// A Small Compiler for a Simple Functional Language

// (it does not include a parser and lexer)

// abstract syntax trees

abstract class Exp

abstract class BExp 

abstract class Decl

// functions and declarations

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

case class Main(e: Exp) extends Decl

// expressions

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 Sequ(e1: Exp, e2: Exp) extends Exp

// boolean expressions

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

// calculating the maximal needed stack size

def max_stack_exp(e: Exp): Int = e match {

  case Call(_, args) => args.map(max_stack_exp).sum

  case If(a, e1, e2) => max_stack_bexp(a) + (List(max_stack_exp(e1), max_stack_exp(e2)).max)

  case Write(e) => max_stack_exp(e) + 1

  case Var(_) => 1

  case Num(_) => 1

  case Aop(_, a1, a2) => max_stack_exp(a1) + max_stack_exp(a2)

  case Sequ(e1, e2) => List(max_stack_exp(e1), max_stack_exp(e2)).max

}

def max_stack_bexp(e: BExp): Int = e match {

  case Bop(_, a1, a2) => max_stack_exp(a1) + max_stack_exp(a2)

}

// compiler - built-in functions 

// copied from http://www.ceng.metu.edu.tr/courses/ceng444/link/jvm-cpm.html

//

val library = """

.class public XXX.XXX

.super java/lang/Object

.method public static write(I)V 

        .limit locals 5 

        .limit stack 5 

        iload 0 

        getstatic java/lang/System/out Ljava/io/PrintStream; 

        swap 

        invokevirtual java/io/PrintStream/println(I)V 

        return 

.end method

"""

// for generating new labels

var counter = -1

def Fresh(x: String) = {

  counter += 1

  x ++ "_" ++ counter.toString()

}

// convenient string interpolations for

// generating instructions, labels etc

import scala.language.implicitConversions

import scala.language.reflectiveCalls

// convenience for code-generation (string interpolations)

implicit def sring_inters(sc: StringContext) = new {

  def i(args: Any*): String = "   " ++ sc.s(args:_*) ++ "\n"  // instructions

  def l(args: Any*): String = sc.s(args:_*) ++ ":\n"          // labels

  def m(args: Any*): String = sc.s(args:_*) ++ "\n"           // methods

}

// variable / index environments

type Env = Map[String, Int]

// compile expressions

def compile_exp(a: Exp, env : Env) : String = a match {

  case Num(i) => i"ldc $i"

  case Var(s) => i"iload ${env(s)}"

  case Aop("+", a1, a2) => compile_exp(a1, env) ++ compile_exp(a2, env) ++ i"iadd"

  case Aop("-", a1, a2) => compile_exp(a1, env) ++ compile_exp(a2, env) ++ i"isub"

  case Aop("*", a1, a2) => compile_exp(a1, env) ++ compile_exp(a2, env) ++ i"imul"

  case Aop("/", a1, a2) => compile_exp(a1, env) ++ compile_exp(a2, env) ++ i"idiv"

  case Aop("%", a1, a2) => compile_exp(a1, env) ++ compile_exp(a2, env) ++ i"irem"

  case If(b, a1, a2) => {

    val if_else = Fresh("If_else")

    val if_end = Fresh("If_end")

    compile_bexp(b, env, if_else) ++

    compile_exp(a1, env) ++

    i"goto $if_end" ++

    l"$if_else" ++

    compile_exp(a2, env) ++

    l"$if_end"

  }

  case Call(name, args) => {

    val is = "I" * args.length

    args.map(a => compile_exp(a, env)).mkString ++

    i"invokestatic XXX/XXX/$name($is)I"

  }

  case Sequ(a1, a2) => {

    compile_exp(a1, env) ++ i"pop" ++ compile_exp(a2, env)

  }

  case Write(a1) => {

    compile_exp(a1, env) ++

    i"dup" ++

    i"invokestatic XXX/XXX/write(I)V"

  }

}

// compile boolean expressions

def compile_bexp(b: BExp, env : Env, jmp: String) : String = b match {

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

    compile_exp(a1, env) ++ compile_exp(a2, env) ++ i"if_icmpne $jmp"

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

    compile_exp(a1, env) ++ compile_exp(a2, env) ++ i"if_icmpeq $jmp"

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

    compile_exp(a1, env) ++ compile_exp(a2, env) ++ i"if_icmpge $jmp"

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

    compile_exp(a1, env) ++ compile_exp(a2, env) ++ i"if_icmpgt $jmp"

}

// compile functions and declarations

def compile_decl(d: Decl) : String = d match {

  case Def(name, args, a) => { 

    val env = args.zipWithIndex.toMap

    val is = "I" * args.length

    m".method public static $name($is)I" ++

    m".limit locals ${args.length.toString}" ++

    m".limit stack ${1 + max_stack_exp(a)}" ++

    l"${name}_Start" ++   

    compile_exp(a, env) ++

    i"ireturn" ++

    m".end method\n"

  }

  case Main(a) => {

    m".method public static main([Ljava/lang/String;)V" ++

    m".limit locals 200" ++

    m".limit stack 200" ++

    compile_exp(a, Map()) ++

    i"invokestatic XXX/XXX/write(I)V" ++

    i"return" ++

    m".end method\n"

  }

}

// the main compilation function

def compile(prog: List[Decl], class_name: String) : String = {

  val instructions = prog.map(compile_decl).mkString

  (library + instructions).replaceAllLiterally("XXX", class_name)

}

// An example program (factorials)

//

// def fact(n) = if n == 0 then 1 else n * fact(n - 1);

// def facT(n, acc) = if n == 0 then acc else facT(n - 1, n * acc);

// 

// fact(10) ; facT(15, 1)

// 

val test_prog = 

  List(Def("fact", List("n"),

         If(Bop("==",Var("n"),Num(0)),

            Num(1),

            Aop("*",Var("n"),

                    Call("fact",List(Aop("-",Var("n"),Num(1))))))),

       Def("facT",List("n", "acc"),

         If(Bop("==",Var("n"),Num(0)),

            Var("acc"),

            Call("facT",List(Aop("-",Var("n"),Num(1)), 

                             Aop("*",Var("n"),Var("acc")))))),

       Main(Sequ(Write(Call("fact",List(Num(10)))),

                 Write(Call("facT",List(Num(10), Num(1)))))))

// prints out the JVM instructions

println(compile(test_prog, "fact"))