// I'm writing ++ for string contactenation. So e.g.
  //
  //   "hello" ++ " " ++ "world"
  //
  // will be the string
  //
  //   "hello world"
  //
  // I use "string interpolation" (see https://en.wikipedia.org/wiki/String_interpolation)
  // to construct strings. For example the string
  //
  //   s"hello ${ prog } world"
  //  
  // (note the s in front of the first quotation mark) is constructed from three parts
  //
  //  - "hello "
  //  - the result of the program prog
  //  - " world"
  //  
  //  If running the program prog returns
  //  
  //     "my lovely"
  //  
  //  then s"hello ${ prog } world" evaluates to
  //  
  //     s"hello my lovely world" 
  //  


  private var labelCount = 0
  def freshLabel ( s : String ) : String = {
    labelCount += 1
    s"${ s }_${ labelCount.toString }" }
  val pad = "   "


  def codegenExp ( exp : Exp ) : String = {
    exp match {
      case IntLit ( n  ) => {
        s"${ pad }LoadImm ${ n.toString }\n" }
      case Var ( x ) => { s"${ pad }Load ${ x }\n" }
      case Add ( l, r ) => {
        codegenExp ( l ) ++
        s"${ pad }Push\n" ++
        codegenExp ( r ) ++        
        s"${ pad }Plus\n" }
      case Subt ( l, r ) => {
        codegenExp ( l ) ++
        s"${ pad }Push\n" ++
        codegenExp ( r ) ++        
        s"${ pad }Minus\n" }
      case Mult ( l, r ) => {
        codegenExp ( l ) ++
        s"${ pad }Push\n" ++
        codegenExp ( r ) ++        
        s"${ pad }Times\n" }
      case Div ( l, r ) => {
        codegenExp ( l ) ++
	"Push\n" ++
        codegenExp ( r ) ++
        s"${ pad }Div\n" }
      case Negate ( e ) => "..." } }


  def codegenM ( p : Program ) : String = {
    p match {
      case Seq ( l, r  ) => {
        codegenM ( l ) ++
        codegenM ( r ) }
      case Assign ( x, e ) => {
        codegenExp ( e ) ++
        s"${ pad }Store ${ x }\n" }
      case For ( x, lo, hi, body ) => { 
        val check = freshLabel ( "check" )
        val loopExit = freshLabel ( "exit" )
        codegenExp ( lo ) ++
        s"${ pad }Store ${ x }\n" ++
        s"${ check }:\n" ++
        codegenExp ( hi ) ++
        s"${ pad }Push\n" ++ 
        s"${ pad }Load ${ x }\n" ++
        s"${ pad }CompGreaterThan \n" ++
        s"${ pad }JumpTrue ${ loopExit }\n" ++
        codegenM ( body ) ++
        s"${ pad }Load ${ x }\n" ++
        s"${ pad }LoadImm 1\n" ++
        s"${ pad }PlusStack \n" ++
        s"${ pad }Store ${ x }\n" ++
        s"${ pad }Jump ${ check }\n" ++
        s"${ loopExit }:\n" } } }