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Simulation of an elevator control system for a job interview coding challenge.

The algorithm aims to optimise an average time needed to fulfil a request. Where request is: a call button, pushed on the floor (pick-ups), or destination button, pushed inside an elevator (drop-offs). The system has no inputs about actual passenger flow, so there are no optimisations in that regard.

Roughly, the algorithm is:

• On each step, the system iterates active pick-ups and assigns each to the closest by ETA elevator
  • ETA is estimated number of steps to reach pick-up floor, considering the below details
• Particular elevators queue consists of its drops-off and the active pick-ups
  • drop-offs are kept in the queue until fulfilled, pick-ups are cleaned and reassigned anew each step
• Elevator keeps moving in the same direction while its queue contains requests in that direction
• Then, if there are requests in opposite direction, the direction is switched
• Elevator stops if:
  • it has drop-offs at current floor
  • system has pick-ups at current floor in current direction
  • When stopped, it can resume after 3 steps
• Elevator with empty queue is idle and has no preferred direction
  • the new direction is chosen by the 1st request assigned
    • if several assigned in one step - by majority of them

To explain, why this algorithm is (close to) optimal, let's consider distance in floors an elevator has to travel to fulfil certain request R. It depends on the floors Q(...) it has to visit before R, and can be expressed as a sum of the distances between consecutive floors, sth like (in pseudo-scala): D = Q.sliding(2)( (prev,next)=>abs(next-prev) ).sum. By moving elevators in the same direction as long as possible, serving requests on the way, we essentially order Q monotonously, thus minimizing every item of the sum.

FCFS would order it only by time, making every item of the sum arbitrary large, i.e. an elevator would cover up to the entire building height between each pair.

The public API is in package object of xko.elevators it follows the challenge proposal, except the elevator status is a separate Elevator class. All the API instances are immutable: the request methods - pickUp and dropOff - as well as proceed (advances to the next step), all return a new copy of ControlSystem, which should be used for further requests and advancing to the next step.

Elevator is implemented by Lift trait and its further implementations for different states - all placed in the same file. The ControlSystem is implemented by Scheduler

The project is sbt-based. All sbt commands are available via included starter script. E.g. ./bin/sbt test.

The tests are located in Spec.scala