Do you have any accessible references on state machines? I’ve just read about them a bit but never coded one in a project. Seems like a useful concept for testable, reliable code.
The video Finite State Machines explained by Abelardo Pardo[1] seems like a good introduction (I'm not familiar with the author; I just searched Finite State machine on youtube and found the first result which wasn't a jumbled mess of abstract or misused jargon).
It may seem simple, but that's truely all there is to finite state machines, a set of finite states and a set of events which cause transitions between states. Their specific applications to various domains is a much larger topic, but with a basic understanding, any application should be clear whenever it comes up. There's no one way to implement a state machine, and you've probably used them all the time, even if you haven't thought about them formally. For example, a simple but common 2-state machine takes the form:
running = true
while running:
…
if <some condition>:
running = false
A finite state machine has a set of input symbols, a set of states, and a transition function, which maps the current state and current input to the next state (and, when programming it, it doesn't have to be a literal function; for example, it could be a 2D array). It also has a start state, and a set of accept states.
It's also common for state machines to have some sort of output; otherwise, all an FSM can do is recognize whether an input string is "good." So, there's also an output function, so that the FSM actually does something while processing the input string. There are two types of FSMs: a Mealy Machine, and a Moore machine. In a Mealy machine, the output function maps the current state and the current input to an output; so, one state could be associated with multiple outputs. In a Moore machine, the output is purely a function of the current state, no matter the input. Usually, a Moore machine has more states than the equivalent Mealy machine. In practice, Moore machines are a lot rarer than Mealy machines.
You can model FSMs in a variety of ways. You could use the state pattern [0]. You could have a table-based state machine; examples in C here [1] [2]. It's also common to model them using a loop and a switch statement with integers to represent the current state - or just use "goto" to go to the next state, if it's available in your language.
Also, this is tangential, but something nice about coroutines is that they save can you from writing an explicit state machine, as the state is implicit in your code. One example of this is how it's easier to write iterators as coroutines that yield results as they run, rather than as structures that update themselves based on their current state, transition to the next state, and yield a result.
So far, what I've covered is basically a deterministic finite automaton (DFA). There are also non-deterministic finite automata (NDFAs), which are used to implement regular expressions (at least when regular expressions aren't extended to require more powerful recognizers) [3], as well as pushdown automata (PDAs), which are basically finite automata with a stack, and can parse e.g. programming languages. Finally, there are Turing machines.
For theory on that, I'd recommend reading Sipser's Introduction to the Theory of Computation. If you look up the book on Google, it's not difficult to find a PDF of it.
