// 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 ( e : Exp, targetRegister : Int ) : String = {
    e match {
      case IntLit ( n  ) => s"${ pad }LoadImm R${ targetRegister } ${ n }\n"
      case Var ( x ) => s"${ pad }Load R${ targetRegister } ${ x }\n"
      case Add ( l, r ) => {
        codegenExp ( l, targetRegister ) ++
        codegenExp ( r, targetRegister+1 ) ++        
        s"${ pad }Plus R${ targetRegister } R${ targetRegister+1 }\n" }
      case Subt ( l, r ) => {
        codegenExp ( l, targetRegister ) ++
        codegenExp ( r, targetRegister+1 ) ++        
        s"${ pad }Minus R${ targetRegister } R${ targetRegister+1 }\n" }
      case Mult ( l, r ) => {
        codegenExp ( l, targetRegister ) ++
        codegenExp ( r, targetRegister+1 ) ++        
        s"${ pad }Times R${ targetRegister } R${ targetRegister+1 }\n" }
      case Div ( l, r ) => {
        codegenExp ( l, targetRegister ) ++
        codegenExp ( r, targetRegister+1 ) ++        
        s"${ pad }Div R${ targetRegister } R${ targetRegister+1 }\n" }
      case Negate ( e ) => "..." } }

  def codegenM ( p : Program, targetRegister : Int ) : String = {
    p match {
      case Seq ( l, r  ) => {
        codegenM ( l, targetRegister ) ++
        codegenM ( r, targetRegister ) }
      case Assign ( x, e ) => {
        codegenExp ( e, targetRegister ) ++
        s"${ pad }Store R${ targetRegister } ${ x }\n" }
      case For ( x, lo, hi, body ) => {
        val check = freshLabel ( "check" )
        val loopExit = freshLabel ( "exit" )
        codegenExp ( lo, targetRegister ) ++
        s"${ pad }Store R${ targetRegister } ${ x }\n" ++
        s"${ check }:\n" ++
        s"${ pad }Load R${ targetRegister } ${ x }\n" ++
        codegenExp ( hi, targetRegister+1 ) ++
        s"${ pad }CompGreaterThan R${ targetRegister } R${ targetRegister+1 }\n" ++
        s"${ pad }JumpTrue R${targetRegister} ${ loopExit }\n" ++
        codegenM ( body, targetRegister+2 ) ++
        s"${ pad }Load R${ targetRegister } ${ x }\n" ++
        s"${ pad }LoadImm R${ targetRegister+2 } 1\n" ++
        s"${ pad }Plus R${ targetRegister } R${ targetRegister+2 }\n" ++
        s"${ pad }Store R${ targetRegister } ${ x }\n" ++
        s"${ pad }Jump ${ check }\n" ++
        s"${ loopExit }:\n" } } }