Walker.scala

trait Infos {
  val validActions = 'r' :: 'l' :: 'a' :: Nil
  val dirs         = Map(0 -> "North", 1 -> "East", 2 -> "South", 3 -> "West")

  def isAction: Char => Boolean = validActions contains _

  implicit class StringCheck(str: String) {
    def isActionString: Boolean = str forall isAction
    def isNaAS:         Boolean = !isActionString
    def isNumber:       Boolean = str forall (x => ('0' to '9') :+ '-' contains x)
    def isNaN:          Boolean = !isNumber
  }
}

case class Coord(x:Int, y:Int, dir:String = "North")

object Walker extends Infos {
  def usage: Unit = println("usage: walker.jar <X_POS_ON_GRID> <Y_POS_ON_GRID> <DIRECTIONS_STRING>")

  def move(action:List[Char], dir:Int = 0)(x:Int, y:Int): List[Coord] =
    action match {
      case h :: t => h match {
        case 'r' | 'l' => move(t, (4 + dir + (if ('r' == h) 1 else -1)) % 4)(x,y)
        case 'a'       => dir match {
          case 0 | 2 => Coord(x,y,dirs(dir)) :: move(t, dir)(x, y + (if (dir == 0) 1 else -1))
          case 1 | 3 => Coord(x,y,dirs(dir)) :: move(t, dir)(x + (if (dir == 1) 1 else -1), y)
        }
      }
      case Nil => Coord(x,y,dirs(dir)) :: Nil
    }

  def putOnGrid(actions:String, x:String, y:String): Unit =
    if (x.isNaN || y.isNaN || actions.isNaAS) usage else {
      val coords: List[Coord] = move(actions.toList)(x.toInt, y.toInt)
      val init: Coord = coords.head
      val last: Coord = coords.last
      val max:Coord   = Coord(coords.map(_.x).max + 2, coords.map(_.y).max + 2)
      val min:Coord   = Coord(coords.map(_.x).min - 2, coords.map(_.y).min - 2)
      println(
        s"Initial position: \u001b[92m[${init.x},${init.y}]\u001b[0m facing ${init.dir}\n" +
        (max.y to min.y by -1).map { y =>
          (min.x to max.x).map { x =>
            if      (Coord(x,y,init.dir) == init) "\u001b[92mD\u001b[0m"
            else if (Coord(x,y,last.dir) == last) "\u001b[91mA\u001b[0m"
            else if (coords.map(e => (e.x,e.y)).contains((x,y))) "o"
            else "."
          }.mkString("  ")
        }.mkString("\n") +
        s"\nCurrent position: \u001b[91m[${last.x},${last.y}]\u001b[0m facing ${last.dir}"
      )
    }

  //usage example: scala walker.jar -3 -6 "RAALAAAARARLLAAAAAAAALRAALRRARRAAAALLLAR"
  def main(av:Array[String]): Unit = av.toList match {
    case x :: y :: actions :: Nil => putOnGrid(actions.toLowerCase, x, y)
    case           actions :: Nil => putOnGrid(actions.toLowerCase, "0", "0")
    case _                        => usage
  }
}