Coding exercise

CodingExercise.scala

package mentoring

object CodingExercise {
  final case class Pos(x: Int, y: Int) {
    def +(other: Pos) = Pos(x + other.x, y + other.y)
  }

  object Pos {
    val Origin = Pos(0, 0)
  }

  val symbolToDelta = Map(
    '<' -> Pos(0, -1),
    '>' -> Pos(0, 1),
    '^' -> Pos(1, 0),
    'v' -> Pos(-1, 0)
  )

  def firstRepeatedPos(input: String): Option[Pos] = {
    val positions     = input.toStream.map(symbolToDelta).scanLeft(Pos.Origin)(_ + _)
    val seenPositions = positions.scanLeft(Set.empty[Pos])(_ + _)
    positions.zip(seenPositions).collectFirst {
      case (position, seen) if seen.contains(position) => position
    }
  }
}

CodingExerciseTest.scala

package mentoring

import mentoring.CodingExercise.Pos
import org.scalatest.{FlatSpec, Matchers}

final class CodingExerciseTest extends FlatSpec with Matchers {

  "The first repeating position" should "not exist for the empty path" in {
    CodingExercise.firstRepeatedPos("") shouldBe empty
  }

  it should "detect when it's crossed" in {
    CodingExercise.firstRepeatedPos(">>^>v<") should ===(Some(Pos(0, 2)))
  }
}