theory UF_Rec

begin

section {* Universal Function *}

text{* coding of the configuration *}

text {*

  @{text "Entry sr i"} returns the @{text "i"}-th entry of a list of natural 

  numbers encoded by number @{text "sr"} using Godel's coding.

  *}

fun Entry where

  "Entry sr i = Lo sr (Pi (Suc i))"

definition 

  "rec_entry = CN rec_lo [Id 2 0, CN rec_pi [CN S [Id 2 1]]]"

lemma entry_lemma [simp]:

  "rec_eval rec_entry [sr, i] = Entry sr i"

by(simp add: rec_entry_def)

fun Listsum2 :: "nat list \<Rightarrow> nat \<Rightarrow> nat"

  where

  "Listsum2 xs 0 = 0"

| "Listsum2 xs (Suc n) = Listsum2 xs n + xs ! n"

fun rec_listsum2 :: "nat \<Rightarrow> nat \<Rightarrow> recf"

  where

  "rec_listsum2 vl 0 = CN Z [Id vl 0]"

| "rec_listsum2 vl (Suc n) = CN rec_add [rec_listsum2 vl n, Id vl n]"

lemma listsum2_lemma [simp]: 

  "length xs = vl \<Longrightarrow> rec_eval (rec_listsum2 vl n) xs = Listsum2 xs n"

by (induct n) (simp_all)

text {*

  @{text "Strt"} corresponds to the @{text "strt"} function on page 90 of the 

  B book, but this definition generalises the original one to deal with multiple 

  input arguments.

  *}

fun Strt' :: "nat list \<Rightarrow> nat \<Rightarrow> nat"

  where

  "Strt' xs 0 = 0"

| "Strt' xs (Suc n) = (let dbound = Listsum2 xs n + n 

                       in Strt' xs n + (2 ^ (xs ! n + dbound) - 2 ^ dbound))"

fun Strt :: "nat list \<Rightarrow> nat"

  where

  "Strt xs = (let ys = map Suc xs in Strt' ys (length ys))"

fun rec_strt' :: "nat \<Rightarrow> nat \<Rightarrow> recf"

  where

  "rec_strt' xs 0 = Z"

| "rec_strt' xs (Suc n) = 

      (let dbound = CN rec_add [rec_listsum2 xs n, constn n] in

       let t1 = CN rec_power [constn 2, dbound] in

       let t2 = CN rec_power [constn 2, CN rec_add [Id xs n, dbound]] in

       CN rec_add [rec_strt' xs n, CN rec_minus [t2, t1]])"

fun rec_map :: "recf \<Rightarrow> nat \<Rightarrow> recf list"

  where

  "rec_map rf vl = map (\<lambda>i. CN rf [Id vl i]) [0..<vl]"

fun rec_strt :: "nat \<Rightarrow> recf"

  where

  "rec_strt xs = CN (rec_strt' xs xs) (rec_map S xs)"

lemma strt'_lemma [simp]:

  "length xs = vl \<Longrightarrow> rec_eval (rec_strt' vl n) xs = Strt' xs n"

by (induct n) (simp_all add: Let_def)

lemma map_suc:

  "map (\<lambda>x. Suc (xs ! x)) [0..<length xs] = map Suc xs"

proof -

  have "Suc \<circ> (\<lambda>x. xs ! x) = (\<lambda>x. Suc (xs ! x))" by (simp add: comp_def)

  then have "map (\<lambda>x. Suc (xs ! x)) [0..<length xs] = map (Suc \<circ> (\<lambda>x. xs ! x)) [0..<length xs]" by simp

  also have "... = map Suc (map (\<lambda>x. xs ! x) [0..<length xs])" by simp

  also have "... = map Suc xs" by (simp add: map_nth)

  finally show "map (\<lambda>x. Suc (xs ! x)) [0..<length xs] = map Suc xs" .

qed

lemma strt_lemma [simp]: 

  "length xs = vl \<Longrightarrow> rec_eval (rec_strt vl) xs = Strt xs"

by (simp add: comp_def map_suc[symmetric])

text {* The @{text "Scan"} function on page 90 of B book. *}

fun Scan :: "nat \<Rightarrow> nat"

  where

  "Scan r = r mod 2"

definition 

  "rec_scan = CN rec_rem [Id 1 0, constn 2]"

lemma scan_lemma [simp]: 

  "rec_eval rec_scan [r] = r mod 2"

by(simp add: rec_scan_def)

text {* The @{text Newleft} and @{text Newright} functions on page 91 of B book. *}

fun Newleft :: "nat \<Rightarrow> nat \<Rightarrow> nat \<Rightarrow> nat"

  where

  "Newleft p r a = (if a = 0 \<or> a = 1 then p 

                    else if a = 2 then p div 2

                    else if a = 3 then 2 * p + r mod 2

                    else p)"

fun Newright :: "nat \<Rightarrow> nat \<Rightarrow> nat \<Rightarrow> nat"

  where

  "Newright p r a  = (if a = 0 then r - Scan r

                      else if a = 1 then r + 1 - Scan r

                      else if a = 2 then 2 * r + p mod 2

                      else if a = 3 then r div 2

                      else r)"

definition

    "rec_newleft =

       (let cond1 = CN rec_disj [CN rec_eq [Id 3 2, Z], CN rec_eq [Id 3 2, constn 1]] in

        let cond2 = CN rec_eq [Id 3 2, constn 2] in

        let cond3 = CN rec_eq [Id 3 2, constn 3] in

        let case3 = CN rec_add [CN rec_mult [constn 2, Id 3 0], 

                                CN rec_rem [Id 3 1, constn 2]] in

        CN rec_if [cond1, Id 3 0, 

          CN rec_if [cond2, CN rec_quo [Id 3 0, constn 2],

            CN rec_if [cond3, case3, Id 3 0]]])"

definition

    "rec_newright =

       (let condn = \<lambda>n. CN rec_eq [Id 3 2, constn n] in

        let case0 = CN rec_minus [Id 3 1, CN rec_scan [Id 3 1]] in

        let case1 = CN rec_minus [CN rec_add [Id 3 1, constn 1], CN rec_scan [Id 3 1]] in

        let case2 = CN rec_add [CN rec_mult [constn 2, Id 3 1],                     

                                CN rec_rem [Id 3 0, constn 2]] in

        let case3 = CN rec_quo [Id 2 1, constn 2] in

        CN rec_if [condn 0, case0, 

          CN rec_if [condn 1, case1,

            CN rec_if [condn 2, case2,

              CN rec_if [condn 3, case3, Id 3 1]]]])"

lemma newleft_lemma [simp]:

  "rec_eval rec_newleft [p, r, a] = Newleft p r a"

by (simp add: rec_newleft_def Let_def)

lemma newright_lemma [simp]:

  "rec_eval rec_newright [p, r, a] = Newright p r a"

by (simp add: rec_newright_def Let_def)

text {*

  The @{text "Actn"} function given on page 92 of B book, which is used to 

  fetch Turing Machine intructions. In @{text "Actn m q r"}, @{text "m"} is 

  the Goedel coding of a Turing Machine, @{text "q"} is the current state of 

  Turing Machine, @{text "r"} is the right number of Turing Machine tape.

  *}

fun Actn :: "nat \<Rightarrow> nat \<Rightarrow> nat \<Rightarrow> nat"

  where

  "Actn m q r = (if q \<noteq> 0 then Entry m (4 * (q - 1) + 2 * Scan r) else 4)"

definition 

  "rec_actn = (let add1 = CN rec_mult [constn 4, CN rec_pred [Id 3 1]] in

               let add2 = CN rec_mult [constn 2, CN rec_scan [Id 3 2]] in

               let entry = CN rec_entry [Id 3 0, CN rec_add [add1, add2]]

               in CN rec_if [Id 3 1, entry, constn 4])"

lemma actn_lemma [simp]:

  "rec_eval rec_actn [m, q, r] = Actn m q r"

by (simp add: rec_actn_def)

fun Newstat :: "nat \<Rightarrow> nat \<Rightarrow> nat \<Rightarrow> nat"

  where

  "Newstat m q r = (if q \<noteq> 0 then Entry m (4 * (q - 1) + 2 * Scan r + 1) else 0)"

definition rec_newstat :: "recf"

  where

  "rec_newstat = (let add1 = CN rec_mult [constn 4, CN rec_pred [Id 3 1]] in

                  let add2 = CN S [CN rec_mult [constn 2, CN rec_scan [Id 3 2]]] in

                  let entry = CN rec_entry [Id 3 0, CN rec_add [add1, add2]]

                  in CN rec_if [Id 3 1, entry, Z])"

lemma newstat_lemma [simp]: 

  "rec_eval rec_newstat [m, q, r] = Newstat m q r"

by (simp add: rec_newstat_def)

fun Trpl :: "nat \<Rightarrow> nat \<Rightarrow> nat \<Rightarrow> nat"

  where

  "Trpl p q r = (Pi 0) ^ p * (Pi 1) ^ q * (Pi 2) ^ r"

definition 

  "rec_trpl = CN rec_mult [CN rec_mult 

       [CN rec_power [constn (Pi 0), Id 3 0], 

        CN rec_power [constn (Pi 1), Id 3 1]],

        CN rec_power [constn (Pi 2), Id 3 2]]"

lemma trpl_lemma [simp]: 

  "rec_eval rec_trpl [p, q, r] = Trpl p q r"

by (simp add: rec_trpl_def)

fun Left where

  "Left c = Lo c (Pi 0)"

definition

  "rec_left = CN rec_lo [Id 1 0, constn (Pi 0)]"

lemma left_lemma [simp]:

  "rec_eval rec_left [c] = Left c" 

by(simp add: rec_left_def)

fun Right where

  "Right c = Lo c (Pi 2)"

definition 

  "rec_right = CN rec_lo [Id 1 0, constn (Pi 2)]"

lemma right_lemma [simp]:

  "rec_eval rec_right [c] = Right c" 

by(simp add: rec_right_def)

fun Stat where

  "Stat c = Lo c (Pi 1)"

definition 

  "rec_stat = CN rec_lo [Id 1 0, constn (Pi 1)]"

lemma stat_lemma [simp]:

  "rec_eval rec_stat [c] = Stat c" 

by(simp add: rec_stat_def)

fun Inpt :: "nat \<Rightarrow> nat list \<Rightarrow> nat"

  where

  "Inpt m xs = Trpl 0 1 (Strt xs)"

fun Newconf :: "nat \<Rightarrow> nat \<Rightarrow> nat"

  where

  "Newconf m c = Trpl (Newleft (Left c) (Right c) (Actn m (Stat c) (Right c)))

                      (Newstat m (Stat c) (Right c)) 

                      (Newright (Left c) (Right c) (Actn m (Stat c) (Right c)))"

definition 

  "rec_newconf = (let act = CN rec_actn [Id 2 0, CN rec_stat [Id 2 1], CN rec_right [Id 2 1]] in

                  let left = CN rec_left [Id 2 1] in

                  let right = CN rec_right [Id 2 1] in

                  let stat = CN rec_stat [Id 2 1] in

                  let one = CN rec_newleft [left, right, act] in

                  let two = CN rec_newstat [Id 2 0, stat, right] in

                  let three = CN rec_newright [left, right, act]

                  in CN rec_trpl [one, two, three])" 

lemma newconf_lemma [simp]: 

  "rec_eval rec_newconf [m, c] = Newconf m c"

by (simp add: rec_newconf_def Let_def)

text {*

  @{text "Conf k m r"} computes the TM configuration after @{text "k"} steps of execution

  of TM coded as @{text "m"} starting from the initial configuration where the left 

  number equals @{text "0"}, right number equals @{text "r"}. 

  *}

fun Conf :: "nat \<Rightarrow> nat \<Rightarrow> nat \<Rightarrow> nat"

  where

  "Conf 0 m r  = Trpl 0 1 r"

| "Conf (Suc k) m r = Newconf m (Conf k m r)"

definition 

  "rec_conf = PR (CN rec_trpl [constn 0, constn 1, Id 2 1])

                 (CN rec_newconf [Id 4 2 , Id 4 1])"

lemma conf_lemma [simp]: 

  "rec_eval rec_conf [k, m, r] = Conf k m r"

by(induct k) (simp_all add: rec_conf_def)

text {*

  @{text "Nstd c"} returns true if the configuration coded 

  by @{text "c"} is not a stardard final configuration.

  *}

fun Nstd :: "nat \<Rightarrow> bool"

  where

  "Nstd c = (Stat c \<noteq> 0 \<or> 

             Left c \<noteq> 0 \<or> 

             Right c \<noteq> 2 ^ (Lg (Suc (Right c)) 2) - 1 \<or> 

             Right c = 0)"

definition 

  "rec_nstd = (let disj1 = CN rec_noteq [rec_stat, constn 0] in

               let disj2 = CN rec_noteq [rec_left, constn 0] in

               let rhs = CN rec_pred [CN rec_power [constn 2, CN rec_lg [CN S [rec_right], constn 2]]] in

               let disj3 = CN rec_noteq [rec_right, rhs] in

               let disj4 = CN rec_eq [rec_right, constn 0] in

               CN rec_disj [CN rec_disj [CN rec_disj [disj1, disj2], disj3], disj4])"

lemma nstd_lemma [simp]:

  "rec_eval rec_nstd [c] = (if Nstd c then 1 else 0)"

by(simp add: rec_nstd_def)

text{* 

  @{text "Nostop t m r"} means that afer @{text "t"} steps of 

  execution, the TM coded by @{text "m"} is not at a stardard 

  final configuration.

  *}

fun Nostop :: "nat \<Rightarrow> nat \<Rightarrow> nat \<Rightarrow> bool"

  where

  "Nostop t m r = Nstd (Conf t m r)"

definition 

  "rec_nostop = CN rec_nstd [rec_conf]"

lemma nostop_lemma [simp]:

  "rec_eval rec_nostop [t, m, r] = (if Nostop t m r then 1 else 0)" 

by (simp add: rec_nostop_def)

fun Value where

  "Value x = (Lg (Suc x) 2) - 1"

definition 

  "rec_value = CN rec_pred [CN rec_lg [S, constn 2]]"

lemma value_lemma [simp]:

  "rec_eval rec_value [x] = Value x"

by (simp add: rec_value_def)

text{*

  @{text "rec_halt"} is the recursive function calculating the steps a TM needs to execute before

  to reach a stardard final configuration. This recursive function is the only one

  using @{text "Mn"} combinator. So it is the only non-primitive recursive function 

  needs to be used in the construction of the universal function @{text "rec_uf"}.

  *}

definition 

  "rec_halt = MN rec_nostop" 

definition 

  "rec_uf = CN rec_value [CN rec_right [CN rec_conf [rec_halt, Id 2 0, Id 2 1]]]"