Finite State Machines

A computational model that transitions between a fixed set of states based on specific inputs.

Finite State Machines (FSMs) are the backbone of predictable logic in systems ranging from vending machines to AAA game AI (like the behavior trees in Halo). By defining a finite set of states (e.g., Idle, Loading, Error) and strict transition rules, developers eliminate the 'illegal state' bugs common in complex boolean logic. Whether you are implementing a regex engine or a TCP network protocol, FSMs provide a mathematical guarantee that your system remains in a known, valid configuration at all times.

https://brilliant.org/wiki/finite-state-machines/