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(*         *   The Coq Proof Assistant / The Coq Development Team       *)
(*  v      *         Copyright INRIA, CNRS and contributors             *)
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(*         *     GNU Lesser General Public License Version 2.1          *)
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Require Import Relations Setoid SetoidList List Multiset PermutSetoid Permutation Lia.

Set Implicit Arguments.

(** This file is similar to [PermutSetoid], except that the equality used here
    is Coq usual one instead of a setoid equality. In particular, we can then
    prove the equivalence between [Permutation.Permutation] and
    [PermutSetoid.permutation].
*)

Section Perm.

  Variable A : Type.
  Hypothesis eq_dec : forall x y:A, {x=y} + {~ x=y}.

  Notation permutation := (permutation _ eq_dec).
  Notation list_contents := (list_contents _ eq_dec).

  (** we can use [multiplicity] to define [In] and [NoDup]. *)

  Lemma multiplicity_In :
    forall l a, In a l <-> 0 < multiplicity (list_contents l) a.
  Proof.
    intros; split; intro H.
    eapply In_InA, multiplicity_InA in H; eauto with typeclass_instances.
    eapply multiplicity_InA, InA_alt in H as (y & -> & H); eauto with typeclass_instances.
  Qed.

  Lemma multiplicity_In_O :
    forall l a, ~ In a l -> multiplicity (list_contents l) a = 0.
  Proof.
    intros l a; rewrite multiplicity_In;
      destruct (multiplicity (list_contents l) a); auto.
    destruct 1; auto with arith.
  Qed.

  Lemma multiplicity_In_S :
    forall l a, In a l -> multiplicity (list_contents l) a >= 1.
  Proof.
    intros l a; rewrite multiplicity_In; auto.
  Qed.

  Lemma multiplicity_NoDup :
    forall l, NoDup l <-> (forall a, multiplicity (list_contents l) a <= 1).
  Proof.
    induction l.
    simpl.
    split; auto with arith.
    intros; apply NoDup_nil.
    split; simpl.
    inversion_clear 1.
    rewrite IHl in H1.
    intros; destruct (eq_dec a a0) as [H2|H2]; simpl; auto.
    subst a0.
    rewrite multiplicity_In_O; auto.
    intros; constructor.
    rewrite multiplicity_In.
    generalize (H a).
    destruct (eq_dec a a) as [H0|H0].
    destruct (multiplicity (list_contents l) a); auto with arith.
    simpl; inversion 1.
    inversion H3.
    destruct H0; auto.
    rewrite IHl; intros.
    generalize (H a0); auto with arith.
    destruct (eq_dec a a0); simpl; auto with arith.
  Qed.

  Lemma NoDup_permut :
    forall l l', NoDup l -> NoDup l' ->
      (forall x, In x l <-> In x l') -> permutation l l'.
  Proof.
    intros.
    red; unfold meq; intros.
    rewrite multiplicity_NoDup in H, H0.
    generalize (H a) (H0 a) (H1 a); clear H H0 H1.
    do 2 rewrite multiplicity_In.
    destruct 3; lia.
  Qed.

  (** Permutation is compatible with In. *)
  Lemma permut_In_In :
    forall l1 l2 e, permutation l1 l2 -> In e l1 -> In e l2.
  Proof.
    unfold PermutSetoid.permutation, meq; intros l1 l2 e P IN.
    generalize (P e); clear P.
    destruct (In_dec eq_dec e l2) as [H|H]; auto.
    rewrite (multiplicity_In_O _ _ H).
    intros.
    generalize (multiplicity_In_S _ _ IN).
    rewrite H0.
    inversion 1.
  Qed.

  Lemma permut_cons_In :
    forall l1 l2 e, permutation (e :: l1) l2 -> In e l2.
  Proof.
    intros; eapply permut_In_In; eauto.
    red; auto.
  Qed.

  (** Permutation of an empty list. *)
  Lemma permut_nil :
    forall l, permutation l nil -> l = nil.
  Proof.
    intro l; destruct l as [ | e l ]; trivial.
    assert (In e (e::l)) by (red; auto).
    intro Abs; generalize (permut_In_In _ Abs H).
    inversion 1.
  Qed.

  (** When used with [eq], this permutation notion is equivalent to
      the one defined in [List.v]. *)

  Lemma permutation_Permutation :
    forall l l', Permutation l l' <-> permutation l l'.
  Proof.
    split.
    induction 1.
    apply permut_refl.
    apply permut_cons; auto.
    change (permutation (y::x::l) ((x::nil)++y::l)).
    apply permut_add_cons_inside; simpl; apply permut_refl.
    apply permut_trans with l'; auto.
    revert l'.
    induction l.
    intros.
    rewrite (permut_nil (permut_sym H)).
    apply Permutation_refl.
    intros.
    destruct (In_split _ _ (permut_cons_In H)) as (h2,(t2,H1)).
    subst l'.
    apply Permutation_cons_app.
    apply IHl.
    apply permut_remove_hd with a; auto with typeclass_instances.
  Qed.

  (** Permutation for short lists. *)

  Lemma permut_length_1:
    forall a b, permutation (a :: nil) (b :: nil)  -> a=b.
  Proof.
    intros a b; unfold PermutSetoid.permutation, meq; intro P;
      generalize (P b); clear P; simpl.
    destruct (eq_dec b b) as [H|H]; [ | destruct H; auto].
    destruct (eq_dec a b); simpl; auto; intros; discriminate.
  Qed.

  Lemma permut_length_2 :
    forall a1 b1 a2 b2, permutation (a1 :: b1 :: nil) (a2 :: b2 :: nil) ->
      (a1=a2) /\ (b1=b2) \/ (a1=b2) /\ (a2=b1).
  Proof.
    intros a1 b1 a2 b2 P.
    assert (H:=permut_cons_In P).
    inversion_clear H.
    left; split; auto.
    apply permut_length_1.
    red; red; intros.
    generalize (P a); clear P; simpl.
    destruct (eq_dec a1 a) as [H2|H2];
      destruct (eq_dec a2 a) as [H3|H3]; auto.
    destruct H3; transitivity a1; auto.
    destruct H2; transitivity a2; auto.
    right.
    inversion_clear H0; [|inversion H].
    split; auto.
    apply permut_length_1.
    red; red; intros.
    generalize (P a); clear P; simpl.
    destruct (eq_dec a1 a) as [H2|H2];
      destruct (eq_dec b2 a) as [H3|H3]; auto.
    simpl; rewrite <- plus_n_Sm; inversion 1; auto.
    destruct H3; transitivity a1; auto.
    destruct H2; transitivity b2; auto.
  Qed.

  (** Permutation is compatible with length. *)
  Lemma permut_length :
    forall l1 l2, permutation l1 l2 -> length l1 = length l2.
  Proof.
    induction l1; intros l2 H.
    rewrite (permut_nil (permut_sym H)); auto.
    destruct (In_split _ _ (permut_cons_In H)) as (h2,(t2,H1)).
    subst l2.
    rewrite app_length.
    simpl; rewrite <- plus_n_Sm; f_equal.
    rewrite <- app_length.
    apply IHl1.
    apply permut_remove_hd with a; auto with typeclass_instances.
  Qed.

  Variable B : Type.
  Variable eqB_dec : forall x y:B, { x=y }+{ ~x=y }.

  (** Permutation is compatible with map. *)

  Lemma permutation_map :
    forall f l1 l2, permutation l1 l2 ->
      PermutSetoid.permutation _ eqB_dec (map f l1) (map f l2).
  Proof.
    intros f; induction l1.
    intros l2 P; rewrite (permut_nil (permut_sym P)); apply permut_refl.
    intros l2 P.
    simpl.
    destruct (In_split _ _ (permut_cons_In P)) as (h2,(t2,H1)).
    subst l2.
    rewrite map_app.
    simpl.
    apply permut_add_cons_inside.
    rewrite <- map_app.
    apply IHl1; auto.
    apply permut_remove_hd with a; auto with typeclass_instances.
  Qed.

End Perm.

