// A parser and interpreter for the While language

// 

import scala.language.implicitConversions

import scala.language.reflectiveCalls

abstract class Parser[I, T](implicit ev: I => Seq[_]) {

  def parse(ts: I): Set[(T, I)]

  def parse_all(ts: I) : Set[T] =

    for ((head, tail) <- parse(ts); if (tail.isEmpty)) yield head

}

class SeqParser[I, T, S](p: => Parser[I, T], q: => Parser[I, S])(implicit ev: I => Seq[_]) extends Parser[I, (T, S)] {

  def parse(sb: I) = 

    for ((head1, tail1) <- p.parse(sb); 

         (head2, tail2) <- q.parse(tail1)) yield ((head1, head2), tail2)

}

class AltParser[I, T](p: => Parser[I, T], q: => Parser[I, T])(implicit ev: I => Seq[_]) extends Parser[I, T] {

  def parse(sb: I) = p.parse(sb) ++ q.parse(sb)   

}

class FunParser[I, T, S](p: => Parser[I, T], f: T => S)(implicit ev: I => Seq[_]) extends Parser[I, S] {

  def parse(sb: I) = 

    for ((head, tail) <- p.parse(sb)) yield (f(head), tail)

}

case class StringParser(s: String) extends Parser[String, String] {

  def parse(sb: String) = {

    val (prefix, suffix) = sb.splitAt(s.length)

    if (prefix == s) Set((prefix, suffix)) else Set()

  }

}

case object NumParser extends Parser[String, Int] {

  val reg = "[0-9]+".r

  def parse(sb: String) = reg.findPrefixOf(sb) match {

    case None => Set()

    case Some(s) => {

      val (head, tail) = sb.splitAt(s.length)

      Set((head.toInt, tail)) 

    }

  }

}

implicit def string2parser(s : String) = StringParser(s)

implicit def ParserOps[I, T](p: Parser[I, T])(implicit ev: I => Seq[_]) = new {

  def || (q : => Parser[I, T]) = new AltParser[I, T](p, q)

  def ==>[S] (f: => T => S) = new FunParser[I, T, S](p, f)

  def ~[S] (q : => Parser[I, S]) = new SeqParser[I, T, S](p, q)

}

implicit def StringOps(s: String) = new {

  def || (q : => Parser[String, String]) = new AltParser[String, String](s, q)

  def || (r: String) = new AltParser[String, String](s, r)

  def ==>[S] (f: => String => S) = new FunParser[String, String, S](s, f)

  def ~[S](q : => Parser[String, S]) = 

    new SeqParser[String, String, S](s, q)

  def ~ (r: String) = 

    new SeqParser[String, String, String](s, r)

}

// the abstract syntax trees for the WHILE language

abstract class Stmt

abstract class AExp

abstract class BExp 

type Block = List[Stmt]

case object Skip extends Stmt

case class If(a: BExp, bl1: Block, bl2: Block) extends Stmt

case class While(b: BExp, bl: Block) extends Stmt

case class Assign(s: String, a: AExp) extends Stmt

case class Var(s: String) extends AExp

case class Num(i: Int) extends AExp

case class Aop(o: String, a1: AExp, a2: AExp) extends AExp

case object True extends BExp

case object False extends BExp

case class Bop(o: String, a1: AExp, a2: AExp) extends BExp

case object IdParser extends Parser[String, String] {

  val reg = "[a-z][a-z,0-9]*".r

  def parse(sb: String) = reg.findPrefixOf(sb) match {

    case None => Set()

    case Some(s) => Set(sb.splitAt(s.length))

  }

}

lazy val AExp: Parser[String, AExp] = 

lazy val Te: Parser[String, AExp] = 

lazy val Fa: Parser[String, AExp] = 

   IdParser ==> Var || 

lazy val BExp: Parser[String, BExp] = 

lazy val Stmt: Parser[String, Stmt] =

  (("skip" ==> ((_) => Skip: Stmt)) ||

   (IdParser ~ ":=" ~ AExp) ==> { case ((x, y), z) => Assign(x, z): Stmt } ||

   ("if" ~ BExp ~ "then" ~ Block ~ "else" ~ Block) ==>

    { case (((((x,y),z),u),v),w) => If(y, u, w): Stmt } ||

   ("while" ~ BExp ~ "do" ~ Block) ==> { case (((x, y), z), w) => While(y, w) }) 

lazy val Stmts: Parser[String, Block] =

  (Stmt ~ ";" ~ Stmts) ==> { case ((x, y), z) => x :: z : Block } ||

  (Stmt ==> ((s) => List(s) : Block))

lazy val Block: Parser[String, Block] =

  (("{" ~ Stmts ~ "}") ==> { case ((x, y), z) => y} || 

   (Stmt ==> ((s) => List(s))))

Stmts.parse_all("x2:=5+3;")

Block.parse_all("{x:=5;y:=8}")

Block.parse_all("if(false)then{x:=5}else{x:=10}")

Block.parse_all(fib)

// an interpreter for the WHILE language

type Env = Map[String, Int]

def eval_aexp(a: AExp, env: Env) : Int = a match {

  case Num(i) => i

  case Var(s) => env(s)

  case Aop("+", a1, a2) => eval_aexp(a1, env) + eval_aexp(a2, env)

  case Aop("-", a1, a2) => eval_aexp(a1, env) - eval_aexp(a2, env)

  case Aop("*", a1, a2) => eval_aexp(a1, env) * eval_aexp(a2, env)

}

def eval_bexp(b: BExp, env: Env) : Boolean = b match {

  case True => true

  case False => false

  case Bop("!=", a1, a2) => !(eval_aexp(a1, env) == eval_aexp(a2, env))

  case Bop(">", a1, a2) => eval_aexp(a1, env) > eval_aexp(a2, env)

  case Bop("<", a1, a2) => eval_aexp(a1, env) < eval_aexp(a2, env)

}

def eval_stmt(s: Stmt, env: Env) : Env = s match {

  case Skip => env

  case Assign(x, a) => env + (x -> eval_aexp(a, env))

  case If(b, bl1, bl2) => if (eval_bexp(b, env)) eval_bl(bl1, env) else eval_bl(bl2, env) 

  case While(b, bl) => 

    if (eval_bexp(b, env)) eval_stmt(While(b, bl), eval_bl(bl, env))

    else env

}

def eval_bl(bl: Block, env: Env) : Env = bl match {

  case Nil => env

  case s::bl => eval_bl(bl, eval_stmt(s, env))

}

def eval(bl: Block) : Env = eval_bl(bl, Map())

// parse + evaluate fib program; then lookup what is

// stored under the variable result 

println(eval(Block.parse_all(fib).head)("result"))