Elevator Controller

Requirements

• Accept hall calls (a floor and a direction) from the landing buttons
• Accept car calls (a destination floor) from the panel inside each car
• Assign every hall call to exactly one of several cars
• Serve each car's stops in its current direction of travel before reversing
• Open the doors on arrival, and never move a car while its doors are open

Assumptions you may make

• One building with a fixed number of floors and cars
• Time advances in discrete ticks; a moving car travels one floor per tick
• No physics, weight limits or emergency modes — those are extensions
• The controller runs on a single thread

Things the brief expects to exist

Candidates, not a design. How they relate is yours to decide.

ElevatorController

Receives hall calls, hands each to one car, and advances time

DispatchStrategy

Decides which car should answer a given hall call

ElevatorCar

Knows its floor and heading, and delegates each tick to its state

StopSet

Holds a car's requested floors and picks the next one to serve

CarState

Decides what a car does on this tick and which state follows

HallCall

Records a request made from a landing

What this exercise is teaching

• Separate choosing which car answers a call from deciding the order a car serves its stops
• Make the doors-open safety rule a property of the structure rather than a check someone must remember
• Put the dispatch rule behind a seam so a different rule can be tried without editing the controller

Once it works, make it survive these

• Add a fire-service mode that recalls every car to the lobby and ignores new calls
• Skip hall calls for a car that reports it is at capacity
• Support destination dispatch, where riders choose their floor in the lobby before boarding