(* Title: Turing machines

   Author: Xu Jian <xujian817@hotmail.com>

   Maintainer: Xu Jian

*)

theory turing_basic

imports Main

begin

section {* Basic definitions of Turing machine *}

datatype action = W0 | W1 | L | R | Nop

datatype cell = Bk | Oc

type_synonym tape = "cell list \<times> cell list"

type_synonym state = nat

type_synonym instr = "action \<times> state"

type_synonym tprog = "instr list \<times> nat"

type_synonym config = "state \<times> tape"

fun nth_of where

  "nth_of xs i = (if i \<ge> length xs then None

                  else Some (xs ! i))"

lemma nth_of_map [simp]:

  shows "nth_of (map f p) n = (case (nth_of p n) of None \<Rightarrow> None | Some x \<Rightarrow> Some (f x))"

apply(induct p arbitrary: n)

apply(auto)

apply(case_tac n)

apply(auto)

done

fun 

  fetch :: "instr list \<Rightarrow> state \<Rightarrow> cell \<Rightarrow> instr"

where

  "fetch p 0 b = (Nop, 0)"

| "fetch p (Suc s) Bk = 

     (case nth_of p (2 * s) of

        Some i \<Rightarrow> i

      | None \<Rightarrow> (Nop, 0))"

|"fetch p (Suc s) Oc = 

     (case nth_of p ((2 * s) + 1) of

         Some i \<Rightarrow> i

       | None \<Rightarrow> (Nop, 0))"

lemma fetch_Nil [simp]:

  shows "fetch [] s b = (Nop, 0)"

apply(case_tac s)

apply(auto)

apply(case_tac b)

apply(auto)

done

fun 

  update :: "action \<Rightarrow> tape \<Rightarrow> tape"

where 

  "update W0 (l, r) = (l, Bk # (tl r))" 

| "update W1 (l, r) = (l, Oc # (tl r))"

| "update L (l, r) = (if l = [] then ([], Bk # r) else (tl l, (hd l) # r))" 

| "update R (l, r) = (if r = [] then (Bk # l, []) else ((hd r) # l, tl r))" 

| "update Nop (l, r) = (l, r)"

abbreviation 

  "read r == if (r = []) then Bk else hd r"

fun step :: "config \<Rightarrow> tprog \<Rightarrow> config"

  where 

  "step (s, l, r) (p, off) = 

     (let (a, s') = fetch p (s - off) (read r) in (s', update a (l, r)))"

fun steps :: "config \<Rightarrow> tprog \<Rightarrow> nat \<Rightarrow> config"

  where

  "steps c p 0 = c" |

  "steps c p (Suc n) = steps (step c p) p n"

lemma step_red [simp]: 

  shows "steps c p (Suc n) = step (steps c p n) p"

by (induct n arbitrary: c) (auto)

lemma steps_add [simp]: 

  shows "steps c p (m + n) = steps (steps c p m) p n"

by (induct m arbitrary: c) (auto)

fun 

  tm_wf :: "tprog \<Rightarrow> bool"

where

  "tm_wf (p, off) = (length p \<ge> 2 \<and> length p mod 2 = 0 \<and> 

                    (\<forall>(a, s) \<in> set p. s \<le> length p div 2

                                             + off \<and> s \<ge> off))"

lemma halt_lemma: 

  "\<lbrakk>wf LE; \<forall>n. (\<not> P (f n) \<longrightarrow> (f (Suc n), (f n)) \<in> LE)\<rbrakk> \<Longrightarrow> \<exists>n. P (f n)"

by (metis wf_iff_no_infinite_down_chain)

abbreviation exponent :: "'a \<Rightarrow> nat \<Rightarrow> 'a list" ("_ \<up> _" [100, 99] 100)

  where "x \<up> n == replicate n x"

consts tape_of :: "'a \<Rightarrow> cell list" ("<_>" 100)

fun tape_of_nat_list :: "nat list \<Rightarrow> cell list" 

  where 

  "tape_of_nat_list [] = []" |

  "tape_of_nat_list [n] = Oc\<up>(Suc n)" |

  "tape_of_nat_list (n#ns) = Oc\<up>(Suc n) @ Bk # (tape_of_nat_list ns)"

defs (overloaded)

  tape_of_nl_abv: "<am> \<equiv> tape_of_nat_list am"

  tape_of_nat_abv : "<(n::nat)> \<equiv> Oc\<up>(Suc n)"

definition tinres :: "cell list \<Rightarrow> cell list \<Rightarrow> bool"

  where

  "tinres xs ys = (\<exists>n. xs = ys @ Bk \<up> n \<or> ys = xs @ Bk \<up> n)"

fun 

  shift :: "instr list \<Rightarrow> nat \<Rightarrow> instr list"

where

  "shift p n = (map (\<lambda> (a, s). (a, (if s = 0 then 0 else s + n))) p)"

lemma length_shift [simp]: 

  "length (shift p n) = length p"

by (simp)

fun 

  adjust :: "instr list \<Rightarrow> instr list"

where

  "adjust p = map (\<lambda> (a, s). (a, if s = 0 then (Suc (length p div 2)) else s)) p"

lemma length_adjust[simp]: 

  shows "length (adjust p) = length p"

by (induct p) (auto)

fun

  tm_comp :: "instr list \<Rightarrow> instr list \<Rightarrow> instr list" ("_ |+| _" [0, 0] 100)

where

  "tm_comp p1 p2 = ((adjust p1) @ (shift p2 ((length p1) div 2)))"

fun

  is_final :: "config \<Rightarrow> bool"

where

  "is_final (s, l, r) = (s = 0)"

lemma is_final_steps:

  assumes "is_final (s, l, r)"

  shows "is_final (steps (s, l, r) (p, off) n)"

using assms by (induct n) (auto)

fun 

  holds_for :: "(tape \<Rightarrow> bool) \<Rightarrow> config \<Rightarrow> bool" ("_ holds'_for _" [100, 99] 100)

where

  "P holds_for (s, l, r) = P (l, r)"  

(*

lemma step_0 [simp]: 

  shows "step (0, (l, r)) p = (0, (l, r))"

by simp

lemma steps_0 [simp]: 

  shows "steps (0, (l, r)) p n = (0, (l, r))"

by (induct n) (simp_all)

*)

lemma is_final_holds[simp]:

  assumes "is_final c"

  shows "Q holds_for (steps c (p, off) n) = Q holds_for c"

using assms 

apply(induct n)

apply(case_tac [!] c)

apply(auto)

done

type_synonym assert = "tape \<Rightarrow> bool"

definition 

  assert_imp :: "assert \<Rightarrow> assert \<Rightarrow> bool" ("_ \<mapsto> _" [0, 0] 100)

where

  "P \<mapsto> Q = (\<forall>l r. P (l, r) \<longrightarrow> Q (l, r))"

lemma holds_for_imp:

  assumes "P holds_for c"

  and "P \<mapsto> Q"

  shows "Q holds_for c"

using assms unfolding assert_imp_def by (case_tac c, auto)

definition

  Hoare :: "assert \<Rightarrow> tprog \<Rightarrow> assert \<Rightarrow> bool" ("({(1_)}/ (_)/ {(1_)})" 50)

where

  "{P} p {Q} \<equiv> 

     (\<forall>l r. P (l, r) \<longrightarrow> (\<exists>n. is_final (steps (1, (l, r)) p n) \<and> Q holds_for (steps (1, (l, r)) p n)))"

lemma HoareI:

  assumes "\<And>l r. P (l, r) \<Longrightarrow> \<exists>n. is_final (steps (1, (l, r)) p n) \<and> Q holds_for (steps (1, (l, r)) p n)"

  shows "{P} p {Q}"

unfolding Hoare_def using assms by auto

lemma step_0 [simp]: 

  shows "step (0, (l, r)) p = (0, (l, r))"

by (case_tac p, simp)

lemma steps_0 [simp]: 

  shows "steps (0, (l, r)) p n = (0, (l, r))"

by (induct n) (simp_all)

declare steps.simps[simp del]

  assumes "steps (1, tp) A n = (0, tp')"

  obtains n' where "\<not> is_final (steps (1, tp) A n')" 

        and "steps (1, tp) A (Suc n') = (0, tp')"

proof -

  from assms have "\<exists> n'. \<not> is_final (steps (1, tp) A n') \<and> 

               steps (1, tp) A (Suc n') = (0, tp')"

  proof(induct n arbitrary: tp', simp add: steps.simps)

    fix n tp'

    assume ind: 

      "\<And>tp'. steps (1, tp) A n = (0, tp') \<Longrightarrow>

      \<exists>n'. \<not> is_final (steps (1, tp) A n') \<and> steps (1, tp) A (Suc n') = (0, tp')"

    and h: " steps (1, tp) A (Suc n) = (0, tp')"

    from h show  "\<exists>n'. \<not> is_final (steps (1, tp) A n') \<and> steps (1, tp) A (Suc n') = (0, tp')"

    proof(simp add: step_red del: steps.simps, 

                     case_tac "(steps (Suc 0, tp) A n)", case_tac "a = 0", simp)

      fix a b c

      assume " steps (Suc 0, tp) A n = (0, tp')"

      hence "\<exists>n'. \<not> is_final (steps (1, tp) A n') \<and> steps (1, tp) A (Suc n') = (0, tp')"

        apply(rule_tac ind, simp)

        done

      thus "\<exists>n'. \<not> is_final (steps (Suc 0, tp) A n') \<and> step (steps (Suc 0, tp) A n') A = (0, tp')"

        apply(simp)

        done

    next

      fix a b c

      assume "steps (Suc 0, tp) A n = (a, b, c)"

             "step (steps (Suc 0, tp) A n) A = (0, tp')"

        "a \<noteq> 0"

      thus "\<exists>n'. \<not> is_final (steps (Suc 0, tp) A n') \<and> 

        step (steps (Suc 0, tp) A n') A = (0, tp')"

        done

    qed

  qed

  thus "(\<And>n'. \<lbrakk>\<not> is_final (steps (1, tp) A n'); 

    steps (1, tp) A (Suc n') = (0, tp')\<rbrakk> \<Longrightarrow> thesis) \<Longrightarrow> thesis"

    by auto

qed

declare tm_comp.simps [simp del] adjust.simps[simp del] shift.simps[simp del]

lemma length_comp:

"length (A |+| B) = length A + length B"

apply(auto simp: tm_comp.simps)

done

lemma tmcomp_fetch_in_first:

  assumes "case (fetch A a x) of (ac, ns) \<Rightarrow> ns \<noteq> 0"

  shows "fetch (A |+| B) a x = fetch A a x"

using assms

apply(case_tac a, case_tac [!] x, 

auto simp: length_comp tm_comp.simps length_adjust nth_append)

apply(simp_all add: adjust.simps)

done

lemma is_final_eq: "is_final (ba, tp) = (ba = 0)"

apply(case_tac tp, simp add: is_final.simps)

done

lemma t_merge_pre_eq_step: 

  assumes step: "step (a, b, c) (A, 0) = cf"

  and     tm_wf: "tm_wf (A, 0)" 

  and     unfinal: "\<not> is_final cf"

  shows "step (a, b, c) (A |+| B, 0) = cf"

proof -

  have "fetch (A |+| B) a (read c) = fetch A a (read c)"

  proof(rule_tac tmcomp_fetch_in_first)

    from step and unfinal show "case fetch A a (read c) of (ac, ns) \<Rightarrow> ns \<noteq> 0"

      apply(auto simp: is_final.simps)

      apply(case_tac "fetch A a (read c)", simp_all add: is_final_eq)

      done

  qed      

  thus "?thesis"

    using step

    apply(auto simp: step.simps is_final.simps)

    done

qed

declare tm_wf.simps[simp del] step.simps[simp del]

lemma t_merge_pre_eq:  

  "\<lbrakk>steps (Suc 0, tp) (A, 0) stp = cf; \<not> is_final cf; tm_wf (A, 0)\<rbrakk>

  \<Longrightarrow> steps (Suc 0, tp) (A |+| B, 0) stp = cf"

proof(induct stp arbitrary: cf, simp add: steps.simps)

  fix stp cf

  assume ind: "\<And>cf. \<lbrakk>steps (Suc 0, tp) (A, 0) stp = cf; \<not> is_final cf; tm_wf (A, 0)\<rbrakk> 

    \<Longrightarrow> steps (Suc 0, tp) (A |+| B, 0) stp = cf"

  and h: "steps (Suc 0, tp) (A, 0) (Suc stp) = cf"

      "\<not> is_final cf" "tm_wf (A, 0)"

  from h show "steps (Suc 0, tp) (A |+| B, 0) (Suc stp) = cf"

  proof(simp add: step_red, case_tac "(steps (Suc 0, tp) (A, 0) stp)", simp)

    fix a b c

    assume g: "steps (Suc 0, tp) (A, 0) stp = (a, b, c)"

      "step (a, b, c) (A, 0) = cf"

    have "(steps (Suc 0, tp) (A |+| B, 0) stp) = (a, b, c)"

    proof(rule ind, simp_all add: h g)

      show "0 < a"

        using g h

        apply(simp add: step_red)

        apply(case_tac a, auto simp: step_0)

        apply(case_tac "steps (Suc 0, tp) (A, 0) stp", simp)

        done

    qed

    thus "step (steps (Suc 0, tp) (A |+| B, 0) stp) (A |+| B, 0) = cf"

      apply(simp)

      apply(rule_tac t_merge_pre_eq_step, simp_all add: g h)

      done

  qed

qed

lemma tmcomp_fetch_in_first2:

  assumes "fetch A a x = (ac, 0)"

          "tm_wf (A, 0)"

          "a \<le> length A div 2" "a > 0"

  shows "fetch (A |+| B) a x = (ac, Suc (length A div 2))"

using assms

apply(case_tac a, case_tac [!] x, 

auto simp: length_comp tm_comp.simps length_adjust nth_append)

apply(simp_all add: adjust.simps)

done

lemma tmcomp_exec_after_first:

  "\<lbrakk>0 < a; step (a, b, c) (A, 0) = (0, tp'); tm_wf (A, 0); 

       a \<le> length A div 2\<rbrakk>

       \<Longrightarrow> step (a, b, c) (A |+| B, 0) = (Suc (length A div 2), tp')"

apply(simp add: step.simps, auto)

apply(case_tac "fetch A a Bk", simp add: tmcomp_fetch_in_first2)

apply(case_tac "fetch A a (hd c)", simp add: tmcomp_fetch_in_first2)

done

lemma step_nothalt_pre: "\<lbrakk>step (aa, ba, ca) (A, 0) = (a, b, c);  0 < a\<rbrakk> \<Longrightarrow> 0 < aa"

apply(case_tac "aa = 0", simp add: step_0, simp)

done

lemma nth_in_set: 

  "\<lbrakk> A ! i = x; i <  length A\<rbrakk> \<Longrightarrow> x \<in> set A"

by auto

lemma step_nothalt: 

  "\<lbrakk>step (aa, ba, ca) (A, 0) = (a, b, c); 0 < a; tm_wf (A, 0)\<rbrakk> \<Longrightarrow> 

  a \<le> length A div 2"

apply(simp add: step.simps)

apply(case_tac aa, case_tac [!] aa, auto split: if_splits simp: tm_wf.simps)

apply(case_tac "A ! (2 * nat)", simp)

apply(erule_tac x = "(aa, a)" in ballE, simp_all add: nth_in_set)

apply(case_tac "hd ca", auto split: if_splits simp: tm_wf.simps)

apply(case_tac "A ! (2 * nat)", simp)

apply(erule_tac x = "(aa, a)" in ballE, simp_all add: nth_in_set)

apply(case_tac "A ! (Suc (2 * nat))")

apply(erule_tac x = "(aa,bb)" in ballE, simp_all add: nth_in_set)

done

lemma steps_in_range: 

  " \<lbrakk>0 < a; steps (Suc 0, tp) (A, 0) stp = (a, b, c); tm_wf (A, 0)\<rbrakk>

  \<Longrightarrow> a \<le> length A div 2"

proof(induct stp arbitrary: a b c)

  fix a b c

  assume h: "0 < a" "steps (Suc 0, tp) (A, 0) 0 = (a, b, c)" 

            "tm_wf (A, 0)"

  thus "a \<le> length A div 2"

    apply(simp add: steps.simps tm_wf.simps, auto)

    done

next

  fix stp a b c

  assume ind: "\<And>a b c. \<lbrakk>0 < a; steps (Suc 0, tp) (A, 0) stp = (a, b, c); 

    tm_wf (A, 0)\<rbrakk> \<Longrightarrow> a \<le> length A div 2"

  and h: "0 < a" "steps (Suc 0, tp) (A, 0) (Suc stp) = (a, b, c)" "tm_wf (A, 0)"

  from h show "a \<le> length A div 2"

  proof(simp add: step_red, case_tac "(steps (Suc 0, tp) (A, 0) stp)", simp)

    fix aa ba ca

    assume g: "step (aa, ba, ca) (A, 0) = (a, b, c)" 

           "steps (Suc 0, tp) (A, 0) stp = (aa, ba, ca)"

    hence "aa \<le> length A div 2"

      apply(rule_tac ind, auto simp: h step_nothalt_pre)

      done

    thus "?thesis"

      using g h

      apply(rule_tac step_nothalt, auto)

      done

  qed

qed

lemma t_merge_pre_halt_same: 

  assumes a_ht: "steps (1, tp) (A, 0) n = (0, tp')"

  and a_wf: "tm_wf (A, 0)"

  obtains n' where "steps (1, tp) (A |+| B, 0) n' = (Suc (length A div 2), tp')"

proof -

  assume a: "\<And>n. steps (1, tp) (A |+| B, 0) n = (Suc (length A div 2), tp') \<Longrightarrow> thesis"

  obtain stp' where "\<not> is_final (steps (1, tp) (A, 0) stp')" and 

                          "steps (1, tp) (A, 0) (Suc stp') = (0, tp')"

  using a_ht before_final by blast

  then have "steps (1, tp) (A |+| B, 0) (Suc stp') = (Suc (length A div 2), tp')"

  proof(simp add: step_red)

    assume "\<not> is_final (steps (Suc 0, tp) (A, 0) stp')"

           " step (steps (Suc 0, tp) (A, 0) stp') (A, 0) = (0, tp')"

    moreover hence "(steps (Suc 0, tp) (A |+| B, 0) stp') = (steps (Suc 0, tp) (A, 0) stp')"

      apply(rule_tac t_merge_pre_eq)

      apply(simp_all add: a_wf a_ht)

      done

    ultimately show "step (steps (Suc 0, tp) (A |+| B, 0) stp') (A |+| B, 0) = (Suc (length A div 2), tp')"

      apply(case_tac " steps (Suc 0, tp) (A, 0) stp'", simp)

      apply(rule tmcomp_exec_after_first, simp_all add: a_wf)

      apply(erule_tac steps_in_range, auto simp: a_wf)

      done

  qed

  with a show thesis by blast

qed

lemma tm_comp_fetch_second_zero:

  "\<lbrakk>fetch B sa' x = (a, 0); tm_wf (A, 0); tm_wf (B, 0); sa' > 0\<rbrakk>

     \<Longrightarrow> fetch (A |+| B) (sa' + (length A div 2)) x = (a, 0)"

apply(case_tac x)

apply(case_tac [!] sa',

  auto simp: fetch.simps length_comp length_adjust nth_append tm_comp.simps

             tm_wf.simps shift.simps split: if_splits)

done 

lemma tm_comp_fetch_second_inst:

  "\<lbrakk>sa > 0; s > 0;  tm_wf (A, 0); tm_wf (B, 0); fetch B sa x = (a, s)\<rbrakk>

     \<Longrightarrow> fetch (A |+| B) (sa + length A div 2) x = (a, s + length A div 2)"

apply(case_tac x)

apply(case_tac [!] sa,

  auto simp: fetch.simps length_comp length_adjust nth_append tm_comp.simps

             tm_wf.simps shift.simps split: if_splits)

done 

lemma t_merge_second_same:

  assumes a_wf: "tm_wf (A, 0)"

  and b_wf: "tm_wf (B, 0)"

  and steps: "steps (Suc 0, l, r) (B, 0) stp = (s, l', r')"

  shows "steps (Suc (length A div 2), l, r)  (A |+| B, 0) stp

       = (if s = 0 then 0

          else s + length A div 2, l', r')"

using a_wf b_wf steps

proof(induct stp arbitrary: s l' r', simp add: steps.simps, simp)

  fix stpa sa l'a r'a

  assume ind: "\<And>s l' r'. steps (Suc 0, l, r) (B, 0) stpa = (s, l', r') \<Longrightarrow>

    steps (Suc (length A div 2), l, r) (A |+| B, 0) stpa = 

                (if s = 0 then 0 else s + length A div 2, l', r')"

  and h: "step (steps (Suc 0, l, r) (B, 0) stpa) (B, 0) = (sa, l'a, r'a)"

  obtain sa' l'' r'' where a: "(steps (Suc 0, l, r) (B, 0) stpa) = (sa', l'', r'')"

    apply(case_tac "steps (Suc 0, l, r) (B, 0) stpa", auto)

    done

  from this have b: "steps (Suc (length A div 2), l, r) (A |+| B, 0) stpa =