# dfa: Deterministic Finite Automata, decidability of language inclusion

Require Import comparisons positives ordinal pair lset.
Require Import monoid boolean prop sups bmx.
Set Implicit Arguments.
Unset Printing Implicit Defensive.

# DFA and associated language

A DFA is given by its number of states, a deterministic transition function, an acceptance condition, and a finite subset of the alphabet.
States are represented by ordinals of the appropriate size.
Making the finite subset of the alphabet explicit avoids us to use ordinals for the alphabet.

Record t := mk {
n: nat;
u: ord n;
M: ord n positive ord n;
v: ord n bool;
vars: list positive
}.
Notation "x ^u" := (u x) (at level 2, left associativity, format "x ^u").
Notation "x ^M" := (M x) (at level 2, left associativity, format "x ^M").
Notation "x ^v" := (v x) (at level 2, left associativity, format "x ^v").

changing the initial state
Definition reroot A i := mk i A^M A^v (vars A).

Lemma reroot_id A: A = reroot A (A^u).
Proof. destruct A; reflexivity. Qed.

language of a DFA A, starting from state i
Fixpoint lang A i w :=
match w with
| nilis_true (A^v i)
| cons a wIn a (vars A) lang A (A^M i a) w
end.

# Reduction of DFA language inclusion to DFA language emptiness

Section diff.

Variables A B: t.

automaton for A\B
Definition diff := mk
(pair.mk (u A) (u B))
(fun p apair.mk (M A (pair.pi1 p) a) (M B (pair.pi2 p) a))
(fun pv A (pair.pi1 p) ! v B (pair.pi2 p))
(vars A).

specification of its language
Lemma diff_spec: vars A vars B
i j, lang A i lang B j lang diff (pair.mk i j) bot.
Proof.
intro H.
cut ( w i j, lang A i w lang B j w ¬ lang diff (pair.mk i j) w).
intros G i j. split. intros Hij w Hw. apply G in Hw as []. apply Hij.
intros Hij w. apply G. intro Hw. elim (Hij _ Hw).
induction w; intros i j; simpl lang; rewrite pair.pi1mk, pair.pi2mk.
case (v A i); case (v B j); firstorder discriminate.
split. intros Hij [HaB Hw]. apply IHw in Hw as []. intro Aw. apply Hij. now split.
intros Hw [Ha Aw]. split. apply H, Ha. eapply IHw. 2: eassumption. tauto.
Qed.

End diff.

# Decidability of DFA language emptiness

We proceed as follows: 1. we forget all transition labels to get a directed graph whose nodes have an accepting status. 2. we compute the reflexive and transitive closure of this graph 3. we deduce the set of all states reachable from the initial state. 4. the DFA is empty iff this set does not contain any accepting states.
All these computations are straightforward, except for 2, for which we exploit Kleene star on Boolean matrices.
The resulting algorithm is not efficient at all. We don't care because this is not the one we execute in the end: this one is just used to establish KA completeness.

Section empty_dec.

Variables A: t.

erased transition graph, represented as a Boolean matrix
Definition step: bmx (n A) (n A) := fun i j\sup_(a\in vars A) eqb_ord (M A i a) j.

reflexive transitive closure of this graph
Definition steps := (@str bmx _ step).

Variable i: ord (n A).

basic properties of this closed graph
Lemma steps_refl: steps i i.
Proof. apply bmx_str_clot. constructor. Qed.

Lemma steps_snoc: j a, steps i j In a (vars A) steps i (M A j a).
Proof.
setoid_rewrite bmx_str_clot. intros. eapply clot_snoc. eassumption.
setoid_rewrite is_true_sup. eexists. split. eassumption. apply eqb_refl.
Qed.

state reached from i by following a word w in the DFA
Fixpoint Ms i w := match w with nili | cons a wMs (M A i a) w end.

each unlabelled path in the erased graph corresponds to a labelled path (word) in the DFA
Lemma steps_least: j, steps i j w, w vars A j = Ms i w.
Proof.
intros j H. apply bmx_str_clot in H. induction H as [i|i j k Hij _ [w [Hw ->]]].
nil. split. lattice. reflexivity.
setoid_rewrite is_true_sup in Hij. destruct Hij as [a [Ha Hij]].
(a::w). split. intros b [<-|Hb]. assumption. now apply Hw.
revert Hij. case eqb_ord_spec. 2: discriminate. now intros <-.
Qed.

can we reach an accepting state from i
Definition empty := \inf_(j<_) (steps i j <<< !v A j).

if not, all states reachable from i map to the empty language
Lemma empty_lang1 j: steps i j empty lang A j bot.
Proof.
intros Hj He. setoid_rewrite is_true_inf in He. setoid_rewrite le_bool_spec in He.
pose proof (fun iHe i (ordinal.in_seq _)) as H. clear He.
intro w. revert j Hj. induction w as [|a w IH]; simpl lang; intros j Hj.
apply (H j), negb_spec in Hj. rewrite Hj. discriminate.
intros [Ha Hj']. apply IH in Hj' as []. now apply steps_snoc.
Qed.

conversely, if i maps to them empty language, then there is no reachable accepting state
Lemma empty_lang2: lang A i bot empty.
Proof.
intro H. setoid_rewrite is_true_inf. intros j _.
rewrite le_bool_spec. intro Hj. apply steps_least in Hj as [w [Hw ->]].
generalize i (H w) Hw. clear. induction w; intros i Hi Hw.
simpl in ×. destruct (v A i). now elim Hi. reflexivity.
apply IHw. intro H. elim Hi. split. apply Hw. now left. assumption.
intros ? ?. apply Hw. now right.
Qed.

decidability of language emptiness follows
Theorem empty_dec: {lang A i bot} + {¬ (lang A i bot)}.
Proof.
case_eq empty; [left|right].
apply (empty_lang1 _ steps_refl H).
intro E. apply empty_lang2 in E. rewrite H in E. discriminate.
Qed.

End empty_dec.

# Decidability of DFA language inclusion

Corollary lang_incl_dec A B: vars A vars B
i j, {lang A i lang B j} + {~(lang A i lang B j)}.
Proof. intros. eapply sumbool_iff. symmetry. now apply diff_spec. apply empty_dec. Qed.