// A Small Compiler for the WHILE Language

// 

// the abstract syntax trees

abstract class Stmt

abstract class AExp

abstract class BExp 

type Block = List[Stmt]

// statements

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 Read(s: String) extends Stmt

// arithmetic expressions

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

// boolean expressions

case object True extends BExp

case object False extends BExp

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

// compiler headers needed for the JVM

// (contains an init method, as well as methods for read and write)

val beginning = """

.class public XXX.XXX

.super java/lang/Object

.method public <init>()V

   aload_0

   invokenonvirtual java/lang/Object/<init>()V

   return

.end method

.method public static write(I)V 

    .limit locals 1 

    .limit stack 2 

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

    iload 0

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

    return 

.end method

.method public static read()I 

    .limit locals 10 

    .limit stack 10

    ldc 0 

    istore 1  ; this will hold our final integer 

Label1: 

    getstatic java/lang/System/in Ljava/io/InputStream; 

    invokevirtual java/io/InputStream/read()I 

    istore 2 

    iload 2 

    ldc 10   ; the newline delimiter 

    isub 

    ifeq Label2 

    iload 2 

    ldc 32   ; the space delimiter 

    isub 

    ifeq Label2

    iload 2 

    ldc 48   ; we have our digit in ASCII, have to subtract it from 48 

    isub 

    ldc 10 

    iload 1 

    imul 

    iadd 

    istore 1 

    goto Label1 

Label2: 

    ;when we come here we have our integer computed in local variable 1 

    iload 1 

    ireturn 

.end method

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

   .limit locals 200

   .limit stack 200

"""

val ending = """

   return

.end method

"""

// for generating new labels

var counter = -1

def Fresh(x: String) = {

  counter += 1

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

}

// environments and instructions

type Env = Map[String, String]

type Instrs = List[String]

// arithmetic expression compilation

def compile_aexp(a: AExp, env : Env) : Instrs = a match {

  case Num(i) => List("ldc " + i.toString + "\n")

  case Var(s) => List("iload " + env(s) + "\n")

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

    compile_aexp(a1, env) ++ 

    compile_aexp(a2, env) ++ List("iadd\n")

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

    compile_aexp(a1, env) ++ compile_aexp(a2, env) ++ List("isub\n")

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

    compile_aexp(a1, env) ++ compile_aexp(a2, env) ++ List("imul\n")

}

// boolean expression compilation

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

  case True => Nil

  case False => List("goto " + jmp + "\n")

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

    compile_aexp(a1, env) ++ compile_aexp(a2, env) ++ 

    List("if_icmpne " + jmp + "\n")

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

    compile_aexp(a1, env) ++ compile_aexp(a2, env) ++ 

    List("if_icmpeq " + jmp + "\n")

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

    compile_aexp(a1, env) ++ compile_aexp(a2, env) ++ 

    List("if_icmpge " + jmp + "\n")

}

// statement compilation

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

  case Skip => (Nil, env)

  case Assign(x, a) => {

    val index = if (env.isDefinedAt(x)) env(x) else 

                    env.keys.size.toString

    (compile_aexp(a, env) ++ 

     List("istore " + index + "\n"), env + (x -> index))

  } 

  case If(b, bl1, bl2) => {

    val if_else = Fresh("If_else")

    val if_end = Fresh("If_end")

    val (instrs1, env1) = compile_block(bl1, env)

    val (instrs2, env2) = compile_block(bl2, env1)

    (compile_bexp(b, env, if_else) ++

     instrs1 ++

     List("goto " + if_end + "\n") ++

     List("\n" + if_else + ":\n\n") ++

     instrs2 ++

     List("\n" + if_end + ":\n\n"), env2)

  }

  case While(b, bl) => {

    val loop_begin = Fresh("Loop_begin")

    val loop_end = Fresh("Loop_end")

    val (instrs1, env1) = compile_block(bl, env)

    (List("\n" + loop_begin + ":\n\n") ++

     compile_bexp(b, env, loop_end) ++

     instrs1 ++

     List("goto " + loop_begin + "\n") ++

     List("\n" + loop_end + ":\n\n"), env1)

  }

  case Write(x) => 

    (List("iload " + env(x) + "\n" + 

           "invokestatic XXX/XXX/write(I)V\n"), env)

  case Read(x) => {

    val index = if (env.isDefinedAt(x)) env(x) else 

                    env.keys.size.toString

    (List("invokestatic XXX/XXX/read()I\n" + 

          "istore " + index + "\n"), env + (x -> index))

  }

}

// compilation of a block (i.e. list of instructions)

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

  case Nil => (Nil, env)

  case s::bl => {

    val (instrs1, env1) = compile_stmt(s, env)

    val (instrs2, env2) = compile_block(bl, env1)

    (instrs1 ++ instrs2, env2)

  }

}

// main compilation function for blocks

def compile(bl: Block, class_name: String) : String = {

  val instructions = compile_block(bl, Map.empty)._1

  (beginning ++ instructions.mkString ++ ending).replaceAllLiterally("XXX", class_name)

}

// Fibonacci numbers as a test-case

val fib_test = 

  List(Read("n"),                       //  read n;                     

       Assign("minus1",Num(0)),         //  minus1 := 0;

       Assign("minus2",Num(1)),         //  minus2 := 1;

       Assign("temp",Num(0)),           //  temp := 0;

       While(Bop("<",Num(0),Var("n")),  //  while n > 0 do  {

          List(Assign("temp",Var("minus2")),    //  temp := minus2;

               Assign("minus2",Aop("+",Var("minus1"),Var("minus2"))), 

                                        //  minus2 := minus1 + minus2;

               Assign("minus1",Var("temp")), //  minus1 := temp;

               Assign("n",Aop("-",Var("n"),Num(1))))), //  n := n - 1 };

       Write("minus1"))                 //  write minus1

// prints out the JVM-assembly program

println(compile(fib_test, "fib"))

// can be assembled with 

//

//    java -jar jvm/jasmin-2.4/jasmin.jar fib.j

//

// and started with

//

//    java fib/fib