(* Copyright (c) 2012-2015, Robbert Krebbers. *)
(* This file is distributed under the terms of the BSD license. *)
(** Merge sort. Adapted from the implementation of Hugo Herbelin in the Coq
standard library, but without using the module system. *)
From Coq Require Export Sorted.
From stdpp Require Export orders list.
Section merge_sort.
Context {A} (R : relation A) `{∀ x y, Decision (R x y)}.
Fixpoint list_merge (l1 : list A) : list A → list A :=
fix list_merge_aux l2 :=
match l1, l2 with
| [], _ => l2
| _, [] => l1
| x1 :: l1, x2 :: l2 =>
if decide_rel R x1 x2 then x1 :: list_merge l1 (x2 :: l2)
else x2 :: list_merge_aux l2
end.
Global Arguments list_merge !_ !_ /.
Local Notation stack := (list (option (list A))).
Fixpoint merge_list_to_stack (st : stack) (l : list A) : stack :=
match st with
| [] => [Some l]
| None :: st => Some l :: st
| Some l' :: st => None :: merge_list_to_stack st (list_merge l' l)
end.
Fixpoint merge_stack (st : stack) : list A :=
match st with
| [] => []
| None :: st => merge_stack st
| Some l :: st => list_merge l (merge_stack st)
end.
Fixpoint merge_sort_aux (st : stack) (l : list A) : list A :=
match l with
| [] => merge_stack st
| x :: l => merge_sort_aux (merge_list_to_stack st [x]) l
end.
Definition merge_sort : list A → list A := merge_sort_aux [].
End merge_sort.
(** ** Properties of the [Sorted] and [StronglySorted] predicate *)
Section sorted.
Context {A} (R : relation A).
Lemma Sorted_StronglySorted `{!Transitive R} l :
Sorted R l → StronglySorted R l.
Proof. by apply Sorted.Sorted_StronglySorted. Qed.
Lemma StronglySorted_unique `{!AntiSymm (=) R} l1 l2 :
StronglySorted R l1 → StronglySorted R l2 → l1 ≡ₚ l2 → l1 = l2.
Proof.
intros Hl1; revert l2. induction Hl1 as [|x1 l1 ? IH Hx1]; intros l2 Hl2 E.
{ symmetry. by apply Permutation_nil. }
destruct Hl2 as [|x2 l2 ? Hx2].
{ by apply Permutation_nil in E. }
assert (x1 = x2); subst.
{ rewrite Forall_forall in Hx1, Hx2.
assert (x2 ∈ x1 :: l1) as Hx2' by (by rewrite E; left).
assert (x1 ∈ x2 :: l2) as Hx1' by (by rewrite <-E; left).
inversion Hx1'; inversion Hx2'; simplify_eq; auto. }
f_equal. by apply IH, (inj (x2 ::)).
Qed.
Lemma Sorted_unique `{!Transitive R, !AntiSymm (=) R} l1 l2 :
Sorted R l1 → Sorted R l2 → l1 ≡ₚ l2 → l1 = l2.
Proof. auto using StronglySorted_unique, Sorted_StronglySorted. Qed.
Global Instance HdRel_dec x `{∀ y, Decision (R x y)} l :
Decision (HdRel R x l).
Proof.
refine
match l with
| [] => left _
| y :: l => cast_if (decide (R x y))
end; abstract first [by constructor | by inversion 1].
Defined.
Global Instance Sorted_dec `{∀ x y, Decision (R x y)} : ∀ l,
Decision (Sorted R l).
Proof.
refine
(fix go l :=
match l return Decision (Sorted R l) with
| [] => left _
| x :: l => cast_if_and (decide (HdRel R x l)) (go l)
end); clear go; abstract first [by constructor | by inversion 1].
Defined.
Global Instance StronglySorted_dec `{∀ x y, Decision (R x y)} : ∀ l,
Decision (StronglySorted R l).
Proof.
refine
(fix go l :=
match l return Decision (StronglySorted R l) with
| [] => left _
| x :: l => cast_if_and (decide (Forall (R x) l)) (go l)
end); clear go; abstract first [by constructor | by inversion 1].
Defined.
Context {B} (f : A → B).
Lemma HdRel_fmap (R1 : relation A) (R2 : relation B) x l :
(∀ y, R1 x y → R2 (f x) (f y)) → HdRel R1 x l → HdRel R2 (f x) (f <$> l).
Proof. destruct 2; constructor; auto. Qed.
Lemma Sorted_fmap (R1 : relation A) (R2 : relation B) l :
(∀ x y, R1 x y → R2 (f x) (f y)) → Sorted R1 l → Sorted R2 (f <$> l).
Proof. induction 2; simpl; constructor; eauto using HdRel_fmap. Qed.
Lemma StronglySorted_fmap (R1 : relation A) (R2 : relation B) l :
(∀ x y, R1 x y → R2 (f x) (f y)) →
StronglySorted R1 l → StronglySorted R2 (f <$> l).
Proof.
induction 2; csimpl; constructor;
rewrite ?Forall_fmap; eauto using Forall_impl.
Qed.
End sorted.
(** ** Correctness of merge sort *)
Section merge_sort_correct.
Context {A} (R : relation A) `{∀ x y, Decision (R x y)} `{!Total R}.
Lemma list_merge_cons x1 x2 l1 l2 :
list_merge R (x1 :: l1) (x2 :: l2) =
if decide (R x1 x2) then x1 :: list_merge R l1 (x2 :: l2)
else x2 :: list_merge R (x1 :: l1) l2.
Proof. done. Qed.
Lemma HdRel_list_merge x l1 l2 :
HdRel R x l1 → HdRel R x l2 → HdRel R x (list_merge R l1 l2).
Proof.
destruct 1 as [|x1 l1 IH1], 1 as [|x2 l2 IH2];
rewrite ?list_merge_cons; simpl; repeat case_decide; auto.
Qed.
Lemma Sorted_list_merge l1 l2 :
Sorted R l1 → Sorted R l2 → Sorted R (list_merge R l1 l2).
Proof.
intros Hl1. revert l2. induction Hl1 as [|x1 l1 IH1];
induction 1 as [|x2 l2 IH2]; rewrite ?list_merge_cons; simpl;
repeat case_decide;
constructor; eauto using HdRel_list_merge, HdRel_cons, total_not.
Qed.
Lemma merge_Permutation l1 l2 : list_merge R l1 l2 ≡ₚ l1 ++ l2.
Proof.
revert l2. induction l1 as [|x1 l1 IH1]; intros l2;
induction l2 as [|x2 l2 IH2]; rewrite ?list_merge_cons; simpl;
repeat case_decide; auto.
- by rewrite (right_id_L [] (++)).
- by rewrite IH2, Permutation_middle.
Qed.
Local Notation stack := (list (option (list A))).
Inductive merge_stack_Sorted : stack → Prop :=
| merge_stack_Sorted_nil : merge_stack_Sorted []
| merge_stack_Sorted_cons_None st :
merge_stack_Sorted st → merge_stack_Sorted (None :: st)
| merge_stack_Sorted_cons_Some l st :
Sorted R l → merge_stack_Sorted st → merge_stack_Sorted (Some l :: st).
Fixpoint merge_stack_flatten (st : stack) : list A :=
match st with
| [] => []
| None :: st => merge_stack_flatten st
| Some l :: st => l ++ merge_stack_flatten st
end.
Lemma Sorted_merge_list_to_stack st l :
merge_stack_Sorted st → Sorted R l →
merge_stack_Sorted (merge_list_to_stack R st l).
Proof.
intros Hst. revert l.
induction Hst; repeat constructor; naive_solver auto using Sorted_list_merge.
Qed.
Lemma merge_list_to_stack_Permutation st l :
merge_stack_flatten (merge_list_to_stack R st l) ≡ₚ
l ++ merge_stack_flatten st.
Proof.
revert l. induction st as [|[l'|] st IH]; intros l; simpl; auto.
by rewrite IH, merge_Permutation, (assoc_L _), (comm (++) l).
Qed.
Lemma Sorted_merge_stack st :
merge_stack_Sorted st → Sorted R (merge_stack R st).
Proof. induction 1; simpl; auto using Sorted_list_merge. Qed.
Lemma merge_stack_Permutation st : merge_stack R st ≡ₚ merge_stack_flatten st.
Proof.
induction st as [|[] ? IH]; intros; simpl; auto.
by rewrite merge_Permutation, IH.
Qed.
Lemma Sorted_merge_sort_aux st l :
merge_stack_Sorted st → Sorted R (merge_sort_aux R st l).
Proof.
revert st. induction l; simpl;
auto using Sorted_merge_stack, Sorted_merge_list_to_stack.
Qed.
Lemma merge_sort_aux_Permutation st l :
merge_sort_aux R st l ≡ₚ merge_stack_flatten st ++ l.
Proof.
revert st. induction l as [|?? IH]; simpl; intros.
- by rewrite (right_id_L [] (++)), merge_stack_Permutation.
- rewrite IH, merge_list_to_stack_Permutation; simpl.
by rewrite Permutation_middle.
Qed.
Lemma Sorted_merge_sort l : Sorted R (merge_sort R l).
Proof. apply Sorted_merge_sort_aux. by constructor. Qed.
Lemma merge_sort_Permutation l : merge_sort R l ≡ₚ l.
Proof. unfold merge_sort. by rewrite merge_sort_aux_Permutation. Qed.
Lemma StronglySorted_merge_sort `{!Transitive R} l :
StronglySorted R (merge_sort R l).
Proof. auto using Sorted_StronglySorted, Sorted_merge_sort. Qed.
End merge_sort_correct.