A super minimal implementation of a thread-pool-executor service / scheduler in Java. The goal is to provide a simple, easy to understand implementation. The implementation is not intended to be used in production, but rather to be used as a learning tool.

Problem description:

- There are 7 elevators in a building with 55 floors.
- Each floor has exactly one button to request an elevator.
- Elevators have the capacity of carrying an arbitrary, given number of people.
- Elevators don't change direction and don't stop mid-trip.
- People can only choose their destination freely, when they are on the ground-floor:
    - up: ground-floor [0] − can only travel to non-ground-floors [1;54]
    - down: non-ground-floor [1;54] − can only travel to the ground-floor [0]
- The requests are handled globally.
- The elevator-scheduler assigns a chosen request to an elevator based on some arbitrary algorithm.
- If there are no elevators available, the request is queued up.

Design decisions:

- Decided to use vanilla Java without any non-default libraries for the sake of simplicity.
- Got rid of the suggested "up"/"down" directions since they can be inferred from the current floor.
- Chose a poison-pill approach to terminate the scheduler (also
  see: https://gitlab.com/niklaswimmer/dc-tower-elevator-challange/-/blob/main/app/src/main/java/me/nikx/dctower/TowerController.java
- Chose JDK-19 to make use of type inferrence with `var` and other nifty features.