From Undecidability.TM Require Import ProgrammingTools.
From Undecidability Require Import BinNumbers.EncodeBinNumbers.
From Undecidability Require Import BinNumbers.PosDefinitions.
Local Open Scope positive_scope.
Definition atBit (t : tape sigPos^+) (lp : positive) (mb : bool) (rs : list bool) :=
exists (ls : list sigPos^+),
t = midtape (map inr (rev (encode lp)) ++ inl START :: ls) (bitToSigPos' mb) (map bitToSigPos' rs ++ [inl STOP]).
Definition atLSB (t : tape sigPos^+) (lp : positive) (mb : bool) := atBit t lp mb nil.
Lemma atLSB_moveRight_contains_rev (t : tape sigPos^+) (lp : positive) (mb : bool) :
atLSB t lp mb ->
tape_move t Rmove ≂ lp ~~ mb.
Proof.
intros (ls&->); cbn. hnf; destruct mb; cbn; eexists; f_equal.
- unfold bitToSigPos'; cbn. simpl_list. rewrite <- map_rev; cbn. simpl_list. f_equal. f_equal.
rewrite !map_rev. f_equal. now rewrite map_id.
- unfold bitToSigPos'; cbn. simpl_list. rewrite <- map_rev; cbn. simpl_list. f_equal. f_equal.
rewrite !map_rev. f_equal. now rewrite map_id.
Qed.
Lemma atBit_moveRight_cons (t : tape sigPos^+) (lp : positive) (b b' : bool) (rs : list bool) :
atBit t lp b (b' :: rs) ->
atBit (tape_move t Rmove) (lp ~~ b) b' rs.
Proof.
intros (ls&->). hnf. cbn.
rewrite Encode_positive_app_xIO. rewrite rev_app_distr. cbn.
destruct b; cbn.
- eexists. f_equal.
- eexists. f_equal.
Qed.
Lemma atBit_moveRight (t : tape sigPos^+) (lp : positive) (b : bool) (rs : list bool) :
atBit t lp b rs ->
match rs with
| nil => tape_move t Rmove ≂ lp ~~ b
| b' :: rs' => atBit (tape_move t Rmove) (lp ~~ b) b' rs'
end.
Proof.
intros H. destruct rs as [ | b' rs'].
- now apply atLSB_moveRight_contains_rev.
- now apply atBit_moveRight_cons.
Qed.
Lemma contains_rev_moveLeft_atLSB (t : tape sigPos^+) (lp : positive) (mb : bool) :
t ≂ (lp ~~ mb) ->
atLSB (tape_move t Lmove) lp mb.
Proof.
intros (ls&->); cbn. hnf.
rewrite Encode_positive_app_xIO.
rewrite <- !map_rev.
destruct mb; cbn.
- rewrite rev_app_distr; cbn. eexists. f_equal. f_equal. f_equal. now rewrite map_id.
- rewrite rev_app_distr; cbn. eexists. f_equal. f_equal. f_equal. now rewrite map_id.
Qed.
Lemma atBit_moveLeft_cons (t : tape sigPos^+) (lp : positive) (b b' : bool) (rs : list bool) :
atBit t (lp ~~ b) b' rs ->
atBit (tape_move t Lmove) lp b (b' :: rs).
Proof.
intros (ls&->); cbn. hnf.
rewrite Encode_positive_app_xIO.
destruct b; cbn.
- exists ls. simpl_list. cbn. f_equal.
- exists ls. simpl_list. cbn. f_equal.
Qed.
Definition atHSB (t : tape sigPos^+) (p : positive) :=
exists (ls : list sigPos^+),
t = midtape (inl START :: ls) (inr sigPos_xH) (map inr (tl (encode p)) ++ [inl STOP]).
Lemma atHSB_moveLeft_contains (t : tape sigPos^+) (p : positive) :
atHSB t p ->
tape_move t Lmove ≃ p.
Proof.
intros (ls&->). hnf. cbn.
pose proof Encode_positive_startsWith_xH p as (str'&H); setoid_rewrite H; clear H; cbn.
eexists. f_equal. f_equal. now rewrite map_id.
Qed.
Lemma contains_moveRight_atHSB (t : tape sigPos^+) (p : positive) :
t ≃ p ->
atHSB (tape_move t Rmove) p.
Proof.
intros (ls&->). hnf. cbn.
pose proof Encode_positive_startsWith_xH p as (str'&H); setoid_rewrite H; clear H; cbn.
eexists. f_equal. f_equal. now rewrite map_id.
Qed.
Lemma atBit_moveLeft_atHSB (t : tape sigPos^+) (b : bool) (rs : list bool) :
atBit t 1 b rs ->
atHSB (tape_move t Lmove) (bits_to_pos (b :: rs)).
Proof.
intros (ls&->). cbn -[bits_to_pos]. hnf. exists ls. f_equal. cbn -[bits_to_pos]. rewrite app_comm_cons. f_equal.
rewrite encode_bits_to_pos. cbn. f_equal. now rewrite map_map.
Qed.
Lemma atBit_ext (t : tape sigPos^+) (p0 : positive) (b0 : bool) (bits0 : list bool) (p1 : positive) (b1 : bool) (bits1 : list bool) :
atBit t p0 b0 bits0 ->
p0 = p1 ->
b0 = b1 ->
bits0 = bits1 ->
atBit t p1 b1 bits1.
Proof. now intros H -> -> ->. Qed.
Lemma atLSB_ext (t : tape sigPos^+) (p0 : positive) (b0 : bool) (p1 : positive) (b1 : bool) :
atLSB t p0 b0 ->
p0 = p1 ->
b0 = b1 ->
atLSB t p1 b1 .
Proof. now intros H -> ->. Qed.
Lemma atHSB_ext (t : tape sigPos^+) (p0 : positive) (p1 : positive) :
atHSB t p0 ->
p0 = p1 ->
atHSB t p1.
Proof. now intros H ->. Qed.
Ltac atBit_ext :=
once lazymatch goal with
| [ H : atBit ?t ?p0 ?b0 ?bits0 |- atBit ?t ?p1 ?b1 ?bits0 ] => apply (atBit_ext H); auto
| [ H : atLSB ?t ?p0 ?b0 |- atLSB ?t ?p1 ?b1 ] => apply (atLSB_ext H); auto
| [ H : atHSB ?t ?p0 |- atHSB ?t ?p1 ] => apply (atHSB_ext H); auto
end.