Section Code.
(* Arbitrary set *)
Variable A : Type.
(* Equality is decidable *)
Variable eqA_dec : forall a b : A, {a = b} + {a <> b}.
(* A code is an association list *)
Definition code := list (A * list bool).
(* Predicate that tells that a message (a list of message) is in the alphabet of the code *)
Definition in_alphabet (m : list A) (c : code) :=
forall a : A, In a m -> exists l : list bool, In (a, l) c.
Proof using.
intros m1 m2 c H H0; red in |- *.
intros a H1; case (H0 a); auto.
Qed.
Proof using.
intros c a H; inversion H.
Qed.
Hint Resolve in_alphabet_nil : core.
Proof using.
intros m c a ca H H0; red in |- *; simpl in |- *.
intros a1 [H1| H1].
exists ca; rewrite <- H1; auto.
case (H0 a1); auto.
Qed.
Hint Resolve in_alphabet_cons : core.
Proof using.
intros c a l H; red in |- *.
intros a1 H1; apply H; simpl in |- *; auto.
Qed.
Definition code_dec :
forall (c : code) a,
{(exists l : list bool, In (a, l) c)} +
{(forall l : list bool, ~ In (a, l) c)}.
intros c a; elim c; simpl in |- *; auto.
intros (a1, l1) l H; case (eqA_dec a1 a); intros H1.
left; exists l1; rewrite H1; auto.
case H.
intros e; left; case e; intros l2 H2; exists l2; auto.
intros n; right; intros l0; red in |- *; intros [H0| H0];
[ case H1 | case (n l0) ]; auto.
injection H0; auto.
Defined.
(* Checking if a code is in an alphabet is decidable *)
Definition in_alphabet_dec :
forall m c, {in_alphabet m c} + {~ in_alphabet m c}.
intros m; elim m; simpl in |- *; auto.
intros a l H c; case (H c); intros H1.
case (code_dec c a); intros H2.
left; red in |- *; simpl in |- *.
intros a1 [H3| H3];
[ case H2; intros l1 Hl1; exists l1; rewrite <- H3 | idtac ];
auto.
right; red in |- *; intros H3; case (H3 a); simpl in |- *; auto.
right; Contradict H1; auto; red in |- *.
intros a0 H0; case (H1 a0); simpl in |- *; auto.
intros x H2; exists x; auto.
Defined.
(* The empty list is associated to none of element of A in the code *)
Definition not_null (c : code) := forall a : A, ~ In (a, nil) c.
Proof using.
intros a l H; red in |- *.
intros a0; red in |- *; intros H0; case (H a0); simpl in |- *; auto.
Qed.
Proof using.
intros a b l H H0; red in |- *.
intros a1; simpl in |- *; red in |- *; intros [H1| H1]; auto.
case H; injection H1; auto.
case (H0 a1); simpl in |- *; auto.
Qed.
Hint Resolve not_null_cons : core.
Proof using.
intros l1; elim l1; simpl in |- *; auto.
intros (a2, l2); case l2; auto.
intros l H l0 H0 H1; case (H0 a2); simpl in |- *; auto.
intros b l l0 H l3 H0 H1; apply not_null_cons; auto.
red in |- *; intros H2; discriminate.
apply H; auto.
apply not_null_inv with (1 := H0).
Qed.
Hint Resolve not_null_app : core.
Proof using.
intros l; elim l; simpl in |- *; auto.
intros b; red in |- *; intros a; red in |- *; intros H; inversion H.
intros (a1, l1) l0 H b; apply not_null_cons; auto.
red in |- *; intros; discriminate.
Qed.
Hint Resolve not_null_map : core.
(* Define the property of list of booleans to be a prefix of another *)
Inductive is_prefix : list bool -> list bool -> Prop :=
| prefixNull : forall l, is_prefix nil l
| prefixCons :
forall (b : bool) l1 l2,
is_prefix l1 l2 -> is_prefix (b :: l1) (b :: l2).
Hint Constructors is_prefix : core.
Proof using.
intros l; elim l; simpl in |- *; auto.
Qed.
Hint Resolve is_prefix_refl : core.
(* A code is unique_prefix, if there is no encoding that is prefix of another *)
Definition unique_prefix (l : code) :=
(forall (a1 a2 : A) (lb1 lb2 : list bool),
In (a1, lb1) l -> In (a2, lb2) l -> is_prefix lb1 lb2 -> a1 = a2) /\
unique_key l.
Proof using.
split; auto.
intros a1 a2 lb1 lb2 H; inversion H; auto.
Qed.
Hint Resolve unique_prefix_nil : core.
Proof using.
intros c a1 a2 lb1 lb2 (H1, H2); apply (H1 a1 a2 lb1 lb2); auto.
Qed.

(* A unique prefix code has unique keys *)
Theorem unique_prefix2 : forall c : code, unique_prefix c -> unique_key c.