Rewrite using induction on list length.

This commit is contained in:
Danila Fedorin 2021-01-02 18:16:00 -08:00
parent 661bcbb557
commit 2b69cbd391

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@ -202,35 +202,54 @@ Module DayEight (Import M:Int).
left. split; auto. apply stuck_prog; auto.
(* A valid input always terminates, either by getting to the end of the program,
or by looping and thus getting stuck. *)
Program Fixpoint valid_input_terminates (pc : fin (S n)) (v : set (fin n)) (acc : t) (Hnd : List.NoDup v)
{ measure (length v) }:
(exists pc', run_noswap inp (pc, v, acc) pc') :=
match valid_input_progress pc v acc with
| or_introl (conj Heq Hdone) => _
| or_intror (ex_intro _ pcs (conj Hw w)) =>
match w with
| or_introl (conj Hnin Hstuck) => _
| or_intror (ex_intro _ pc' (ex_intro _ acc' (conj Hin Hst))) =>
match valid_input_terminates pc' (set_remove Fin.eq_dec pcs v) acc' (set_remove_nodup Fin.eq_dec pcs Hnd) with
| ex_intro _ pc'' Hrun => _
Obligation 1. eexists. apply run_noswap_ok. assumption. Qed.
Obligation 2. eexists. apply run_noswap_fail. assumption. Qed.
Obligation 3.
clear Heq_anonymous. clear valid_input_terminates. clear Hst.
induction v.
- inversion Hin.
- destruct (Fin.eq_dec pcs a) eqn:Heq_dec.
+ simpl. rewrite Heq_dec. lia.
+ inversion Hnd; subst.
inversion Hin. subst. exfalso. apply n0. auto.
specialize (IHv H2 H).
simpl. rewrite Heq_dec. simpl. lia.
Theorem list_length_induction {X : Type} (P : list X -> Prop) :
(forall l, (forall l', length l' < length l -> P l') -> P l) ->
forall l, P l.
intros Hrec.
assert (forall (l l' : list X), length l' <= length l -> P l').
{ induction l; intros l' Hlen; apply Hrec; intros l'0 Hlen0.
- simpl in Hlen. lia.
- apply IHl. simpl in Hlen. lia. }
intros l. apply H with l. lia.
Theorem set_remove_length : forall (f : fin n) (s : set (fin n)),
set_In f s ->
length (set_remove Fin.eq_dec f s) < length s.
intros f s Hin.
induction s.
- inversion Hin.
- simpl. destruct (Fin.eq_dec f a) eqn:Heq.
+ unfold lt. apply le_n. (* Why couldn't lia get this one? *)
+ inversion Hin; subst. exfalso. apply n0. auto.
apply IHs in H. simpl. lia.
Theorem valid_input_terminates : forall (pc : fin (S n)) (v : set (fin n)) (acc : t),
(exists pc', run_noswap inp (pc, v, acc) pc').
intros pc v. generalize dependent pc.
induction v using list_length_induction.
intros pc acc.
destruct (valid_input_progress pc l acc) as [[_ Hd]|[pc' [Hw [[_ Hst]|[pc'' [acc'' [Hin Hst]]]]]]].
- (* We're done. *)
eexists. apply run_noswap_ok. apply Hd.
- (* We're stuck. *)
eexists. apply run_noswap_fail. apply Hst.
- (* We can make a step. This will remove our current PC from the valid list, *)
edestruct (H (set_remove Fin.eq_dec pc' l)).
(* Since the PC must be in the list, removing it makes the list smaller. *)
apply (set_remove_length _ _ Hin).
(* Without the current PC, our valid set shrinks.
Since this is the inductive step, we have assumed
that programs with smaller sets of valid PCs always
terminate. Thus, after we make the step, we're done. *)
exists x. subst. eapply run_noswap_trans.
+ auto.
+ apply Hst.
+ apply H0.
Obligation 4. eexists. eapply run_noswap_trans; auto. apply Hst. apply Hrun. Qed.
End ValidInput.
End DayEight.