import scala.language.implicitConversions    
import scala.language.reflectiveCalls
import scala.annotation.tailrec   

abstract class Rexp 
case object NULL extends Rexp
case object EMPTY extends Rexp
case class CHAR(c: Char) extends Rexp
case class ALT(r1: Rexp, r2: Rexp) extends Rexp 
case class SEQ(r1: Rexp, r2: Rexp) extends Rexp 
case class STAR(r: Rexp) extends Rexp 
case class RECD(x: String, r: Rexp) extends Rexp

abstract class Val
case object Void extends Val
case class Chr(c: Char) extends Val
case class Sequ(v1: Val, v2: Val) extends Val
case class Left(v: Val) extends Val
case class Right(v: Val) extends Val
case class Stars(vs: List[Val]) extends Val
case class Rec(x: String, v: Val) extends Val
   
// some convenience for typing in regular expressions
def charlist2rexp(s : List[Char]): Rexp = s match {
  case Nil => EMPTY
  case c::Nil => CHAR(c)
  case c::s => SEQ(CHAR(c), charlist2rexp(s))
}
implicit def string2rexp(s : String) : Rexp = charlist2rexp(s.toList)

implicit def RexpOps(r: Rexp) = new {
  def | (s: Rexp) = ALT(r, s)
  def % = STAR(r)
  def ~ (s: Rexp) = SEQ(r, s)
}

implicit def stringOps(s: String) = new {
  def | (r: Rexp) = ALT(s, r)
  def | (r: String) = ALT(s, r)
  def % = STAR(s)
  def ~ (r: Rexp) = SEQ(s, r)
  def ~ (r: String) = SEQ(s, r)
  def $ (r: Rexp) = RECD(s, r)
}

// size of a regular expressions - for testing purposes 
def size(r: Rexp) : Int = r match {
  case NULL => 1
  case EMPTY => 1
  case CHAR(_) => 1
  case ALT(r1, r2) => 1 + size(r1) + size(r2)
  case SEQ(r1, r2) => 1 + size(r1) + size(r2)
  case STAR(r) => 1 + size(r)
  case RECD(_, r) => 1 + size(r)
}


// nullable function: tests whether the regular 
// expression can recognise the empty string
def nullable (r: Rexp) : Boolean = r match {
  case NULL => false
  case EMPTY => true
  case CHAR(_) => false
  case ALT(r1, r2) => nullable(r1) || nullable(r2)
  case SEQ(r1, r2) => nullable(r1) && nullable(r2)
  case STAR(_) => true
  case RECD(_, r1) => nullable(r1)
}

// derivative of a regular expression w.r.t. a character
def der (c: Char, r: Rexp) : Rexp = r match {
  case NULL => NULL
  case EMPTY => NULL
  case CHAR(d) => if (c == d) EMPTY else NULL
  case ALT(r1, r2) => ALT(der(c, r1), der(c, r2))
  case SEQ(r1, r2) => 
    if (nullable(r1)) ALT(SEQ(der(c, r1), r2), der(c, r2))
    else SEQ(der(c, r1), r2)
  case STAR(r) => SEQ(der(c, r), STAR(r))
  case RECD(_, r1) => der(c, r1)
}

// derivative w.r.t. a string (iterates der)
def ders (s: List[Char], r: Rexp) : Rexp = s match {
  case Nil => r
  case c::s => ders(s, der(c, r))
}

// extracts a string from value
def flatten(v: Val) : String = v match {
  case Void => ""
  case Chr(c) => c.toString
  case Left(v) => flatten(v)
  case Right(v) => flatten(v)
  case Sequ(v1, v2) => flatten(v1) + flatten(v2)
  case Stars(vs) => vs.map(flatten).mkString
  case Rec(_, v) => flatten(v)
}

// extracts an environment from a value
def env(v: Val) : List[(String, String)] = v match {
  case Void => Nil
  case Chr(c) => Nil
  case Left(v) => env(v)
  case Right(v) => env(v)
  case Sequ(v1, v2) => env(v1) ::: env(v2)
  case Stars(vs) => vs.flatMap(env)
  case Rec(x, v) => (x, flatten(v))::env(v)
}

def mkeps_all(r: Rexp) : Set[Val] = r match {
  case EMPTY => Set(Void)
  case ALT(r1, r2) => (nullable(r1), nullable(r2)) match {
    case (true, true) => mkeps_all(r1).map(Left) ++ mkeps_all(r2).map(Right)
    case (true, false) => mkeps_all(r1).map(Left)
    case (false, true) => mkeps_all(r2).map(Right)
  }
  case SEQ(r1, r2) => for (v1 <- mkeps_all(r1);
                           v2 <- mkeps_all(r2)) yield Sequ(v1, v2)
  case STAR(r) => Set(Stars(Nil), Stars(List(mkeps(r))))
  case RECD(x, r) => for (v <-  mkeps_all(r)) yield Rec(x, v)
}

def inj(r: Rexp, c: Char, v: Val) : Val = (r, v) match {
  case (STAR(r), Sequ(v1, Stars(vs))) => Stars(inj(r, c, v1)::vs)
  case (SEQ(r1, r2), Sequ(v1, v2)) => Sequ(inj(r1, c, v1), v2)
  case (SEQ(r1, r2), Left(Sequ(v1, v2))) => Sequ(inj(r1, c, v1), v2)
  case (SEQ(r1, r2), Right(v2)) => Sequ(mkeps(r1), inj(r2, c, v2))
  case (ALT(r1, r2), Left(v1)) => Left(inj(r1, c, v1))
  case (ALT(r1, r2), Right(v2)) => Right(inj(r2, c, v2))
  case (CHAR(d), Void) => Chr(c) 
  case (RECD(x, r1), _) => Rec(x, inj(r1, c, v))
}

def inj_all(r: Rexp, c: Char, vs: Set[Val]) : Set[Val] =
  for (v <- vs) yield inj(r, c, v)

// main lexing function (produces a value)
def lex(r: Rexp, s: List[Char]) : Val = s match {
  case Nil => if (nullable(r)) mkeps(r) else throw new Exception("Not matched")
  case c::cs => inj(r, c, lex(der(c, r), cs))
}

def lexing(r: Rexp, s: String) : Val = lex(r, s.toList)

// Examples
val K: Rexp = "a" | "b"
val I: Rexp = "ab" | "ba"  

val R0 = (K | I).%

lexing(R0, "abab")

val K: Rexp = ("key" $ "a" | "b")
val I: Rexp = ("id" $ ("ab" | "ba"))  

val R0 = (K | I).%
lexing(R0, "abaa")
env(lexing(R0, "abaa"))

val r0: Rexp = (K | I).%
val r1 = der('a', r0)
val r1_simp = simp2(r1)
val r2 = der('b', r1)
val r2_simp = simp2(r2)
nullable(r2)
val v2 = mkeps(r2)
val v1 = inj(r1, 'b', v2)
val v0 = inj(r0, 'a', v1)
env(v0)
env(lexing(r0, "abab"))