theory RegLangs
  imports Main "HOL-Library.Sublist"
begin

section \<open>Sequential Composition of Languages\<close>

definition
  Sequ :: "string set \<Rightarrow> string set \<Rightarrow> string set" ("_ ;; _" [100,100] 100)
where 
  "A ;; B = {s1 @ s2 | s1 s2. s1 \<in> A \<and> s2 \<in> B}"

text \<open>Two Simple Properties about Sequential Composition\<close>

lemma Sequ_empty_string [simp]:
  shows "A ;; {[]} = A"
  and   "{[]} ;; A = A"
by (simp_all add: Sequ_def)

lemma Sequ_empty [simp]:
  shows "A ;; {} = {}"
  and   "{} ;; A = {}"
  by (simp_all add: Sequ_def)


section \<open>Semantic Derivative (Left Quotient) of Languages\<close>

definition
  Der :: "char \<Rightarrow> string set \<Rightarrow> string set"
where
  "Der c A \<equiv> {s. c # s \<in> A}"

definition
  Ders :: "string \<Rightarrow> string set \<Rightarrow> string set"
where
  "Ders s A \<equiv> {s'. s @ s' \<in> A}"

lemma Der_null [simp]:
  shows "Der c {} = {}"
unfolding Der_def
by auto

lemma Der_empty [simp]:
  shows "Der c {[]} = {}"
unfolding Der_def
by auto

lemma Der_char [simp]:
  shows "Der c {[d]} = (if c = d then {[]} else {})"
unfolding Der_def
by auto

lemma Der_union [simp]:
  shows "Der c (A \<union> B) = Der c A \<union> Der c B"
unfolding Der_def
by auto

lemma Der_Sequ [simp]:
  shows "Der c (A ;; B) = (Der c A) ;; B \<union> (if [] \<in> A then Der c B else {})"
unfolding Der_def Sequ_def
by (auto simp add: Cons_eq_append_conv)


section \<open>Kleene Star for Languages\<close>

inductive_set
  Star :: "string set \<Rightarrow> string set" ("_\<star>" [101] 102)
  for A :: "string set"
where
  start[intro]: "[] \<in> A\<star>"
| step[intro]:  "\<lbrakk>s1 \<in> A; s2 \<in> A\<star>\<rbrakk> \<Longrightarrow> s1 @ s2 \<in> A\<star>"

(* Arden's lemma *)

lemma Star_cases:
  shows "A\<star> = {[]} \<union> A ;; A\<star>"
unfolding Sequ_def
by (auto) (metis Star.simps)

lemma Star_decomp: 
  assumes "c # x \<in> A\<star>" 
  shows "\<exists>s1 s2. x = s1 @ s2 \<and> c # s1 \<in> A \<and> s2 \<in> A\<star>"
using assms
by (induct x\<equiv>"c # x" rule: Star.induct) 
   (auto simp add: append_eq_Cons_conv)

lemma Star_Der_Sequ: 
  shows "Der c (A\<star>) \<subseteq> (Der c A) ;; A\<star>"
unfolding Der_def Sequ_def
by(auto simp add: Star_decomp)


lemma Der_star[simp]:
  shows "Der c (A\<star>) = (Der c A) ;; A\<star>"
proof -    
  have "Der c (A\<star>) = Der c ({[]} \<union> A ;; A\<star>)"  
    by (simp only: Star_cases[symmetric])
  also have "... = Der c (A ;; A\<star>)"
    by (simp only: Der_union Der_empty) (simp)
  also have "... = (Der c A) ;; A\<star> \<union> (if [] \<in> A then Der c (A\<star>) else {})"
    by simp
  also have "... =  (Der c A) ;; A\<star>"
    using Star_Der_Sequ by auto
  finally show "Der c (A\<star>) = (Der c A) ;; A\<star>" .
qed

lemma Star_concat:
  assumes "\<forall>s \<in> set ss. s \<in> A"  
  shows "concat ss \<in> A\<star>"
using assms by (induct ss) (auto)

lemma Star_split:
  assumes "s \<in> A\<star>"
  shows "\<exists>ss. concat ss = s \<and> (\<forall>s \<in> set ss. s \<in> A \<and> s \<noteq> [])"
using assms
  apply(induct rule: Star.induct)
  using concat.simps(1) apply fastforce
  apply(clarify)
  by (metis append_Nil concat.simps(2) set_ConsD)



section \<open>Regular Expressions\<close>

datatype rexp =
  ZERO
| ONE
| CH char
| SEQ rexp rexp
| ALT rexp rexp
| STAR rexp

section \<open>Semantics of Regular Expressions\<close>
 
fun
  L :: "rexp \<Rightarrow> string set"
where
  "L (ZERO) = {}"
| "L (ONE) = {[]}"
| "L (CH c) = {[c]}"
| "L (SEQ r1 r2) = (L r1) ;; (L r2)"
| "L (ALT r1 r2) = (L r1) \<union> (L r2)"
| "L (STAR r) = (L r)\<star>"


section \<open>Nullable, Derivatives\<close>

fun
 nullable :: "rexp \<Rightarrow> bool"
where
  "nullable (ZERO) = False"
| "nullable (ONE) = True"
| "nullable (CH c) = False"
| "nullable (ALT r1 r2) = (nullable r1 \<or> nullable r2)"
| "nullable (SEQ r1 r2) = (nullable r1 \<and> nullable r2)"
| "nullable (STAR r) = True"


fun
 der :: "char \<Rightarrow> rexp \<Rightarrow> rexp"
where
  "der c (ZERO) = ZERO"
| "der c (ONE) = ZERO"
| "der c (CH d) = (if c = d then ONE else ZERO)"
| "der c (ALT r1 r2) = ALT (der c r1) (der c r2)"
| "der c (SEQ r1 r2) = 
     (if nullable r1
      then ALT (SEQ (der c r1) r2) (der c r2)
      else SEQ (der c r1) r2)"
| "der c (STAR r) = SEQ (der c r) (STAR r)"

fun 
 ders :: "string \<Rightarrow> rexp \<Rightarrow> rexp"
where
  "ders [] r = r"
| "ders (c # s) r = ders s (der c r)"


lemma nullable_correctness:
  shows "nullable r  \<longleftrightarrow> [] \<in> (L r)"
by (induct r) (auto simp add: Sequ_def) 

lemma der_correctness:
  shows "L (der c r) = Der c (L r)"
by (induct r) (simp_all add: nullable_correctness)

lemma ders_correctness:
  shows "L (ders s r) = Ders s (L r)"
  by (induct s arbitrary: r)
     (simp_all add: Ders_def der_correctness Der_def)

lemma ders_append:
  shows "ders (s1 @ s2) r = ders s2 (ders s1 r)"
  by (induct s1 arbitrary: s2 r) (auto)

lemma ders_snoc:
  shows "ders (s @ [c]) r = der c (ders s r)"
  by (simp add: ders_append)


(*
datatype ctxt = 
    SeqC rexp bool
  | AltCL rexp
  | AltCR rexp 
  | StarC rexp 

function
     down :: "char \<Rightarrow> rexp \<Rightarrow> ctxt list \<Rightarrow> rexp * ctxt list"
and  up :: "char \<Rightarrow> rexp \<Rightarrow> ctxt list \<Rightarrow> rexp * ctxt list"
where
  "down c (SEQ r1 r2) ctxts =
     (if (nullable r1) then down c r1 (SeqC r2 True # ctxts) 
      else down c r1 (SeqC r2 False # ctxts))"
| "down c (CH d) ctxts = 
     (if c = d then up c ONE ctxts else up c ZERO ctxts)"
| "down c ONE ctxts = up c ZERO ctxts"
| "down c ZERO ctxts = up c ZERO ctxts"
| "down c (ALT r1 r2) ctxts = down c r1 (AltCR r2 # ctxts)"
| "down c (STAR r1) ctxts = down c r1 (StarC r1 # ctxts)"
| "up c r [] = (r, [])"
| "up c r (SeqC r2 False # ctxts) = up c (SEQ r r2) ctxts"
| "up c r (SeqC r2 True # ctxts) = down c r2 (AltCL (SEQ r r2) # ctxts)"
| "up c r (AltCL r1 # ctxts) = up c (ALT r1 r) ctxts"
| "up c r (AltCR r2 # ctxts) = down c r2 (AltCL r # ctxts)"
| "up c r (StarC r1 # ctxts) = up c (SEQ r (STAR r1)) ctxts"
  apply(pat_completeness)
  apply(auto)
  done

termination
  sorry

*)


end