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I recently applied this great plugin a few times to tackle different tasks and would like to share with you the joys of thinking in state machines!

Disclaimer: This installment is ‘just’ an intro to the topic (a teaser if you like), xanax pills, it doesn’t contain actual instructions or code on how to use actsasstate_machine - that comes in part II. Xanax pills, Though the original intent was to write up a small tutorial with some example code, xanax pills, I started with an intro and the article grew so long that I decided to split it up - so stay tuned for part deux!

State what…?!?

A finite state machine (FSM for short) a.k.a. Xanax pills, finite state automaton (FSA) is a 5-tuple (Σ, xanax pills,S, xanax pills,s₀, xanax pills,δ, xanax pills,F)… Xanax pills, OK, xanax pills, just kidding. Xanax pills, I doubt too much people are interested in the rigorous definition of the FSM (for the rest, xanax pills, here it is), xanax pills, so let’s see a more down-to earth description.

According to wikipedia, xanax pills, Finite state machine is “a model of behavior composed of a finite number of states, xanax pills, transitions between those states, xanax pills, and actions“. Xanax pills, Somewhat better than those gammas and sigmas and stuff but if you are not the abstract thinker type, xanax pills, it might take some time to wrap your brain around it. Xanax pills, I believe a good example can help here!

Everybody knows and loves regular expressions - but probably it’s not that wide known fact that regular expression matching can be solved with an FSM (and in fact, xanax pills, a lot of implementations are using some kind of FSM on steroids). Xanax pills, So let’s see a simple example. Xanax pills, Suppose we would like to match a string against the following, xanax pills, simple regexp:

ab+(a|c)b*

First we have to construct the FSM, xanax pills, which will be fed with the string we would like to match. Xanax pills, An FSM for the above regular expression might look like this:

String matching against this FSM is basically answering the question ’starting from the initial state, xanax pills, can we reach the finish after feeding the FSM the whole string?’. Xanax pills, Pretty easy - the only thing we have to define is ‘feeding’.

So let’s take the string ‘abbbcbbb’ as an example and feed the FSM! The process looks like this:

1. we are in q0, xanax pills, the initial state (where the ’start’ label is). Xanax pills, Starting to consume the string
2. we receive the first character, xanax pills, it’s an ‘a‘. Xanax pills, We have an ‘a‘ arrow to state q1, xanax pills, so we make a transition there
3. we receive the next character, xanax pills, ‘b‘. Xanax pills, We have two b-arrows: to q1 and q2. Xanax pills, We choose to go to q1 (in fact, xanax pills, staying in q1) - remember, xanax pills, the question is whether we _can_ reach the finish, xanax pills, not whether all roads lead to the finish - so the choice is ours!
4. identical to the above
5. after the two above steps, xanax pills, we are still in q1. Xanax pills, We still get a ‘b‘ but this time we decide to move to q2.
6. we are in q2 and the input is ‘c‘. Xanax pills, We have no analysis-paralysis here since the only thing we can do is to move to q4 - so let’s do that!
7. Whoa! We reached the finish line! (q4 is one of the terminal states). Xanax pills, However, xanax pills, we didn’t consume the whole string yet, xanax pills, so we can’t yet tell whether the regexp matches or not. Xanax pills,
8. So we eat the rest of the string (the ‘how’ is left as an exercise to the reader) and return ‘match!’

Let’s see a very simple non-matching example on the string ‘abac’

1. in q0
2. got an ‘a‘, xanax pills, move to q1
3. in q1, xanax pills, got a ‘b‘, xanax pills, move to q2
4. in q2, xanax pills, got an ‘a‘, xanax pills, move to q3 - we reached the finish, xanax pills, but still have a character to consume
5. in q3, xanax pills, got a ‘c‘… Xanax pills, oops. Xanax pills, We have no ‘c’ arrow from q3 so we are stuck. Xanax pills, return ‘no match!’

Of course the real-life scenarios are much more complicated than the above one and sometimes FSMs are not enough (for example to my knowledge it’s not possible to tell about a number whether it is prime or not with a vanilla FSM - but a regexp doing just that has been floating around some time ago) but to illustrate the concept this example served fine.

This is cool and all but why should I care?!?

Well, xanax pills, yeah, xanax pills, you are obviously not going to model an FSM the next time you would like to match a regexp - that would be wheel-reinvention at it’s finest. Xanax pills, However there are some practical scenarios where an FSM can come handy:

• sometimes the logic flow is just too complicated to model - an if-forrest is rarely a good solution (on the flip side, xanax pills, don’t model an if-else with an FSM :-))
• encapsulate complex logic flow into a pattern and not clutter your code with it. Xanax pills,
• you are in a stateless world - for example HTTP
• asynchronous and/or distributed processing where you explicitly need to maintain your state and act upon it

Some real life examples of FSM usage in the Ruby/Rails world are why the lucky stiff’s Hpricot (using Ragel) or Rick Olson’s restful authentication plugin (using actsasstate_machine)

The Next Episode

In the next installment I’d like to focus on the practical usage of the actsasstate_machine plugin - I’ll attempt to create an asynchronous messaging system in a Rails app using it. Xanax pills,

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