// Parser Combinators:
// Simple Version for WHILE-language
//====================================
//
// with some added convenience for
// map-parsers and grammar rules
//
// call with
//
//    amm comb2.sc


// more convenience for the map parsers later on;
// it allows writing nested patterns as
// case x ~ y ~ z => ...



case class ~[+A, +B](x: A, y: B)

abstract class Parser[I, T](using is: I => Seq[_])  {
  def parse(in: I): Set[(T, I)]  

  def parse_all(in: I) : Set[T] =
    for ((hd, tl) <- parse(in); 
        if is(tl).isEmpty) yield hd
}

// parser combinators

// alternative parser
class AltParser[I, T](p: => Parser[I, T], 
                      q: => Parser[I, T])(using I => Seq[_]) extends Parser[I, T] {
  def parse(in: I) = p.parse(in) ++ q.parse(in)   
}

// sequence parser
class SeqParser[I, T, S](p: => Parser[I, T], 
                         q: => Parser[I, S])(using I => Seq[_]) extends Parser[I, ~[T, S]] {
  def parse(in: I) = 
    for ((hd1, tl1) <- p.parse(in); 
         (hd2, tl2) <- q.parse(tl1)) yield (new ~(hd1, hd2), tl2)
}

// map parser
class maparser[I, T, S](p: => Parser[I, T], 
                         f: T => S)(using I => Seq[_]) extends Parser[I, S] {
  def parse(in: I) = for ((hd, tl) <- p.parse(in)) yield (f(hd), tl)
}



// atomic parser for (particular) strings
case class StrParser(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()
  }
}

// atomic parser for identifiers (variable names)
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))
  }
}


// atomic parser for numbers (transformed into ints)
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 (hd, tl) = sb.splitAt(s.length)
      Set((hd.toInt, tl)) 
    }
  }
}

// the following string interpolation allows us to write 
// StrParser(_some_string_) more conveniently as 
//
// p"<_some_string_>" 

extension (sc: StringContext) 
  def p(args: Any*) = StrParser(sc.s(args:_*))


// more convenient syntax for parser combinators
extension [I, T](p: Parser[I, T])(using I => Seq[_]) {
  def ||(q : => Parser[I, T]) = new AltParser[I, T](p, q)
  def ~[S] (q : => Parser[I, S]) = new SeqParser[I, T, S](p, q)
  def map[S](f: => T => S) = new maparser[I, T, S](p, f)
}




// 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 Write(s: String) 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 class And(b1: BExp, b2: BExp) extends BExp
case class Or(b1: BExp, b2: BExp) extends BExp


// arithmetic expressions
lazy val AExp: Parser[String, AExp] = 
  (Te ~ p"+" ~ AExp).map[AExp]{ case x ~ _ ~ z => Aop("+", x, z) } ||
  (Te ~ p"-" ~ AExp).map[AExp]{ case x ~ _ ~ z => Aop("-", x, z) } || Te
lazy val Te: Parser[String, AExp] = 
  (Fa ~ p"*" ~ Te).map[AExp]{ case x ~ _ ~ z => Aop("*", x, z) } || 
  (Fa ~ p"/" ~ Te).map[AExp]{ case x ~ _ ~ z => Aop("/", x, z) } || Fa  
lazy val Fa: Parser[String, AExp] = 
   (p"(" ~ AExp ~ p")").map{ case _ ~ y ~ _ => y } || 
   IdParser.map(Var) || 
   NumParser.map(Num)

// boolean expressions with some simple nesting
lazy val BExp: Parser[String, BExp] = 
   (AExp ~ p"==" ~ AExp).map[BExp]{ case x ~ _ ~ z => Bop("==", x, z) } || 
   (AExp ~ p"!=" ~ AExp).map[BExp]{ case x ~ _ ~ z => Bop("!=", x, z) } || 
   (AExp ~ p"<" ~ AExp).map[BExp]{ case x ~ _ ~ z => Bop("<", x, z) } || 
   (AExp ~ p">" ~ AExp).map[BExp]{ case x ~ _ ~ z => Bop(">", x, z) } ||
   (p"(" ~ BExp ~ p")" ~ p"&&" ~ BExp).map[BExp]{ case _ ~ y ~ _ ~ _ ~ v => And(y, v) } ||
   (p"(" ~ BExp ~ p")" ~ p"||" ~ BExp).map[BExp]{ case _ ~ y ~ _ ~ _ ~ v => Or(y, v) } ||
   (p"true".map[BExp]{ _ => True }) || 
   (p"false".map[BExp]{ _ => False }) ||
   (p"(" ~ BExp ~ p")").map[BExp]{ case _ ~ x ~ _ => x }

// a single statement 
lazy val Stmt: Parser[String, Stmt] =
  ((p"skip".map[Stmt]{_ => Skip }) ||
   (IdParser ~ p":=" ~ AExp).map[Stmt]{ case x ~ _ ~ z => Assign(x, z) } ||
   (p"write(" ~ IdParser ~ p")").map[Stmt]{ case _ ~ y ~ _ => Write(y) } ||
   (p"if" ~ BExp ~ p"then" ~ Block ~ p"else" ~ Block)
     .map[Stmt]{ case _ ~ y ~ _ ~ u ~ _ ~ w => If(y, u, w) } ||
   (p"while" ~ BExp ~ p"do" ~ Block).map[Stmt]{ case _ ~ y ~ _ ~ w => While(y, w) })   
 
 
// statements
lazy val Stmts: Parser[String, Block] =
  (Stmt ~ p";" ~ Stmts).map[Block]{ case x ~ _ ~ z => x :: z } ||
  (Stmt.map[Block]{ s => List(s) })

// blocks (enclosed in curly braces)
lazy val Block: Parser[String, Block] =
  ((p"{" ~ Stmts ~ p"}").map{ case _ ~ y ~ _ => y } || 
   (Stmt.map(s => List(s))))


// Examples
Stmt.parse_all("x2:=5+3")
Block.parse_all("{x:=5;y:=8}")
Block.parse_all("if(false)then{x:=5}else{x:=10}")


val fib = """n := 10;
             minus1 := 0;
             minus2 := 1;
             temp := 0;
             while (n > 0) do {
                 temp := minus2;
                 minus2 := minus1 + minus2;
                 minus1 := temp;
                 n := n - 1
             };
             result := minus2""".replaceAll("\\s+", "")

Stmts.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)
  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)
  case Bop("<", a1, a2) => eval_aexp(a1, env) < eval_aexp(a2, env)
  case And(b1, b2) => eval_bexp(b1, env) && eval_bexp(b2, env)
  case Or(b1, b2) => eval_bexp(b1, env) || eval_bexp(b2, 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
  case Write(x) => { println(env(x)) ; 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(Stmts.parse_all(fib).head)("result"))


// more examles

// calculate and print all factors bigger 
// than 1 and smaller than n
println("Factors")

val factors =  
   """n := 12;
      f := 2;
      while (f < n / 2 + 1) do {
        if ((n / f) * f == n) then  { write(f) } else { skip };
        f := f + 1
      }""".replaceAll("\\s+", "")

println(eval(Stmts.parse_all(factors).head))


// calculate all prime numbers up to a number 
println("Primes")

val primes =  
   """end := 100;
      n := 2;
      while (n < end) do {
        f := 2;
        tmp := 0;
        while ((f < n / 2 + 1) && (tmp == 0)) do {
          if ((n / f) * f == n) then  { tmp := 1 } else { skip };
          f := f + 1
        };
        if (tmp == 0) then { write(n) } else { skip };
        n  := n + 1
      }""".replaceAll("\\s+", "")

println(eval(Stmts.parse_all(primes).head))