BagPermInsertion Sort With Bags

We have seen how to specify algorithms on "collections", such as sorting algorithms, using Permutations. Instead of using permutations, another way to specify these algorithms is to use bags (also called multisets), which we introduced in Lists. A set of values is like a list with no repeats where the order does not matter. A multiset is like a list, possibly with repeats, where the order does not matter. Whereas the principal query on a set is whether a given element appears in it, the principal query on a bag is how many times a given element appears in it.

From Coq Require Import Strings.String. (* for manual grading *)
From Coq Require Import Setoid Morphisms.
From VFA Require Import Perm.
From VFA Require Import Sort.
To keep this chapter more self-contained, we restate the critical definitions from Lists.
Definition bag := list nat.
Fixpoint count (v:nat) (s:bag) : nat :=
  match s with
  | nil ⇒ 0
  | h :: t ⇒
      (if h =? v then 1 else 0) + count v t
  end.
We will say two bags are equivalent if they have the same number of copies of every possible element.

Definition bag_eqv (b1 b2: bag) : Prop :=
  ∀ n, count n b1 = count n b2.

Exercise: 2 stars, standard (bag_eqv_properties)


(* It is easy to prove bag_eqv is an equivalence relation. *)
Lemma bag_eqv_refl : ∀ b, bag_eqv b b.
Proof.
(* FILL IN HERE *) Admitted.
Lemma bag_eqv_sym: ∀ b1 b2, bag_eqv b1 b2 → bag_eqv b2 b1.
Proof.
(* FILL IN HERE *) Admitted.
Lemma bag_eqv_trans: ∀ b1 b2 b3, bag_eqv b1 b2 → bag_eqv b2 b3 → bag_eqv b1 b3.
Proof.
(* FILL IN HERE *) Admitted.
The following little lemma is handy in a couple of places.

Lemma bag_eqv_cons : ∀ x b1 b2, bag_eqv b1 b2 → bag_eqv (x::b1) (x::b2).
Proof.
  (* FILL IN HERE *) Admitted.
☐

Correctness

A sorting algorithm must rearrange the elements into a list that is totally ordered. But let's say that a different way: the algorithm must produce a list with the same multiset of values, and this list must be totally ordered.

Definition is_a_sorting_algorithm' (f: list nat → list nat) :=
  ∀ al, bag_eqv al (f al) ∧ sorted (f al).

Exercise: 3 stars, standard (insert_bag)

First, prove the auxiliary lemma insert_bag, which will be useful for proving sort_bag below. Your proof will be by induction.

Lemma insert_bag: ∀ x l, bag_eqv (x::l) (insert x l).
Proof.
(* FILL IN HERE *) Admitted.
☐

Exercise: 2 stars, standard (sort_bag)

Now prove that sort preserves bag contents.
Theorem sort_bag: ∀ l, bag_eqv l (sort l).
(* FILL IN HERE *) Admitted.
☐
Now we wrap it all up.

Theorem insertion_sort_correct:
  is_a_sorting_algorithm' sort.
Proof.
split. apply sort_bag. apply sort_sorted.
Qed.

Exercise: 1 star, standard (permutations_vs_multiset)

Compare your proofs of insert_perm, sort_perm with your proofs of insert_bag, sort_bag. Which proofs are simpler?
  • easier with permutations,
  • easier with multisets
  • about the same.
Regardless of "difficulty", which do you prefer / find easier to think about?
  • permutations or
  • multisets
Put an X in one box in each list.
(* Do not modify the following line: *)
Definition manual_grade_for_permutations_vs_multiset : option (nat×string) := None.
☐

Permutations and Multisets

The two specifications of insertion sort are equivalent. One reason is that permutations and multisets are closely related. We're going to prove:
Permutation al bl ↔ bag_eqv al bl.

Exercise: 3 stars, standard (perm_bag)

The forward direction is straighforward, by induction on the evidence for Permutation:
Lemma perm_bag:
  ∀ al bl : list nat,
   Permutation al bl → bag_eqv al bl.
(* FILL IN HERE *) Admitted.
☐
The other direction, bag_eqv al bl → Permutation al bl, is surprisingly difficult. This proof approach is due to Zhong Sheng Hu. The first three lemmas are used to prove the fourth one.

Exercise: 2 stars, advanced (bag_nil_inv)

Lemma bag_nil_inv : ∀ b, bag_eqv [] b → b = [].
Proof.
  (* FILL IN HERE *) Admitted.
☐

Exercise: 3 stars, advanced (bag_cons_inv)

Lemma bag_cons_inv : ∀ l x n,
    S n = count x l →
    ∃ l1 l2,
      l = l1 ++ x :: l2
      ∧ count x (l1 ++ l2) = n.
Proof.
  (* FILL IN HERE *) Admitted.
☐

Exercise: 2 stars, advanced (count_insert_other)

Lemma count_insert_other : ∀ l1 l2 x y,
    y ≠ x → count y (l1 ++ x :: l2) = count y (l1 ++ l2).
Proof.
  (* FILL IN HERE *) Admitted.
☐

Exercise: 3 stars, advanced (bag_perm)

Lemma bag_perm:
  ∀ al bl, bag_eqv al bl → Permutation al bl.
Proof.
(* FILL IN HERE *) Admitted.
☐

The Main Theorem: Equivalence of Multisets and Permutations

Theorem bag_eqv_iff_perm:
  ∀ al bl, bag_eqv al bl ↔ Permutation al bl.
Proof.
  intros. split. apply bag_perm. apply perm_bag.
Qed.
Therefore, it doesn't matter whether you prove your sorting algorithm using the Permutations method or the multiset method.

Corollary sort_specifications_equivalent:
    ∀ sort, is_a_sorting_algorithm sort ↔ is_a_sorting_algorithm' sort.
Proof.
  unfold is_a_sorting_algorithm, is_a_sorting_algorithm'.
  split; intros;
  destruct (H al); split; auto;
  apply bag_eqv_iff_perm; auto.
Qed.

(* Mon May 11 15:03:15 EDT 2020 *)