// A parser for the Fun language

//================================

//

// call with 

//

//     amm fun_parser.sc fact.fun

//     amm fun_parser.sc defs.fun

//

// this will generate a parse-tree from a list

// of tokens

import $file.fun_tokens, fun_tokens._ 

import scala.language.implicitConversions    

import scala.language.reflectiveCalls

// Parser combinators

//    type parameter I needs to be of Seq-type

//

type IsSeq[I] = I => Seq[?]

abstract class Parser[I, T](using is: I => Seq[?]) {

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

  def parse_single(ts: I) : T = 

    parse(ts).partition(p => is(p._2).isEmpty) match {

      case (good, _) if !good.isEmpty => good.head._1

      case (_, err) => { println (s"Parse Error\n${err.minBy(p => is(p._2).length)}") ; sys.exit(-1) }

    }

}

// convenience for writing grammar rules

case class ~[+A, +B](_1: A, _2: B)

// parser combinators

// alternative parser

class AltParser[I : IsSeq, T](p: => Parser[I, T], 

                              q: => Parser[I, T]) extends Parser[I, T] {

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

}

// sequence parser

class SeqParser[I : IsSeq, T, S](p: => Parser[I, T], 

                                 q: => Parser[I, S]) 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 MapParser[I : IsSeq, T, S](p: => Parser[I, T], 

                                f: T => S) extends Parser[I, S] {

  def parse(in: I) = for ((hd, tl) <- p.parse(in)) yield (f(hd), tl)

}

// more convenient syntax for parser combinators

extension [I : IsSeq, T](p: Parser[I, T]) {

  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 MapParser[I, T, S](p, f)

}

def ListParser[I, T, S](p: => Parser[I, T], q: => Parser[I, S])(using is: I => Seq[?]): Parser[I, List[T]] = {

  (p ~ q ~ ListParser(p, q)).map{ case (x:T) ~ (y:S) ~ (z:List[T]) => x :: z } ||

  (p.map[List[T]]{s => List(s)})

}

case class TokParser(tok: Token) extends Parser[List[Token], Token] {

  def parse(ts: List[Token]) = ts match {

    case t::ts if (t == tok) => Set((t, ts)) 

    case _ => Set ()

  }

}

implicit def token2tparser(t: Token) : Parser[List[Token], Token] = TokParser(t)

extension (t: Token) {

  def || (q : => Parser[List[Token], Token]) = new AltParser[List[Token], Token](t, q)

  def map[S] (f: => Token => S) = new MapParser[List[Token], Token, S](t, f)

  def ~[S](q : => Parser[List[Token], S]) = new SeqParser[List[Token], Token, S](t, q)

}

case object NumParser extends Parser[List[Token], Int] {

  def parse(ts: List[Token]) = ts match {

    case T_NUM(n)::ts => Set((n, ts)) 

    case _ => Set ()

  }

}

case object IdParser extends Parser[List[Token], String] {

  def parse(ts: List[Token]) = ts match {

    case T_ID(s)::ts => Set((s, ts)) 

    case _ => Set ()

  }

}

// Abstract syntax trees for the Fun language

abstract class Exp 

abstract class BExp

abstract class Decl

case class Def(name: String, args: List[String], body: Exp) extends Decl

case class Main(e: Exp) extends Decl

case class Call(name: String, args: List[Exp]) extends Exp

case class If(a: BExp, e1: Exp, e2: Exp) extends Exp

case class Write(e: Exp) extends Exp

case class Var(s: String) extends Exp

case class Num(i: Int) extends Exp

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

case class Sequence(e1: Exp, e2: Exp) extends Exp

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

// Grammar Rules for the Fun language

// arithmetic expressions

lazy val Exp: Parser[List[Token], Exp] = 

  (T_KWD("if") ~ BExp ~ T_KWD("then") ~ Exp ~ T_KWD("else") ~ Exp).map{ case _ ~ x ~ _ ~ y ~ _ ~ z => If(x, y, z): Exp } ||

  (M ~ T_SEMI ~ Exp).map{ case x ~ _ ~ y => Sequence(x, y): Exp } || M

lazy val M: Parser[List[Token], Exp] =

  (T_KWD("write") ~ L).map{ case _ ~ y => Write(y): Exp } || L

lazy val L: Parser[List[Token], Exp] = 

  (T ~ T_OP("+") ~ Exp).map{ case x ~ _ ~ z => Aop("+", x, z): Exp } ||

  (T ~ T_OP("-") ~ Exp).map{ case x ~ _ ~ z => Aop("-", x, z): Exp } || T  

lazy val T: Parser[List[Token], Exp] = 

  (F ~ T_OP("*") ~ T).map{ case x ~ _ ~ z => Aop("*", x, z): Exp } || 

  (F ~ T_OP("/") ~ T).map{ case x ~ _ ~ z => Aop("/", x, z): Exp } || 

  (F ~ T_OP("%") ~ T).map{ case x ~ _ ~ z => Aop("%", x, z): Exp } || F

lazy val F: Parser[List[Token], Exp] = 

  (IdParser ~ T_LPAREN ~ ListParser(Exp, T_COMMA) ~ T_RPAREN).map

    { case x ~ _ ~ z ~ _ => Call(x, z): Exp } ||

  (T_LPAREN ~ Exp ~ T_RPAREN).map{ case _ ~ y ~ _ => y: Exp } || 

  IdParser.map{ case x => Var(x): Exp } || 

  NumParser.map{ case x => Num(x): Exp }

// boolean expressions

lazy val BExp: Parser[List[Token], BExp] = 

  (Exp ~ T_OP("==") ~ Exp).map{ case x ~ _ ~ z => Bop("==", x, z): BExp } || 

  (Exp ~ T_OP("!=") ~ Exp).map{ case x ~ _ ~ z => Bop("!=", x, z): BExp } || 

  (Exp ~ T_OP("<") ~ Exp) .map{ case x ~ _ ~ z => Bop("<",  x, z): BExp } || 

  (Exp ~ T_OP(">") ~ Exp) .map{ case x ~ _ ~ z => Bop("<",  z, x): BExp } || 

  (Exp ~ T_OP("<=") ~ Exp).map{ case x ~ _ ~ z => Bop("<=", x, z): BExp } || 

  (Exp ~ T_OP("=>") ~ Exp).map{ case x ~ _ ~ z => Bop("<=", z, x): BExp }  

lazy val Defn: Parser[List[Token], Decl] =

   (T_KWD("def") ~ IdParser ~ T_LPAREN ~ ListParser(IdParser, T_COMMA) ~ T_RPAREN ~ T_OP("=") ~ Exp).map{ case _ ~ y ~ _ ~ w ~ _ ~ _ ~ r => Def(y, w, r): Decl }

lazy val Prog: Parser[List[Token], List[Decl]] =

  (Defn ~ T_SEMI ~ Prog).map{ case x ~ _ ~ z => x :: z : List[Decl] } ||

  (Exp.map((s) => List(Main(s)) : List[Decl]))

// Reading tokens and Writing parse trees

def parse_tks(tks: List[Token]) : List[Decl] = 

  Prog.parse_single(tks)

@main

def main(fname: String) : Unit = {

  val tks = tokenise(os.read(os.pwd / fname))

  println(parse_tks(tks))

}