--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/thys3/src/RegLangs.thy	Sat Apr 30 00:50:08 2022 +0100
@@ -0,0 +1,236 @@
+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
+  | AltCH 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 (AltCH 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 (AltCH 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
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