// regular expressions
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 NOT(r: Rexp) extends Rexp


// 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)


// 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 NOT(r) => !(nullable(r))
}

// tests whether a regular expression 
// recognises nothing
def zeroable (r: Rexp) : Boolean = r match {
  case NULL => true
  case EMPTY => false
  case CHAR(_) => false
  case ALT(r1, r2) => zeroable(r1) && zeroable(r2)
  case SEQ(r1, r2) => if (nullable(r1)) (zeroable(r1) && zeroable(r2)) else zeroable(r1)
      //zeroable(r1) || zeroable(r2)
  case STAR(_) => false
  case NOT(r) => !(zeroable(r))
}


// 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 NOT(r) => NOT(der (c, r))
}

// 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))
}

// main matcher function
def matcher(r: Rexp, s: String) : Boolean = nullable(ders(s.toList, r))


// regular expression for specifying 
// ranges of characters
def RANGE(s : List[Char]) : Rexp = s match {
  case Nil => NULL
  case c::Nil => CHAR(c)
  case c::s => ALT(CHAR(c), RANGE(s))
}

//one or more
def PLUS(r: Rexp) = SEQ(r, STAR(r))


//some regular expressions
val LOWERCASE = RANGE("abcdefghijklmnopqrstuvwxyz".toList)
val UPPERCASE = RANGE("ABCDEFGHIJKLMNOPQRSTUVWXYZ".toList)
val LETTER = ALT(LOWERCASE, UPPERCASE)
val DIGITS = RANGE("0123456789".toList)
val NONZERODIGITS = RANGE("123456789".toList)

val IDENT = SEQ(LETTER, STAR(ALT(LETTER,DIGITS)))
val NUMBER = ALT(SEQ(NONZERODIGITS, STAR(DIGITS)), "0")
val WHITESPACE = RANGE(" \n".toList)

val ALL = ALT(ALT(LETTER, DIGITS), WHITESPACE)

val COMMENT = SEQ(SEQ("/*", NOT(SEQ(SEQ(STAR(ALL), "*/"), STAR(ALL)))), "*/")

abstract class Token

case object T_WHITESPACE extends Token
case class T_IDENT(s: String) extends Token
case class T_OP(s: String) extends Token
case class T_NUM(n: Int) extends Token
case class T_KEYWORD(s: String) extends Token


// an example list of rules
type Rule = (Rexp, List[Char] => Token)

val rules: List[Rule]= 
  List(("if", (s) => T_KEYWORD(s.mkString)),
       ("then", (s) => T_KEYWORD(s.mkString)),
       ("else", (s) => T_KEYWORD(s.mkString)),
       ("+", (s) => T_OP(s.mkString)),
       (IDENT, (s) => T_IDENT(s.mkString)),
       (NUMBER, (s) => T_NUM(s.mkString.toInt)),
       (WHITESPACE, (s) => T_WHITESPACE))


def error (s: String) = throw new IllegalArgumentException ("Could not lex " + s)

def munch(r: Rexp, action: List[Char] => Token, s: List[Char], t: List[Char]) : Option[(List[Char], Token)] = 
  s match {
    case Nil if (nullable(r)) => Some(Nil, action(t))
    case Nil => None
    case c::s if (zeroable(der (c, r)) && nullable(r)) => Some(c::s, action(t))
    case c::s if (zeroable(der (c, r))) => None
    case c::s => munch(der (c, r), action, s, t ::: List(c))
  }

def lex_one (rs: List[Rule], s: List[Char]) : (List[Char], Token) = {
 val somes = rs.map { (r) => munch(r._1, r._2, s, Nil) } .flatten
 if (somes == Nil) error(s.mkString) else (somes sortBy (_._1.length) head)
}

def lex_all (rs: List[Rule], s: List[Char]) : List[Token] = s match {
  case Nil => Nil
  case _ => lex_one(rs, s) match {
    case (rest, t) => t :: lex_all(rs, rest) 
  }
}



println(lex_all(rules, "if true then 42 else +".toList))
println(lex_all(rules, "ifff if     34 34".toList))
println(lex_all(rules, "ifff $ if 34".toList))