// A parser and evaluator for the while language

// 

import matcher._

import parser._

// some regular expressions

val SYM = RANGE("ABCDEFGHIJKLMNOPQRSTUVXYZabcdefghijklmnopqrstuvwxyz_")

val DIGIT = RANGE("0123456789")

val ID = SEQ(SYM, STAR(ALT(SYM, DIGIT))) 

val NUM = PLUS(DIGIT)

val KEYWORD = ALTS("skip", "while", "do", "if", "then", "else", "true", "false") 

val SEMI: Rexp = ";"

val OP: Rexp = ALTS(":=", "=", "-", "+", "*", "!=", "<", ">")

val WHITESPACE = PLUS(RANGE(" \n"))

val RPAREN: Rexp = ")"

val LPAREN: Rexp = "("

val BEGIN: Rexp = "{"

val END: Rexp = "}"

// tokens for classifying the strings that have been recognised

abstract class Token

case object T_WHITESPACE extends Token

case object T_SEMI extends Token

case object T_LPAREN extends Token

case object T_RPAREN extends Token

case object T_BEGIN extends Token

case object T_END extends Token

case class T_ID(s: String) extends Token

case class T_OP(s: String) extends Token

case class T_NUM(s: String) extends Token

case class T_KWD(s: String) extends Token

val lexing_rules: List[(Rexp, List[Char] => Token)] = 

  List((KEYWORD, (s) => T_KWD(s.mkString)),

       (ID, (s) => T_ID(s.mkString)),

       (OP, (s) => T_OP(s.mkString)),

       (NUM, (s) => T_NUM(s.mkString)),

       (SEMI, (s) => T_SEMI),

       (LPAREN, (s) => T_LPAREN),

       (RPAREN, (s) => T_RPAREN),

       (BEGIN, (s) => T_BEGIN),

       (END, (s) => T_END),

       (WHITESPACE, (s) => T_WHITESPACE))

// the tokenizer

val Tok = Tokenizer(lexing_rules, List(T_WHITESPACE))

// the abstract syntax trees

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

// atomic parsers

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) = TokParser(t)

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

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

    case T_NUM(s)::ts => Set((s.toInt, 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 ()

  }

}

def len(xs: List[(Int, Int)]): Int = xs match {

  case Nil => 0

  case (1, x)::xs => len(xs) + 1

  case (_, x)::xs => len(xs) 

}

def fst(p: (Int, Int)): Int = p match {

  case Nil => 0

  case (1, x)::xs => len(xs) + 1

  case (_, x)::xs => len(xs) 

}

// arithmetic expressions

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

  (T ~ T_OP("+") ~ AExp) ==> { case ((x, y), z) => Aop("+", x, z): AExp } ||

  (T ~ T_OP("-") ~ AExp) ==> { case ((x, y), z) => Aop("-", x, z): AExp } || T  

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

  (F ~ T_OP("*") ~ T) ==> { case ((x, y), z) => Aop("*", x, z): AExp } || F

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

  (T_LPAREN ~> AExp <~ T_RPAREN) || 

  IdParser ==> Var || 

  NumParser ==> Num

// boolean expressions

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

  (AExp ~ T_OP("=") ~ AExp) ==> { case ((x, y), z) => Bop("=", x, z): BExp } || 

  (AExp ~ T_OP("!=") ~ AExp) ==> { case ((x, y), z) => Bop("!=", x, z): BExp } || 

  (AExp ~ T_OP("<") ~ AExp) ==> { case ((x, y), z) => Bop("<", x, z): BExp } || 

  (AExp ~ T_OP(">") ~ AExp) ==> { case ((x, y), z) => Bop(">", x, z): BExp } || 

  (T_KWD("true") ==> ((_) => True)) || 

  (T_KWD("false") ==> ((_) => False: BExp))

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

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

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

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

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

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

lazy val Stmts: Parser[List[Token], Block] =

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

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

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

  (T_BEGIN ~> Stmts <~ T_END) || 

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

// examples

val p1 = "x := 5"

val p1_toks = Tok.fromString(p1) 

val p1_ast = Block.parse_all(p1_toks)

println(p1_toks)

println(p1_ast)

val p1a = "{ x := 5; y := 8}"

val p1a_toks = Tok.fromString(p1a) 

val p1a_ast = Block.parse_all(p1a_toks)

println(p1a_ast)

val p2 = "5 = 6"

val p2_toks = Tok.fromString(p2) 

val p2_ast = BExp.parse_all(p2_toks)

println(p2_ast)

val p2a = "true"

val p2a_toks = Tok.fromString(p2a) 

val p2a_ast = BExp.parse_all(p2a_toks)

println(p2a_ast)

val p3 = "if true then skip else skip"

val p3_toks = Tok.fromString(p3) 

val p3_ast = Stmt.parse_all(p3_toks)

println(p3_ast)

val p3a = "if true then x := 5 else x := 10"

val p3a_toks = Tok.fromString(p3a) 

val p3a_ast = Stmt.parse_all(p3a_toks)

println(p3a_ast)

val p3b = "if false then x := 5 else x := 10"

val p3b_toks = Tok.fromString(p3b) 

val p3b_ast = Stmt.parse_all(p3b_toks)

println(p3b_ast)

// multiplication

val p4 = """{ x := 5;

              y := 4;

              r := 0;

              while y > 0 do {

                 r := r + x;

                 y := y - 1

              } 

            }"""

val p4_toks = Tok.fromString(p4) 

val p4_ast = Block.parse_all(p4_toks)

println(p4_ast)

val p5 = """

 n := 9;

 minus1 := 0;

 minus2 := 1;

 temp := 0;

 while n > 0 do  {

   temp := minus2;

   minus2 := minus1 + minus2;

   minus1 := temp;

   n := n - 1

 };

 fib_res := minus2

"""

val p5_toks = Tok.fromString(p5) 

val p5_ast = Stmts.parse_all(p5_toks)

// interpreter

type Env = Map[String, Int]

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)

}

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_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))

}

//examples

println(eval_stmt(p3a_ast.head, Map.empty))

println(eval_stmt(p3b_ast.head, Map.empty))

println(eval_bl(p4_ast.head, Map.empty))

println(eval_bl(p5_ast.head, Map.empty))