import RexpRelated._
import RexpRelated.Rexp._
import Spiral._
import scala.collection.mutable.ArrayBuffer
abstract class BRexp
case object BZERO extends BRexp
case object BONE extends BRexp
case class BCHAR(c: Char) extends BRexp
case class BALTS(b: Bit, rs: List[BRexp]) extends BRexp 
case class BSEQ(r1: BRexp, r2: BRexp) extends BRexp 
case class BSTAR(r: BRexp) extends BRexp 

object BRexp{
  def brternalise(r: Rexp) : BRexp = r match {//remember the initial shadowed bit should be set to Z as there 
  //are no enclosing STAR or SEQ
    case ZERO => BZERO
    case ONE => BONE
    case CHAR(c) => BCHAR(c)
    case ALTS(rs) => BALTS(Z,  rs.map(brternalise))
    case SEQ(r1, r2) => BSEQ( brternalise(r1), brternalise(r2) )
    case STAR(r) => BSTAR(brternalise(r))
    case RECD(x, r) => brternalise(r)
  }
  def brnullable (r: BRexp) : Boolean = r match {
    case BZERO => false
    case BONE => true
    case BCHAR(_) => false
    case BALTS(bs, rs) => rs.exists(brnullable)
    case BSEQ(r1, r2) => brnullable(r1) && brnullable(r2)
    case BSTAR(_) => true
  }
  //this function tells bspill when converting a brexp into a list of rexps
  //the conversion : (a+b)*c -> {a*c, b*c} can only happen when a+b is generated from scratch (e.g. when deriving a seq)
  //or is from the original regex but have been "touched" (i.e. have been derived)
  //Why make this distinction? Look at the following example:
  //r = (c+cb)+c(a+b)
  // r\c = (1+b)+(a+b)
  //after simplification
  // (r\c)simp= 1+b+a
  //we lost the structure that tells us 1+b should be grouped together and a grouped as itself
  //however in our brexp simplification, 
  //(r\c)br_simp = 1+b+(a+b)
  //we do not allow the bracket to be opened as it is from the original expression and have not been touched
  def brder(c: Char, r: BRexp) : BRexp = r match {
    case BZERO => BZERO
    case BONE => BZERO
    case BCHAR(d) => if (c == d) BONE else BZERO
    case BALTS(bs, rs) => BALTS(S, rs.map(brder(c, _)))//After derivative: Splittable in the sense of PD
    case BSEQ(r1, r2) => 
      if (brnullable(r1)) BALTS(S, List(BSEQ(brder(c, r1), r2), brder(c, r2) ) )//in r1\c~r2 r2's structure is maintained together with its splittablility bit
      else BSEQ(brder(c, r1), r2)
    case BSTAR(r) => BSEQ(brder(c, r), BSTAR(r))
  }
  def bflat(rs: List[BRexp]): List[BRexp] = {
    rs match {
      case Nil => Nil//TODO: Look at this! bs should have been exhausted one bit earlier.
      case BZERO :: rs1 => bflat(rs1)
      case BALTS(S, rs1) :: rs2 => rs1 ::: bflat(rs2)
      case BALTS(Z, rs1) :: rs2 => BALTS(Z, rs1) :: bflat(rs2)
      case r1 :: rs2 => r1 :: bflat(rs2)
    }
  }
  def stflat(rs: List[BRexp]): List[BRexp] = {
    //println("bs: " + bs + "rs: "+ rs + "length of bs, rs" + bs.length + ", " + rs.length)
    //assert(bs.length == rs.length - 1 || bs.length == rs.length)//bs == Nil, rs == Nil  May add early termination later.
    rs match {
      case Nil => Nil//TODO: Look at this! bs should have been exhausted one bit earlier.
      case BZERO :: rs1 => bflat(rs1)
      case BALTS(_, rs1) :: rs2 => rs1 ::: bflat(rs2)
      case r1 :: rs2 => r1 :: bflat(rs2)
    }
  }
  def berase(r:BRexp): Rexp = r match{
    case BZERO => ZERO
    case BONE => ONE
    case BCHAR(f) => CHAR(f)
    case BALTS(bs, rs) => ALTS(rs.map(berase(_)))
    case BSEQ(r1, r2) => SEQ (berase(r1), berase(r2))
    case BSTAR(r)=> STAR(berase(r))
  }
  def br_simp(r: BRexp): BRexp = r match {
    case BSEQ(r1, r2) => (br_simp(r1), br_simp(r2)) match {
        case (BZERO, _) => BZERO
        case (_, BZERO) => BZERO
        case (BONE, r2s) => r2s 
        case (r1s, r2s) => BSEQ(r1s, r2s)
    }
    case BALTS(bs, rs) => {
      //assert(bs.length == rs.length - 1)
      val dist_res = if(bs == S) {//all S
        val rs_simp = rs.map(br_simp)
        val flat_res = bflat(rs_simp)
        distinctBy(flat_res, berase)
      }
      else{//not allowed to simplify (all Z)
        rs
      }
      dist_res match {
        case Nil => {BZERO}
        case s :: Nil => { s}
        case rs => {BALTS(bs, rs)}
      }
    }
    //case BSTAR(r) => BSTAR(r)
    case r => r
  }

  def strong_br_simp(r: BRexp): BRexp = r match {
    case BSEQ(r1, r2) => (strong_br_simp(r1), strong_br_simp(r2)) match {
        case (BZERO, _) => BZERO
        case (_, BZERO) => BZERO
        case (BONE, r2s) => r2s 
        case (r1s, r2s) => BSEQ(r1s, r2s)
    }
    case BALTS(bs, rs) => {
      //assert(bs.length == rs.length - 1)
      val dist_res = {//all S
        val rs_simp = rs.map(strong_br_simp)
        val flat_res = stflat(rs_simp)
        distinctBy(flat_res, berase)
      }
      dist_res match {
        case Nil => {BZERO}
        case s :: Nil => { s}
        case rs => {BALTS(bs, rs)}
      }
    }
    case BSTAR(r) => BSTAR(strong_br_simp(r))
    case r => r
  }
  //we want to bound the size by a function bspill s.t.
  //bspill(ders_simp(r,s)) ⊂  PD(r) 
  //and bspill is size-preserving (up to a constant factor)
  //so we bound |ders_simp(r,s)| by |PD(r)|
  //where we already have a nice bound for |PD(r)|: t^2*n^2 in Antimirov's paper

  //the function bspill converts a brexp into a list of rexps
  //the conversion mainly about this: (r1+r2)*r3*r4*.....rn -> {r1*r3*r4*....rn, r2*r3*r4*.....rn} 
  //but this is not always allowed
  //sometimes, we just leave the expression as it is: 
  //eg1: (a+b)*c -> {(a+b)*c}
  //eg2: r1+r2 -> {r1+r2} instead of {r1, r2}
  //why?
  //if we always return {a, b} when we encounter a+b, the property
  //bspill(ders_simp(r,s)) ⊂  PD(r) 
  //does not always hold
  //for instance 
  //if we call the bspill that always returns {r1, r2} when encountering r1+r2 "bspilli"
  //then bspilli( ders_simp( (c+cb)+c(a+b), c ) ) == bspilli(1+b+a) = {1, b, a}
  //However the letter a does not belong to PD( (c+cb)+c(a+b) )
  //then we can no longer say ders_simp(r,s)'s size is bounded by PD(r) because the former contains something the latter does not have
  //In order to make sure the inclusion holds, we have to find out why new terms appeared in the bspilli set that don't exist in the PD(r) set
  //Why a does not belong to PD((c+cb)+c(a+b))?
  //PD(r1+r2) = PD(r1) union PD(r2) => PD((c+cb)+c(a+b)) = PD(c+cb) union PD(c(a+b))
  //So the only possibility for PD to include a must be in the latter part of the regex, namely, c(a+b)
  //we have lemma that PD(r) = union of pders(s, r) where s is taken over all strings whose length does not exceed depth(r)
  //so PD(r) ⊂ pder(empty_string, r) union pder(c, r) union pder(ca, r) union pder(cb, r) where r = c(a+b)
  //RHS = {1} union pder(c, c(a+b))
  //Observe that pder(c, c(a+b)) == {a+b}
  //a and b are together, from the original regular expression (c+cb)+c(a+b).
  //but by our simplification, we first flattened this a+b into the same level with 1+b, then
  //removed duplicates of b, thereby destroying the structure in a+b and making this term a, instead of a+b
  //But in PD(r) we only have a+b, no a
  //one ad hoc solution might be to try this bspill(ders_simp(r,s)) ⊂  PD(r) union {all letters}
  //But this does not hold either according to experiment.
  //So we need to make sure the structure r1+r2 is not simplified away if it is from the original expression
  

  
  def bspill(r: BRexp): List[Rexp] = {
      r match {
          case BALTS(b, rs) => {
            if(b == S)
              rs.flatMap(bspill)
            else
              List(ALTS(rs.map(berase)))
          }
          case BSEQ(r2, r3) => bspill(r2).map( a => if(a == ONE) berase(r3) else SEQ(a, berase(r3)) )
          case BZERO => List()
          case r => List(berase(r))
      }
    
  }

}