).

Defining Pick functions

(* (pick_any n P) returns some number < n that satisfies P, or 0 if it cannot be found. *)
Definition pick_any n (P: pred nat) := default0 (pick (to_pred_ord n P)).

(* (pick_min n P) returns the smallest number < n that satisfies P, or 0 if it cannot be found. *)
Definition pick_min n (P: pred nat) := default0 (pick (pred_min_nat n (to_pred_ord n P))).

(* (pick_max n P) returns the largest number < n that satisfies P, or 0 if it cannot be found. *)
Definition pick_max n (P: pred nat) := default0 (pick (pred_max_nat n (to_pred_ord n P))).

Improved notation

(* Next we provide the following notation for the variations of pick:
     pick-any x N | Ppick-any x < N | P
     pick-min x N | Ppick-min x < N | P
     pick-max x N | Ppick-max x < N | P. *)

Notation "[ 'pick-any' x <= N | P ]" :=
  (pick_any N.+1 (fun x : natP%B))
  (at level 0, x ident, only parsing) : form_scope.

Notation "[ 'pick-any' x < N | P ]" :=
  (pick_any N (fun x : natP%B))
  (at level 0, x ident, only parsing) : form_scope.

Notation "[ 'pick-min' x <= N | P ]" :=
  (pick_min N.+1 (fun x : natP%B))
  (at level 0, x ident, only parsing) : form_scope.

Notation "[ 'pick-min' x < N | P ]" :=
  (pick_min N (fun x : natP%B))
  (at level 0, x ident, only parsing) : form_scope.

Notation "[ 'pick-max' x <= N | P ]" :=
  (pick_max N.+1 (fun x : natP%B))
  (at level 0, x ident, only parsing) : form_scope.

Notation "[ 'pick-max' x < N | P ]" :=
  (pick_max N (fun x : natP%B))
  (at level 0, x ident, only parsing) : form_scope.

Lemmas about pick_any

Section PickAny.

  Variable n: nat.
  Variable p: pred nat.

  Variable P: nat Prop.

  Hypothesis EX: x, x < n p x.

  Hypothesis HOLDS: x, p x P x.

  (* First, we show that any property P of (pick_any n p) can be proven by showing
     that P holds for any number < n that satisfies p. *)

  Lemma pick_any_holds: P (pick_any n p).
  Proof.
    rewrite /pick_any /default0.
    case: pickP; first by intros x PRED; apply HOLDS.
    intros NONE; red in NONE; exfalso.
    move: EX ⇒ [x [LTN PRED]].
    by specialize (NONE (Ordinal LTN)); rewrite /to_pred_ord /= PRED in NONE.
  Qed.

End PickAny.

Lemmas about pick_min
Section PickMin.

  Variable n: nat.
  Variable p: pred nat.

  Variable P: nat Prop.

  (* Assume that there is some number < n that satisfies p. *)
  Hypothesis EX: x, x < n p x.

  Section Bound.

    (* First, we show that (pick_min n p) < n. *)
    Lemma pick_min_ltn: pick_min n p < n.
    Proof.
      rewrite /pick_min /odflt /oapp.
      case: pickP.
      {
        movex /andP [PRED /forallP ALL].
        by rewrite /default0.
      }
      {
        intros NONE; red in NONE; exfalso.
        move: EX ⇒ [x [LT PRED]]; clear EX.
        set argmin := arg_min (Ordinal LT) p id.
        specialize (NONE argmin).
        suff ARGMIN: (pred_min_nat n p) argmin by rewrite ARGMIN in NONE.
        rewrite /argmin; case: arg_minnP; first by done.
        intros y Py MINy.
        apply/andP; split; first by done.
        by apply/forallP; intros y0; apply/implyP; intros Py0; apply MINy.
      }
    Qed.

  End Bound.

  Section Minimum.

    Hypothesis MIN:
       x,
        x < n
        p x
        ( y, y < n p y x y)
        P x.

    (* Next, we show that any property P of (pick_min n p) can be proven by showing
       that P holds for the smallest number < n that satisfies p. *)

    Lemma pick_min_holds: P (pick_min n p).
    Proof.
      rewrite /pick_min /odflt /oapp.
      case: pickP.
      {
        movex /andP [PRED /forallP ALL].
        apply MIN; [by rewrite /default0 | by done |].
        intros y LTy Py; specialize (ALL (Ordinal LTy)).
        by move: ALL ⇒ /implyP ALL; apply ALL.
      }
      {
        intros NONE; red in NONE; exfalso.
        move: EX ⇒ [x [LT PRED]]; clear EX.
        set argmin := arg_min (Ordinal LT) p id.
        specialize (NONE argmin).
        suff ARGMIN: (pred_min_nat n p) argmin by rewrite ARGMIN in NONE.
        rewrite /argmin; case: arg_minnP; first by done.
        intros y Py MINy.
        apply/andP; split; first by done.
        by apply/forallP; intros y0; apply/implyP; intros Py0; apply MINy.
      }
    Qed.

  End Minimum.

End PickMin.

Lemmas about pick_max
Section PickMax.

  Variable n: nat.
  Variable p: pred nat.

  Variable P: nat Prop.

  (* Assume that there is some number < n that satisfies p. *)
  Hypothesis EX: x, x < n p x.

  Section Bound.

    (* First, we show that (pick_max n p) < n... *)
    Lemma pick_max_ltn: pick_max n p < n.
    Proof.
      rewrite /pick_max /odflt /oapp.
      case: pickP.
      {
        movex /andP [PRED /forallP ALL].
        by rewrite /default0.
      }
      {
        intros NONE; red in NONE; exfalso.
        move: EX ⇒ [x [LT PRED]]; clear EX.
        set argmax := arg_max (Ordinal LT) p id.
        specialize (NONE argmax).
        suff ARGMAX: (pred_max_nat n p) argmax by rewrite ARGMAX in NONE.
        rewrite /argmax; case: arg_maxnP; first by done.
        intros y Py MAXy.
        apply/andP; split; first by done.
        by apply/forallP; intros y0; apply/implyP; intros Py0; apply MAXy.
      }
    Qed.

  End Bound.

  Section Maximum.

    Hypothesis MAX:
       x,
        x < n
        p x
        ( y, y < n p y x y)
        P x.

    (* Next, we show that any property P of (pick_max n p) can be proven by showing that
       P holds for the largest number < n that satisfies p. *)

    Lemma pick_max_holds: P (pick_max n p).
    Proof.
      rewrite /pick_max /odflt /oapp.
      case: pickP.
      {
        movex /andP [PRED /forallP ALL].
        apply MAX; [by rewrite /default0 | by rewrite /default0 |].
        intros y LTy Py; specialize (ALL (Ordinal LTy)).
        by move: ALL ⇒ /implyP ALL; apply ALL.
      }
      {
        intros NONE; red in NONE; exfalso.
        move: EX ⇒ [x [LT PRED]]; clear EX.
        set argmax := arg_max (Ordinal LT) p id.
        specialize (NONE argmax).
        suff ARGMAX: (pred_max_nat n p) argmax by rewrite ARGMAX in NONE.
        rewrite /argmax; case: arg_maxnP; first by done.
        intros y Py MAXy.
        apply/andP; split; first by done.
        by apply/forallP; intros y0; apply/implyP; intros Py0; apply MAXy.
      }
    Qed.

  End Maximum.

End PickMax.

Section Predicate.

  Variable n: nat.
  Variable p: pred nat.

  Hypothesis EX: x, x < n p x.

  (* Here we prove that pick_any satiesfies the predicate p, ... *)
  Lemma pick_any_pred: p (pick_any n p).
  Proof.
    by apply pick_any_holds.
  Qed.

  (* ...and the same holds for pick_min... *)
  Lemma pick_min_pred: p (pick_min n p).
  Proof.
    by apply pick_min_holds.
  Qed.

  (* ...and pick_max. *)
  Lemma pick_max_pred: p (pick_max n p).
  Proof.
    by apply pick_max_holds.
  Qed.

End Predicate.

Section PickMinCompare.

  Variable n: nat.
  Variable p1 p2: pred nat.

  Hypothesis EX1 : x, x < n p1 x.
  Hypothesis EX2 : x, x < n p2 x.

  Hypothesis OUT:
     x y, x < n y < n p1 x p2 y ~~ p1 y x y.

  Lemma pick_min_compare: pick_min n p1 pick_min n p2.
  Proof.
    set m1:= pick_min _ _.
    set m2:= pick_min _ _.
    case IN: (p1 m2).
    {
      apply pick_min_holds; first by done.
      intros x Px LTN ALL.
      by apply ALL; first by apply pick_min_ltn.
    }
    {
      apply (OUT m1 m2).
      - by apply pick_min_ltn.
      - by apply pick_min_ltn.
      - by apply pick_min_pred.
      - by apply pick_min_pred.
      - by apply negbT.
    }
  Qed.

End PickMinCompare.