snail kata

snail.scala

import cats.data.NonEmptyList

object Snail {

  case class Coordinate(i: Int, j: Int)
  case class Navigation(visitedSofar: NonEmptyList[Coordinate])

  sealed trait Direction
  object Direction {
    case object Up    extends Direction
    case object Right extends Direction
    case object Down  extends Direction
    case object Left  extends Direction

    def rightHandedRotate(currentDir: Direction): Direction = currentDir match {
        case Direction.Right =>
          Direction.Down
        case Direction.Left =>
          Direction.Up
        case Direction.Up =>
          Direction.Right
        case Direction.Down =>
          Direction.Left
      }
  }

  def coordinateAhead(coordinate: Coordinate, currentDirection: Direction): Coordinate = {
    val iValue = coordinate.i
    val jValue = coordinate.j

    currentDirection match {
      case Direction.Up =>
        coordinate.copy(j = jValue - 1)
      case Direction.Right =>
        coordinate.copy(i = iValue + 1)
      case Direction.Down =>
        coordinate.copy(j = jValue + 1)
      case Direction.Left =>
        coordinate.copy(i = iValue - 1)
    }
  }

  def maybeCoordinateAhead(navigation: Navigation, currentDirection: Direction, squareSide: Int): Option[Coordinate] = {
    val ahead = coordinateAhead(navigation.visitedSofar.head, currentDirection)
    Option.when(
      !navigation.visitedSofar.toList.contains(ahead) &&
        ahead.i <= squareSide &&
        ahead.j <= squareSide &&
        0 < ahead.i &&
        0 < ahead.j
    )(ahead)
  }

  def getNumberAt(xs: List[List[Int]])(coordinate: Coordinate): Int = xs(coordinate.j - 1)(coordinate.i - 1)

  def snail(xs: List[List[Int]]): List[Int] = {

    def update(currentDirection: Direction, navigation: Navigation): Navigation =
      if (navigation.visitedSofar.size == xs.size * xs.size)
        navigation
      else
        maybeCoordinateAhead(navigation, currentDirection, xs.size) match {
          case Some(nextCoordinate) =>
            update(currentDirection, Navigation(nextCoordinate :: navigation.visitedSofar))
          case None =>
            update(Direction.rightHandedRotate(currentDirection), navigation)
        }

    xs match {
      case _ if xs.flatten.isEmpty =>
        Nil
      case _ =>
        val start = Coordinate(1, 1)
        update(Direction.Right, Navigation(NonEmptyList.one(start)))
          .visitedSofar
          .reverse
          .map(getNumberAt(xs))
          .toList
    }
  }

}