Module Share <: SHARE_MODEL.
(* Definition of the trees and operations on them *)
Inductive ShareTree : Set :=
| Leaf : bool -> ShareTree
| Node : ShareTree -> ShareTree -> ShareTree
.
Fixpoint nonFullTree (x:ShareTree) : Prop :=
match x with
| Leaf b   => b = false
| Node l r => nonFullTree l \/ nonFullTree r
end.
Fixpoint nonEmptyTree (x:ShareTree) : Prop :=
match x with
| Leaf b   => b = true
| Node l r => nonEmptyTree l \/ nonEmptyTree r
end.
Fixpoint canonicalTree (x:ShareTree) : Prop :=
match x with
| Leaf _   => True
| Node l r => nonFullTree x /\ nonEmptyTree x /\
canonicalTree l /\ canonicalTree r
end.
Fixpoint union_tree (x y:ShareTree) { struct x } : ShareTree :=
match x with
| Leaf true  => Leaf true
| Leaf false => y
| Node l1 r1 =>
match y with
| Leaf true  => Leaf true
| Leaf false => x
| Node l2 r2 =>
Node (union_tree l1 l2) (union_tree r1 r2)
end
end.
Fixpoint intersect_tree (x y:ShareTree) { struct x } : ShareTree :=
match x with
| Leaf false => Leaf false
| Leaf true  => y
| Node l1 r1 =>
match y with
| Leaf false => Leaf false
| Leaf true  => x
| Node l2 r2 =>
Node (intersect_tree l1 l2) (intersect_tree r1 r2)
end
end.
Fixpoint comp_tree (x:ShareTree) : ShareTree :=
match x with
| Leaf b => Leaf (negb b)
| Node l r => Node (comp_tree l) (comp_tree r)
end.
Fixpoint mkCanon (x:ShareTree) : ShareTree :=
match x with
| Leaf b => Leaf b
| Node l r =>
let l' := mkCanon l in
let r' := mkCanon r in
match l', r' with
| Leaf b1, Leaf b2 =>
if bool_dec b1 b2
then Leaf b1
else Node l' r'
| _, _ => Node l' r'
end
end.
Fixpoint relativ_tree (z a:ShareTree) {struct z} : ShareTree :=
match z with
| Leaf true  => a
| Leaf false => Leaf false
| Node l r   => Node (relativ_tree l a) (relativ_tree r a)
end.
(* The ordering relation on trees, and its induced isomorphism. *)
Inductive shareTreeOrd : ShareTree -> ShareTree -> Prop :=
| Leaf_Ord : forall b1 b2, implb b1 b2 = true ->
shareTreeOrd (Leaf b1) (Leaf b2)
| LeafNode_Ord : forall b l r,
shareTreeOrd (Node (Leaf b) (Leaf b)) (Node l r) ->
shareTreeOrd (Leaf b) (Node l r)
| NodeLeaf_Ord : forall b l r,
shareTreeOrd (Node l r) (Node (Leaf b) (Leaf b)) ->
shareTreeOrd (Node l r) (Leaf b)
| Node_Ord : forall l1 l2 r1 r2,
shareTreeOrd l1 l2 ->
shareTreeOrd r1 r2 ->
shareTreeOrd (Node l1 r1) (Node l2 r2)
.
Hint Constructors shareTreeOrd : core.
Definition shareTreeEq (x y:ShareTree) :=
shareTreeOrd x y /\ shareTreeOrd y x.
Hint Unfold shareTreeEq : core.
Ltac destruct_bool :=
repeat (match goal with [ b:bool |- _ ] => destruct b end).
Ltac invert_ord :=
repeat (
match goal with
| [ H:shareTreeEq _ _ |- _ ] => destruct H
| [ H:shareTreeOrd (Leaf _) (Leaf _) |- _ ] => inversion H; clear H
| [ H:shareTreeOrd (Leaf _) (Node _ _) |- _ ] => inversion H; clear H
| [ H:shareTreeOrd (Node _ _) (Leaf _) |- _ ] => inversion H; clear H
| [ H:shareTreeOrd (Node _ _) (Node _ _) |- _ ] => inversion H; clear H
end; subst).
(* Utility lemmas about full and empty trees, and the top and bottom elements. *)
Hint Resolve geTrueFull leFalseEmpty emptyLeFalse fullGeTrue
falseLeaf_bottom trueLeaf_top : core.
Hint Resolve eqFalseLeaf_empty eqTrueLeaf_full emptyTree_canonical_falseLeaf
fullTree_canonical_trueLeaf : core.
(* Show that shareTreeOrd is a preorder (reflexive and transitive). *)
(* Show that shareTreeEq is an equivalance relation. *)
(* Show that mkCanon generates canonical trees that are equivalant to the original. *)
Hint Resolve mkCanon_nonEmpty mkCanon_correct mkCanon_eq : core.
(* Show that union and intersection are the LUB and GLB for the lattice, respectively. *)

Lemma union_least_bound : forall x y z,
shareTreeOrd x z ->
shareTreeOrd y z ->
shareTreeOrd (union_tree x y) z.