type rexp =

   ZERO 

 | ONE 

 | CHAR of char

 | ALT of rexp * rexp

 | SEQ of rexp * rexp 

 | STAR of rexp 

 | RECD of string * rexp;;

type value =

   Empty

 | Chr of char

 | Sequ of value * value

 | Left of value

 | Right of value

 | Stars of value list

 | Rec of string * value;;

(* some helper functions for strings *)   

let explode s = [for c in s -> c];;

let string_repeat s n =  String.replicate n s;;

(* some helper functions for rexps *)

let rec seq s = match s with

  | [] -> ONE

  | [c] -> CHAR(c)

  | c::cs -> SEQ(CHAR(c), seq cs);;

let chr c = CHAR(c)

let str s = seq(explode s);;

let plus r = SEQ(r, STAR(r));;

let (++) r1 r2 = ALT(r1, r2);;

let (--) r1 r2 = SEQ(r1, r2);;

let ($) x r = RECD(x, r);;

let alts rs = match rs with 

  | [] -> ZERO

  | [r] -> r

  | r::rs -> List.fold (++) r rs;;

(* size of a regular expressions - for testing purposes *)

let rec size r = match r with

  | ZERO -> 1

  | ONE -> 1

  | CHAR(_) -> 1

  | ALT(r1, r2) -> 1 + (size r1) + (size r2)

  | SEQ(r1, r2) -> 1 + (size r1) + (size r2)

  | STAR(r) -> 1 + (size r)

  | RECD(_, r) -> 1 + (size r);;

(* nullable function: tests whether the regular 

   expression can recognise the empty string *)

let rec nullable r = match r with

  | ZERO -> false

  | ONE -> true

  | CHAR(_) -> false

  | ALT(r1, r2) -> nullable(r1) || nullable(r2)

  | SEQ(r1, r2) -> nullable(r1) && nullable(r2)

  | STAR(_) -> true

  | RECD(_, r) -> nullable(r);;

(* derivative of a regular expression r w.r.t. a character c *)

let rec der c r = match r with 

  | ZERO -> ZERO

  | ONE -> ZERO

  | CHAR(d) -> if c = d then ONE else ZERO

  | ALT(r1, r2) -> ALT(der c r1, der c r2)

  | SEQ(r1, r2) -> 

      if nullable r1 then ALT(SEQ(der c r1, r2), der c r2)

      else SEQ(der c r1, r2)

  | STAR(r) -> SEQ(der c r, STAR(r))

  | RECD(_, r) -> der c r;;

(* derivative w.r.t. a list of chars (iterates der) *)

let rec ders s r = match s with 

  | [] -> r

  | c::s -> ders s (der c r);;

(* extracts a string from value *)

let rec flatten v = match v with 

  | Empty -> ""

  | Chr(c) -> System.Convert.ToString(c)

  | Left(v) -> flatten v

  | Right(v) -> flatten v

  | Sequ(v1, v2) -> flatten v1 ^ flatten v2

  | Stars(vs) -> String.concat "" (List.map flatten vs)

  | Rec(_, v) -> flatten v;;

(* extracts an environment from a value *)

let rec env v = match v with

  | Empty -> []

  | Chr(c) -> []

  | Left(v) -> env v

  | Right(v) -> env v

  | Sequ(v1, v2) -> env v1 @ env v2

  | Stars(vs) -> List.fold (@) [] (List.map env vs)

  | Rec(x, v) -> (x, flatten v) :: env v;;

let string_of_pair (x, s) = "(" ^ x ^ "," ^ s ^ ")";;

let string_of_env xs = String.concat "," (List.map string_of_pair xs);;

(* the value for a nullable rexp *)

let rec mkeps r = match r with

  | ONE -> Empty

  | ALT(r1, r2) -> 

      if nullable r1 then Left(mkeps r1) else Right(mkeps r2)

  | SEQ(r1, r2) -> Sequ(mkeps r1, mkeps r2)

  | STAR(r) -> Stars([])

  | RECD(x, r) -> Rec(x, mkeps r);;

(* injection of a char into a value *)

let rec inj r c v = match r, v with

  | STAR(r), Sequ(v1, Stars(vs)) -> Stars(inj r c v1 :: vs)

  | SEQ(r1, r2), Sequ(v1, v2) -> Sequ(inj r1 c v1, v2)

  | SEQ(r1, r2), Left(Sequ(v1, v2)) -> Sequ(inj r1 c v1, v2)

  | SEQ(r1, r2), Right(v2) -> Sequ(mkeps r1, inj r2 c v2)

  | ALT(r1, r2), Left(v1) -> Left(inj r1 c v1)

  | ALT(r1, r2), Right(v2) -> Right(inj r2 c v2)

  | CHAR(d), Empty -> Chr(d) 

  | RECD(x, r1), _ -> Rec(x, inj r1 c v);;

(* some "rectification" functions for simplification *)

let f_id v = v;;

let f_right f = fun v -> Right(f v);;

let f_left f = fun v -> Left(f v);;

let f_alt f1 f2 = fun v -> match v with 

    Right(v) -> Right(f2 v)

  | Left(v) -> Left(f1 v);;

let f_seq f1 f2 = fun v -> match v with 

  Sequ(v1, v2) -> Sequ(f1 v1, f2 v2);;

let f_seq_Empty1 f1 f2 = fun v -> Sequ(f1 Empty, f2 v);;

let f_seq_Empty2 f1 f2 = fun v -> Sequ(f1 v, f2 Empty);;

let f_rec f = fun v -> match v with

    Rec(x, v) -> Rec(x, f v);;

(* simplification of regular expressions returning also an 

   rectification function; no simplification under STARs *)

let rec simp r = match r with

    ALT(r1, r2) -> 

      let (r1s, f1s) = simp r1 in 

      let (r2s, f2s) = simp r2 in

      (match r1s, r2s with

          ZERO, _ -> (r2s, f_right f2s)

        | _, ZERO -> (r1s, f_left f1s)

        | _, _    -> if r1s = r2s then (r1s, f_left f1s)

                     else (ALT (r1s, r2s), f_alt f1s f2s)) 

  | SEQ(r1, r2) -> 

      let (r1s, f1s) = simp r1 in

      let (r2s, f2s) = simp r2 in

      (match r1s, r2s with

          ZERO, _  -> (ZERO, f_right f2s)

        | _, ZERO  -> (ZERO, f_left f1s)

        | ONE, _ -> (r2s, f_seq_Empty1 f1s f2s)

        | _, ONE -> (r1s, f_seq_Empty2 f1s f2s)

        | _, _     -> (SEQ(r1s, r2s), f_seq f1s f2s))

  | RECD(x, r1) -> 

      let (r1s, f1s) = simp r1 in

      (RECD(x, r1s), f_rec f1s)

  | r -> (r, f_id)

;;

(* matcher function *)

let matcher r s = nullable(ders (explode s) r);;

(* lexing function (produces a value) *)

exception LexError;;

let rec lex r s = match s with

    [] -> if (nullable r) then mkeps r else raise LexError

  | c::cs -> inj r c (lex (der c r) cs);;

let lexing r s = lex r (explode s);;

(* lexing with simplification *)

let rec lex_simp r s = match s with

    [] -> if (nullable r) then mkeps r else raise LexError

  | c::cs -> 

    let (r_simp, f_simp) = simp (der c r) in

    inj r c (f_simp (lex_simp r_simp cs));;

let lexing_simp r s = lex_simp r (explode s);;

(* Lexing rules for a small WHILE language *)

let sym = alts (List.map chr (explode "abcdefghijklmnopqrstuvwxyz"));;

let digit = alts (List.map chr (explode "0123456789"));;

let idents =  sym -- STAR(sym ++ digit);;

let nums = plus(digit);;

let keywords = alts (List.map str ["skip"; "while"; "do"; "if"; "then"; "else"; "read"; "write"; "true"; "false"]);;

let semicolon = str ";"

let ops = alts (List.map str [":="; "=="; "-"; "+"; "*"; "!="; "<"; ">"; "<="; ">="; "%"; "/"]);;

let whitespace = plus(str " " ++ str "\n" ++ str "\t");;

let rparen = str ")";;

let lparen = str "(";;

let begin_paren = str "{";;

let end_paren = str "}";;

let while_regs = STAR(("k" $ keywords) ++

                      ("i" $ idents) ++

                      ("o" $ ops) ++ 

                      ("n" $ nums) ++ 

                      ("s" $ semicolon) ++ 

                      ("p" $ (lparen ++ rparen)) ++ 

                      ("b" $ (begin_paren ++ end_paren)) ++ 

                      ("w" $ whitespace));;

(* Some Tests

  ============ *)

let time f x =

  let t = System.DateTime.Now in

  let f_x = f x in

  (printfn "%O" (System.DateTime.Now - t); f_x);;

let prog0 = "read n";;

string_of_env (env (lexing while_regs prog0));;

let prog1 = "read  n; write (n)";;

string_of_env (env (lexing_simp while_regs prog1));;

let prog2 = "

i := 2;

max := 100;

while i < max do {

  isprime := 1;

  j := 2;

  while (j * j) <= i + 1  do {

    if i % j == 0 then isprime := 0  else skip;

    j := j + 1

  };

  if isprime == 1 then write i else skip;

  i := i + 1

}";;

for i = 1 to 100 do

  printf "%i: " i ;

  time (lexing_simp while_regs) (string_repeat prog2 i);

done;;