(* Title: thys/Turing.thy
   Author: Jian Xu, Xingyuan Zhang, and Christian Urban
*)

header {* Turing Machines *}

theory Turing2
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 \<times> instr) list"

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))"

fun 
  fetch :: "tprog \<Rightarrow> state \<Rightarrow> cell \<Rightarrow> instr"
where
  "fetch p 0 b = (Nop, 0)"
| "fetch p (Suc s) b = 
     (case nth_of p s of
        Some i \<Rightarrow> (case b of Bk \<Rightarrow> fst i | Oc \<Rightarrow> snd i)
      | None \<Rightarrow> (Nop, 0))"

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 = 
     (let (a, s') = fetch p s (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"

(* well-formedness of Turing machine programs *)

fun 
  tm_wf :: "tprog \<Rightarrow> bool"
where
  "tm_wf p = (length p \<ge> 1 \<and> (\<forall>((_, s1), (_, s2)) \<in> set p. s1 \<le> length p \<and> s2 \<le> length p))"

end