Solution

import scala.collection.mutable
import scala.util.Try

object Solution extends App {

  /**
    * Implement the following interface for an elevator
    */
  trait Elevator {

    /**
      * Returns the number of floors that the elevator services
      */
    def numberOfFloors: Int

    /**
      * Request the elevator to visit the given floor
      */
    def requestFloor(floor: Int): Unit

    /**
      * Move the elevator to the next floor or remain if no movement is necessary.
      * Returns the floor the elevator is located at after the movement, i.e. either 0 or 1 floors different from before.
      * The elevator continues to travel in its current direction (up or down) until there are no more requests in that direction.
      * Then it continues in the opposite direction, or stops if there are no more requested floors.
      */
    def stepAdvanceFloor(): Int

    /**
      * Returns the floor the elevator is currently located at
      */
    def currentFloor: Int

  }

  val elevator: Elevator = new ElevatorImpl1(8)

  class ElevatorImpl1(reachableFloors: Int) extends Elevator {

    private var floor: Int = 0
    private val requests: mutable.PriorityQueue[Int] = mutable.PriorityQueue[Int]()
    private var request: Option[Int] = None

    /**
      * Returns the number of floors that the elevator services
      */
    override def numberOfFloors: Int = reachableFloors

    /**
      * Request the elevator to visit the given floor
      */
    override def requestFloor(floor: Int): Unit =
      if (floor >= 0 && floor <= reachableFloors)
        requests.enqueue(floor)

    /**
      * Returns the floor the elevator is currently located at
      */
    override def currentFloor: Int = floor

    /**
      * Move the elevator to the next floor or remain if no movement is necessary.
      * Returns the floor the elevator is located at after the movement, i.e. either 0 or 1 floors different from before.
      * The elevator continues to travel in its current direction (up or down) until there are no more requests in that direction.
      * Then it continues in the opposite direction, or stops if there are no more requested floors.
      */
    override def stepAdvanceFloor(): Int = {
      if (request.isEmpty)
        request = Try(requests.dequeue()).toOption
      request.foreach { r =>
        if (r > floor)
          floor += 1
        else if (r < floor)
          floor -= 1
        if (r == floor)
          request = None
      }
      currentFloor
    }
  }


  // Ensure this example returns the correct results

  assert(elevator.numberOfFloors == 8, "numberOfFloors != 8")
  assert(elevator.currentFloor == 0, "currentFloor != 0, stopped")
  elevator.requestFloor(2)
  assert(elevator.stepAdvanceFloor() == 1, "currentFloor != 1, moving up")
  elevator.requestFloor(6)
  assert(elevator.stepAdvanceFloor() == 2, "currentFloor != 2, moving up")
  assert(elevator.stepAdvanceFloor() == 3, "currentFloor != 3, moving up")
  assert(elevator.stepAdvanceFloor() == 4, "currentFloor != 4, moving up")
  elevator.requestFloor(3)
  assert(elevator.stepAdvanceFloor() == 5, "currentFloor != 5, moving up")
  assert(elevator.stepAdvanceFloor() == 6, "currentFloor != 6, moving up")
  assert(elevator.stepAdvanceFloor() == 5, "currentFloor != 5, moving down")
  assert(elevator.stepAdvanceFloor() == 4, "currentFloor != 4, moving down")
  assert(elevator.stepAdvanceFloor() == 3, "currentFloor != 3, moving down")
  assert(elevator.stepAdvanceFloor() == 3, "currentFloor != 3, stopped")
  elevator.requestFloor(4)
  elevator.requestFloor(2)
  assert(elevator.stepAdvanceFloor() == 4, "currentFloor != 4, moving up again")
  assert(elevator.stepAdvanceFloor() == 3, "currentFloor != 3, moving down again")
  assert(elevator.stepAdvanceFloor() == 2, "currentFloor != 3, moving down again")
  // Current floor == 2

  elevator.requestFloor(1)
  elevator.requestFloor(5)
  elevator.requestFloor(0)
  assert(elevator.stepAdvanceFloor() == 1, "currentFloor != 1, moving down again?")
  assert(elevator.stepAdvanceFloor() == 0, "currentFloor != 0, moving down again?")
  assert(elevator.stepAdvanceFloor() == 1, "currentFloor != 1, moving up again?")

}