Walkthrough: elevator system

1 const Direction = { UP: "UP", DOWN: "DOWN", IDLE: "IDLE" };
2 
3 // Observer: Floor Display
4 class DisplayObserver {
5   onFloorArrival(carId, floor) {
6     console.log(`[Display] Car ${carId} arrived at floor ${floor}`);
7   }
8 }
9 
10 // Elevator Car Entity
11 class ElevatorCar {
12   #id;
13   #currentFloor = 1;
14   #direction = Direction.IDLE;
15   #destinations = new Set();
16   #listeners = [];
17 
18   constructor(id) {
19     this.#id = id;
20   }
21 
22   addArrivalListener(listener) {
23     this.#listeners.push(listener);
24   }
25 
26   addDestination(floor) {
27     this.#destinations.add(floor);
28     if (this.#direction === Direction.IDLE) {
29       this.#direction = floor > this.#currentFloor ? Direction.UP : Direction.DOWN;
30     }
31   }
32 
33   step() {
34     if (this.#destinations.size === 0) {
35       this.#direction = Direction.IDLE;
36       return;
37     }
38 
39     // Move 1 floor in active direction
40     if (this.#direction === Direction.UP) this.#currentFloor++;
41     else if (this.#direction === Direction.DOWN) this.#currentFloor--;
42 
43     // Check arrival
44     if (this.#destinations.has(this.#currentFloor)) {
45       this.#destinations.delete(this.#currentFloor);
46       for (const listener of this.#listeners) {
47         listener.onFloorArrival(this.#id, this.#currentFloor);
48       }
49     }
50   }
51 
52   get id() { return this.#id; }
53   get currentFloor() { return this.#currentFloor; }
54   get direction() { return this.#direction; }
55   get targetCount() { return this.#destinations.size; }
56 }
57 
58 // Strategy: Dispatch algorithm (finds closest idle or matching direction car)
59 class NearestCarStrategy {
60   chooseElevator(cars, requestedFloor, requestedDir) {
61     let bestCar = null;
62     let minDistance = Infinity;
63 
64     for (const car of cars) {
65       const dist = Math.abs(car.currentFloor - requestedFloor);
66       // Prefer IDLE cars or cars already moving toward this floor in same direction
67       const canPick = car.direction === Direction.IDLE || car.direction === requestedDir;
68       if (canPick && dist < minDistance) {
69         minDistance = dist;
70         bestCar = car;
71       }
72     }
73     return bestCar ?? cars[0]; // Fallback to first car
74   }
75 }
76 
77 // System Coordinator
78 class ElevatorController {
79   #cars = [];
80   #strategy;
81 
82   constructor(cars, strategy = new NearestCarStrategy()) {
83     this.#cars = cars;
84     this.#strategy = strategy;
85   }
86 
87   requestElevator(floor, direction) {
88     const car = this.#strategy.chooseElevator(this.#cars, floor, direction);
89     car.addDestination(floor);
90     return car.id;
91   }
92 
93   tick() {
94     for (const car of this.#cars) car.step();
95   }
96 }
97 
98 // Verification Test
99 const car1 = new ElevatorCar("Car-1");
100 const car2 = new ElevatorCar("Car-2");
101 const display = new DisplayObserver();
102 car1.addArrivalListener(display);
103 
104 const controller = new ElevatorController([car1, car2]);
105 
106 // Passenger on Floor 3 presses UP:
107 const assignedId = controller.requestElevator(3, Direction.UP);
108 console.assert(assignedId === "Car-1");
109 
110 // Simulate elevator ticks:
111 controller.tick(); // Moves to floor 2
112 controller.tick(); // Arrives at floor 3 (Triggers display listener!)
113 console.assert(car1.currentFloor === 3);
114 console.log("Current car 1 floor:", car1.currentFloor);

1 public class Example {
2   enum Direction { UP, DOWN, IDLE }
3 
4   static final class ElevatorCar {
5     final String id;
6     int currentFloor = 1;
7     Direction direction = Direction.IDLE;
8     final java.util.Set<Integer> targets = new java.util.HashSet