elevator-challenge

elevator coding challenge

How to build/run it

To build and run all tests use the following command:

sbt test

How to use it

Elevator simulation interface is the following:

trait ElevatorControlSystem {

  def status(): Seq[(ElevatorId, FloorNumber, Direction, Set[FloorNumber])]

  def pickup(pickupFloor: FloorNumber, goalFloor: FloorNumber): ElevatorControlSystem

  def step(): ElevatorControlSystem
}

• To query the status - call status() method, it returns sequence of elevators states with the following data: elevatorId, currFloor and goalFloors for each elevator in the system

• To request a pickup - call pickup() method, it accepts two parameters: pickupFloor and goalFloor, the direction is the following:

  • pickupFloor < goalFloor - going up (+)
  • pickupFloor > goalFloor - going down (-)

• By calling step() method the system will simulate time-stepping:

  • elevator that is empty (no goal floors) - is unchanged (only if there is no new pickup requests)
  • elevator that is between floors - going to the next floor (currFloor +/- 1)
  • for elevator that is on either pickupFloor or goalFloor the system will either add or remove goal floors accordingly

• Since both pickup() and step() methods alter system state they both return new ElevatorControlSystem instance, the old instance remains unchanged.

• You can chain several calls:

  controlSystem
    .pickup(...)
    .pickup(...)
    .step()
    .step()
    .status()

Setting initial state

To set initial state - call ElevatorSimulator constructor:

val controlSystem = ElevatorSimulator(List(
  (ElevatorId(1), FloorNumber(0), Up, Set(FloorNumber(1))),
  (ElevatorId(2), FloorNumber(-1), Down, Set(FloorNumber(-2))),
  ...
))

Notes:

• at least one elevator should be provided, maximum is 16
• FloorNumber should be between -9 and 9
• for simplicity elevators has unlimited capacity (number of people)

How it's implemented

The Challenge

Then challenge is to schedule and process the pickup requests. There are few different options, though:

1. First requests handled first, FCFS (first-come, first-served). But, it may lead to both:

   • inefficient elevator usage, and
   • delays of requests

   Elevators may go to the far floors first, ignoring more close but less priority requests.

2. Schedule requests for elevators that are:

   • the closest to the pickup floor, and
   • going the same direction has a priority

Ofcourse, there are many other possible options and optimisations. They are not considered here for the simplicity of the solution.

The Solution

For handling request in more natural and good enough way, the 2nd option was chosen.

The pickup requests are scheduled at the moment of pickup request, by adding pickupFloor to the goalFloors of the closest elevator with giving priority to the elevators going the same direction as pickup direction.

ElevatorSimulator => tests

Example ride

	Pickups	Elevator 1	Elevator 2
initial		0 Up (2)	0 Down (-2)
pickup(1, 3)	1 -> (3)	0 Up (1, 2)	0 Down (-2)
pickup(-1, -3)	1 -> (3), -1 -> (-3)	0 Up (1, 2)	0 Down (-1, -2)
step()		1 Up (2, 3)	-1 Down (-2, -3)
step()		2 Up (3)	-2 Down (-3)
step()		3 Up	-3 Down