import scala.annotation.tailrec    

import scala.language.implicitConversions

abstract class Rexp {

  def simp : Rexp = this

}

case object ZERO extends Rexp

case object ONE extends Rexp

case class CHAR(c: Char) extends Rexp

case class ALT(r1: Rexp, r2: Rexp) extends Rexp {

  override def simp = (r1.simp, r2.simp) match {

    case (ZERO, r) => r

    case (r, ZERO) => r

    case (r, ONE) => if (nullable(r)) r else ALT(r, ONE)

    case (ONE, r) => if (nullable(r)) r else ALT(r, ONE)

    case (r1, r2) => if (r1 == r2) r1 else ALT(r1, r2)

  }

}

case class SEQ(r1: Rexp, r2: Rexp) extends Rexp {

  override def simp = (r1.simp, r2.simp) match {

    case (ZERO, _) => ZERO

    case (_, ZERO) => ZERO

    case (ONE, r) => r

    case (r, ONE) => r

    case (r1, r2) => SEQ(r1, r2)

  }

}

case class STAR(r: Rexp) extends Rexp {

  override def simp = r.simp match {

    case ZERO => ONE

    case ONE => ONE

    case r => STAR(r)

  }

}

case class NTIMES(r: Rexp, n: Int) extends Rexp {

  override def simp = if (n == 0) ONE else 

    r.simp match {

      case ZERO => ZERO

      case ONE => ONE

      case r => NTIMES(r, n)

    }

}

// some convenience for typing in regular expressions

def charlist2rexp(s : List[Char]) : Rexp = s match {

  case Nil => ONE

  case c::Nil => CHAR(c)

  case c::s => SEQ(CHAR(c), charlist2rexp(s))

}

implicit def string2rexp(s : String) : Rexp = charlist2rexp(s.toList)

// nullable function: tests whether the regular 

// expression can recognise the empty string

def nullable (r: Rexp) : Boolean = r match {

  case ZERO => false

  case ONE => true

  case CHAR(_) => false

  case ALT(r1, r2) => nullable(r1) || nullable(r2)

  case SEQ(r1, r2) => nullable(r1) && nullable(r2)

  case STAR(_) => true

  case NTIMES(r, i) => if (i == 0) true else nullable(r)

}

// derivative of a regular expression w.r.t. a character

def der (c: Char, r: Rexp) : Rexp = r match {

  case ZERO => ZERO

  case ONE => ZERO

  case CHAR(d) => if (c == d) ONE else ZERO

  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 NTIMES(r, i) => 

    if (i == 0) ZERO else SEQ(der(c, r), NTIMES(r, i - 1))

}

// derivative w.r.t. a string (iterates der)

@tailrec

def ders (s: List[Char], r: Rexp) : Rexp = s match {

  case Nil => r

  case c::s => ders(s, der(c, r).simp)

}

def ders2 (s: List[Char], r: Rexp) : Rexp = (s, r) match {

  case (Nil, r) => r

  case (s, ZERO) => ZERO

  case (s, ONE) => if (s == Nil) ONE else ZERO

  case (s, CHAR(c)) => if (s == List(c)) ONE else 

                       if (s == Nil) CHAR(c) else ZERO

  case (s, ALT(r1, r2)) => ALT(ders2(s, r2), ders2(s, r2))

  case (c::s, r) => ders2(s, der(c, r).simp)

}

// main matcher function

def matcher(r: Rexp, s: String) : Boolean = nullable(ders2(s.toList, r))

//one or zero

def OPT(r: Rexp) = ALT(r, ONE)

def EVIL1(n: Int) = SEQ(NTIMES(OPT("a"), n), NTIMES("a", n))

val EVIL2 = SEQ(STAR("a"), "b")

def time_needed[T](i: Int, code: => T) = {

  val start = System.nanoTime()

  for (j <- 1 to i) code

  val end = System.nanoTime()

  (end - start)/(i * 1.0e9)

}

val i = 5000

println(i + " " + "%.5f".format(time_needed(10, matcher(EVIL1(i), "a" * i))))

//for (i <- 1 to 10000 by 1000) {

//  println(i + " " + "%.5f".format(time_needed(10, matcher(EVIL1(i), "a" * i))))

//}

for (i <- 1 to 6000000 by 500000) {

  println(i + " " + "%.5f".format(time_needed(10, matcher(EVIL2, "a" * i))))

}