From: eachus@spectre.mitre.org (Robert I. Eachus)
Subject: Re: GoTo in FSM (was Help with Exceptions)
Date: 1996/05/15
Date: 1996-05-15T00:00:00+00:00 [thread overview]
Message-ID: <EACHUS.96May15171124@spectre.mitre.org> (raw)
In-Reply-To: 9605151336.AA03830@most
In article <9605151336.AA03830@most> "W. Wesley Groleau (Wes)" <wwgrol@PSESERV3.FW.HAC.COM> writes:
> I keep hearing about the value of GoTo for a finite state machine.
> Perhaps my understanding of an FSM is incorrect, but every time
> I've coded what I THOUGHT was a state machine, I used neither GoTo
> nor recursion and didn't think the code was exceptionally ugly.
> With Ada 83 the awkward part was defining (and _maintaining_) an
> enumerated type with one literal for each action (plus error).
> Another type enumerated the states. A record type contains one of
> each.
The problem is that you have conceptually a large two dimensional
array indexed by current state and transition. In a parser with
several hundred tokens and a thousand or so states that array gets big
fast. So when you get to this line:
> case State_Transition ( Current_State, Trigger ).Action is
What you want to do is to make State_Transition a function which
returns a tuple, and store the array in a compact form. The usual
form for doing that is to have a large one-dimensional array with
entries (trigger, action, next state), (default, action, next_state),
(error, ..., resume in state), or (continue, ... , state to continue
with).
In a parser or scanner it is the routine you call State_Transition
that ends up containing the gotos. You can also implement a (usually
less than a dozen states) state machine by having each state correpond
to a label. In fact this is usually how the state transition routine
or parser driver is coded. There are a few states, including panic
mode error recovery, and the parser "walks" between them.
> Now I wouldn't call that beautiful, but I wouldn't call it ugly
> either.
Agreed. I could pretty it up a bit, but if you can afford the
space to implement the state table as an array, there isn't a problem.
--
Robert I. Eachus
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Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
1996-05-15 0:00 GoTo in FSM (was Help with Exceptions) W. Wesley Groleau (Wes)
1996-05-15 0:00 ` Robert I. Eachus [this message]
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