Elevator

Design and implement an elevator control system.

My approach is a really simple one:

The first person on the elevator who presses the floor button first is the one who sets the direction.
We give priority to the stops that are already in the direction.
When a person outside wants to use the elevator we try to find the closest elevator to the pickup floor according to the goal floor.

Two persons enter an standby elevator:
- The first person that presses the button sets the direction. Example: both persons are on the third floor.
  - Person A presses the button 1
  - Person B presses the button 4
  - Result: Elevator first goes to floor 1, then to floor 4.
Elevator already has people:
- Case 1:
  - Goal of elevator is floor 6.
  - Person A on floor 2 enters and presses button 3.
  - Elevator stops on floor 3 and continues to floor 6.
- Case 2.1:
  - Goal of elevator is floor 6.
  - Person A enters on floor 2 enters and presses button 3.
  - Person B enters on floor 3 and presses button 1.
  - Person C enters on floor 3 and presses button 4.
  - Elevator goes to floor 3, then goes to floor 4, then goes to floor 6 and finally goes to floor 1.
- Case 2.2:
  - Goal of elevator is floor 1.
  - Person A enters on floor 5 and presses button 4.
  - Person B enters on floor 4 and presses button 5.
  - Elevator goes to floor 4, then to floor 1, then to floor 5.
Person is in floor 3. Many elevators.
- Elevator A has as goal to reach floor 0 and is currently on floor 2.
- Elevator B has as goal to reach floor 1 and is currently on floor 4.
- Algorithm selects elevator B.
- Explanation: we pick the elevator that minimizes distance between pickup floor and elevator goal floor.

Entity	Description	API
Elevator	A class that represents an elevator. It holds the elevator id, and current state such as current floor, current goal floor, requests queue, and current direction.	`step()`, `addStop()`, `state()`
ElevatorStopsQueue	A special queue that gives priority to stops already in the direction. It is backed by two priority queues. It works as follows: 1) Add a stop to the queue: if the elevator is ascending then add the stop to the ascending queue, else add the stop to the descending queue. 2) Consuming the queue: if the elevator is going upwards, then dequeue from the ascending queue, else dequeue from the descending queue. If the current queue is empty, then dequeue from the non empty queue and change direction.	`addStop()`, `nextStop()`, `isEmpty()`
ElevatorSystem	This class controls and holds all the elevators. `bestElevatorToScheduleForPickup()`, is used to request an elevator for any floor. It tries to find the elevator that is closest to the pickup floor.	`elevatorState()`, `elevatorsState()`, `addStopToElevator()`, `bestElevatorToScheduleForPickup()`, `step()`

In order to run tests run sbt test.

ramomar / elevator