Designing an Elevator System

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Requirements:

• The elevator system should consist of multiple elevators serving multiple floors.
• Each elevator should have a capacity limit and should not exceed it.
• Users should be able to request an elevator from any floor and select a destination floor.
• The elevator system should efficiently handle user requests and optimize the movement of elevators to minimize waiting time.
• The system should prioritize requests based on the direction of travel and the proximity of the elevators to the requested floor.
• The elevators should be able to handle multiple requests concurrently and process them in an optimal order.
• The system should ensure thread safety and prevent race conditions when multiple threads interact with the elevators.

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Sequnece Diagram

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System Components:

• Elevator: Represents an individual elevator with attributes like capacity, current floor, direction, and request queue.
• ElevatorSystem: Manages the elevator(s) and handles user requests.
• UserRequest: Represents a user's request, containing the source floor, destination floor, and user ID.
• Direction: Enum to track the direction of the elevator (UP, DOWN, IDLE).

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Key Design Considerations:

• Request Handling: Requests should be queued and processed based on the proximity of elevators and the direction of travel.
• Capacity Management: Each elevator should have a capacity limit, and the system should ensure that the limit is not exceeded.
• Thread Safety: Synchronization mechanisms must be implemented to handle concurrent requests and prevent race conditions.
• Optimization: The elevator system should prioritize requests based on the direction of travel and minimize unnecessary movements.

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Code Implementation:

import java.util.*;
import java.util.concurrent.*;

// Enum to represent the direction of the elevator
enum Direction {
    UP, DOWN, IDLE
}

// Class representing a user request
class UserRequest {
    int sourceFloor;
    int destinationFloor;

    public UserRequest(int sourceFloor, int destinationFloor) {
        this.sourceFloor = sourceFloor;
        this.destinationFloor = destinationFloor;
    }
}

// Class representing the elevator
class Elevator {
    private int capacity;
    private int currentFloor;
    private Direction direction;
    private List requestQueue;

    public Elevator(int capacity) {
        this.capacity = capacity;
        this.currentFloor = 0; // Assume starts at floor 0
        this.direction = Direction.IDLE;
        this.requestQueue = new LinkedList<>();
    }

    public synchronized void addRequest(UserRequest request) {
        // Add a request to the elevator's queue
        if (requestQueue.size() < capacity) {
            requestQueue.add(request);
            System.out.println("Request added to elevator on floor " + currentFloor);
        } else {
            System.out.println("Elevator at capacity, request cannot be added.");
        }
    }

    public void processRequests() {
        // Process the elevator's request queue
        while (!requestQueue.isEmpty()) {
            UserRequest request = requestQueue.remove(0);
            moveToFloor(request.sourceFloor);
            System.out.println("Elevator moving to floor " + request.sourceFloor);
            moveToFloor(request.destinationFloor);
            System.out.println("Elevator moving to floor " + request.destinationFloor);
        }
    }

    private void moveToFloor(int floor) {
        // Simulate elevator movement
        while (currentFloor != floor) {
            if (floor > currentFloor) {
                currentFloor++;
                direction = Direction.UP;
            } else {
                currentFloor--;
                direction = Direction.DOWN;
            }
            System.out.println("Elevator moving " + direction + " to floor " + currentFloor);
        }
        direction = Direction.IDLE;
    }
}

// Class representing the ElevatorSystem
class ElevatorSystem {
    private List elevators;
    private ExecutorService executorService;

    public ElevatorSystem(int numElevators, int elevatorCapacity) {
        elevators = new ArrayList<>();
        for (int i = 0; i < numElevators; i++) {
            elevators.add(new Elevator(elevatorCapacity));
        }
        executorService = Executors.newFixedThreadPool(numElevators);
    }

    public void requestElevator(UserRequest request) {
        Elevator closestElevator = findClosestElevator(request.sourceFloor);
        executorService.submit(() -> {
            closestElevator.addRequest(request);
            closestElevator.processRequests();
        });
    }

    private Elevator findClosestElevator(int sourceFloor) {
        Elevator closestElevator = elevators.get(0);
        int minDistance = Math.abs(closestElevator.currentFloor - sourceFloor);

        for (Elevator elevator : elevators) {
            int distance = Math.abs(elevator.currentFloor - sourceFloor);
            if (distance < minDistance) {
                minDistance = distance;
                closestElevator = elevator;
            }
        }
        return closestElevator;
    }
}

// Main application to simulate elevator system usage
public class ElevatorApp {
    public static void main(String[] args) {
        ElevatorSystem elevatorSystem = new ElevatorSystem(3, 5); // 3 elevators, each with a capacity of 5 requests

        // Users requesting elevators
        elevatorSystem.requestElevator(new UserRequest(0, 5));
        elevatorSystem.requestElevator(new UserRequest(2, 7));
        elevatorSystem.requestElevator(new UserRequest(5, 1));
    }
}

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Explanation of the Code:

• Elevator Class:
  • Represents the elevator with attributes like capacity, currentFloor, and direction.
  • The addRequest method adds a request to the elevator's queue, while processRequests handles the movement to each requested floor.
  • The moveToFloor method simulates the elevator's movement from one floor to another, updating its state.
• ElevatorSystem Class:
  • Manages multiple elevators and assigns user requests to the closest available elevator.
  • The requestElevator method finds the closest elevator based on the source floor of the user's request and assigns it to process the request.
  • Uses a fixed thread pool ExecutorService to handle concurrent requests and ensure efficient processing.
• ExecutorService: Used to manage concurrent execution of requests in different elevators. Each elevator processes its own queue independently in a separate thread.
• UserRequest Class: Represents a user's request, including the source floor and destination floor.

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Design Considerations:

• Capacity Management: Each elevator has a fixed capacity limit, and the system ensures that the elevator does not exceed it when processing requests.
• Request Distribution: Requests are assigned to the closest elevator to optimize waiting times. This can be further optimized with a more complex strategy based on load balancing.
• Concurrency: The system uses ExecutorService to process requests concurrently. Each elevator processes its own requests in separate threads, avoiding race conditions by ensuring thread safety when adding requests to the queue.

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Improvements & Extensions:

• Implement better optimization strategies for assigning requests based on the direction of travel and the load in each elevator.
• Handle more complex scenarios like elevators reaching capacity and unable to accept more requests.
• Implement a more sophisticated scheduling algorithm to prioritize requests (e.g., considering the current direction of the elevator).
• Add logging and error handling mechanisms for more robust production-level code.

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