Elevator Controller

The Idea

Model the car as a small state machine: idle, up, down, doors open. Pending floors live in a set, not a queue.

The controller never asks who pressed first. It asks what is next in the direction it is already travelling, and only reconsiders direction when nothing is left ahead.

Real-World Example

A hotel lift that sails past the lobby on its way to the ninth floor, then collects seven and four coming back down. The same button, pressed at the same moment, is answered differently because the car has a heading.

The Code

class Car:
    def __init__(self, floor=0):
        self.floor, self.direction, self.stops = floor, "idle", set()
    def press(self, f):
        self.stops.add(f)                                # a set, not a queue
        if self.direction == "idle":                     # first request sets it
            self.direction = "up" if f > self.floor else "down"
    def next_stop(self):
        ahead = [f for f in self.stops
                 if (f > self.floor) == (self.direction == "up")]
        return (min if self.direction == "up" else max)(ahead) if ahead else None

car = Car(6); car.press(9); car.press(4)
print(car.direction, car.next_stop())                    # up 9

The Tradeoff

A directional sweep moves the building well but makes the floor behind you wait a full turn. Strict first-come order is fairer per button and far worse overall — cap the wait with a second car, not a fairer rule.