Event-driven finite-state machine

In computation, a finite state machine (FSM) is event driven if the creator of the FSM intends to think of the machine as consuming events or messages. This is in contrast to the parsing-theory origins of the term finite-state machine where the machine is described as consuming characters or tokens.

Often these machines are implemented as threads or processes communicating with one another as part of a larger application. For example, an individual car in a traffic simulation might be implemented as an event-driven finite-state machine.

This is a common and useful idiom, though not as precisely defined and theoretically grounded as the application of finite state machines to parsing. By abuse of terminology, programmers may refer to code created while thinking of this idiom as a finite state machine even if the space required for the state grows with the size of the input.