* Hierarchical States Machines @ 2004-04-28 18:48 Fabien 2004-04-28 19:39 ` Marius Amado Alves ` (2 more replies) 0 siblings, 3 replies; 12+ messages in thread From: Fabien @ 2004-04-28 18:48 UTC (permalink / raw) Hi everyone !!! Pretty new to ADA dev., i am currently trying to implement a hierarchical state machine for my application. So far, i must confess that i do not really get on well with it. I am thus looking for some people who might have tried to do it and who could give me some useful tips . Fabien pS : I don't look for a ready-to-use solution, just some help !!!!! ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Hierarchical States Machines 2004-04-28 18:48 Hierarchical States Machines Fabien @ 2004-04-28 19:39 ` Marius Amado Alves 2004-04-28 19:57 ` Robert I. Eachus 2004-04-29 3:58 ` Steve 2 siblings, 0 replies; 12+ messages in thread From: Marius Amado Alves @ 2004-04-28 19:39 UTC (permalink / raw) To: comp.lang.ada > Pretty new to ADA dev., Welcome! You can now start writing "Ada" not "ADA" as your first rite of passage :-) > i am currently trying to implement a > hierarchical state machine for my application.... I write state machines a lot in Ada. For example package XML_Automaton, which I've just put for your viewing pleasure at the following secret URLs: http://www.liacc.up.pt/~maa/mneson/xml_automaton.ads http://www.liacc.up.pt/~maa/mneson/xml_automaton.adb http://www.liacc.up.pt/~maa/mneson/xml2mntext.adb The last unit is an real life example application of the package. (Soon I'll update the Mneson site with *all* the cool latest developments. As you might guess from above, XML import/export is one of them.) ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Hierarchical States Machines 2004-04-28 18:48 Hierarchical States Machines Fabien 2004-04-28 19:39 ` Marius Amado Alves @ 2004-04-28 19:57 ` Robert I. Eachus 2004-04-29 3:00 ` Randy Brukardt 2004-04-29 12:10 ` Wojtek Narczynski 2004-04-29 3:58 ` Steve 2 siblings, 2 replies; 12+ messages in thread From: Robert I. Eachus @ 2004-04-28 19:57 UTC (permalink / raw) Fabien wrote: > Pretty new to ADA dev., i am currently trying to implement a > hierarchical state machine for my application. So far, i must confess > that i do not really get on well with it. I am thus looking for some > people who might have tried to do it and who could give me some useful > tips . > > Fabien > > pS : I don't look for a ready-to-use solution, just some help !!!!! Unlike any other type of software, goto is your friend. The reason that goto is normally deprecated is that it results in spaghetti code. However, if you are implementing a state machine, you need to reflect the structure of the actual state diagram, which may look like spaghetti. Also if you are implementing a table driven state machine, the best way to do it is to build a driver that is its own state machine. (Sort of like using a universal Turing machine to read a description of a Turing machine and emulate it.) Again, the use of gotos will allow you to write a driver so that its structure corresponds to the algorithm you are implementing. Incidentally, as far as I know, or for that matter most Ada experts know, this is the only area in Ada where you should use gotos. But goto has been kept in the language primarily because it is _necessary_ to create maintainable finite state machines, push-down automata, and the like. -- Robert I. Eachus "The terrorist enemy holds no territory, defends no population, is unconstrained by rules of warfare, and respects no law of morality. Such an enemy cannot be deterred, contained, appeased or negotiated with. It can only be destroyed--and that, ladies and gentlemen, is the business at hand." -- Dick Cheney ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Hierarchical States Machines 2004-04-28 19:57 ` Robert I. Eachus @ 2004-04-29 3:00 ` Randy Brukardt 2004-04-29 7:25 ` Martin Krischik 2004-04-29 12:10 ` Wojtek Narczynski 1 sibling, 1 reply; 12+ messages in thread From: Randy Brukardt @ 2004-04-29 3:00 UTC (permalink / raw) "Robert I. Eachus" <rieachus@comcast.net> wrote in message news:jrOdnQYzWKMxkQ3dRVn-hQ@comcast.com... > Incidentally, as far as I know, or for that matter most Ada experts > know, this is the only area in Ada where you should use gotos. But goto > has been kept in the language primarily because it is _necessary_ to > create maintainable finite state machines, push-down automata, and the like. Ah, baloney. :-) I use gotos whenever they prevent duplicating work that otherwise would have to be done. It's silly to have two copies of code to do something just to avoid a goto (or to make such code into a subprogram with no purpose other than to avoid a goto). A common case is to emulate the missing "continue" that Ada doesn't have. It usually shows up in some deeply nested code: loop <bunch of conditions> <Do some processing> if <unusual condition> then <Handle unusual case> goto Continue; end if; <bunch of else conditions> end if; <normal processing>; <<continue>> null; end loop; You can avoid the goto by making a hash out of the code, but why bother? Gotos are efficient, not particularly unstructured in Ada (most of the bad cases are illegal) - compiler optimizers are unlikely to be able to get rid of batches of boolean flags and turn them into the branches that really were intended in the first place. Randy. ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Hierarchical States Machines 2004-04-29 3:00 ` Randy Brukardt @ 2004-04-29 7:25 ` Martin Krischik 2004-04-29 20:37 ` Randy Brukardt 0 siblings, 1 reply; 12+ messages in thread From: Martin Krischik @ 2004-04-29 7:25 UTC (permalink / raw) Randy Brukardt wrote: > loop > <bunch of conditions> > <Do some processing> > if <unusual condition> then > <Handle unusual case> > goto Continue; > end if; > <bunch of else conditions> > end if; > <normal processing>; > <<continue>> null; > end loop; Is there any deepter reason why you did not use: loop <<continue>> <bunch of conditions> <Do some processing> if <unusual condition> then <Handle unusual case> goto Continue; end if; <bunch of else conditions> end if; <normal processing>; end loop; i.E. A style guide or a RM I am not aware of? With Regards Martin -- mailto://krischik@users.sourceforge.net http://www.ada.krischik.com ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Hierarchical States Machines 2004-04-29 7:25 ` Martin Krischik @ 2004-04-29 20:37 ` Randy Brukardt 0 siblings, 0 replies; 12+ messages in thread From: Randy Brukardt @ 2004-04-29 20:37 UTC (permalink / raw) "Martin Krischik" <krischik@users.sourceforge.net> wrote in message news:1649831.NvcpCQcTn1@linux1.krischik.com... ... > Is there any deepter reason why you did not use: > > loop > <<continue>> > <bunch of conditions> > <Do some processing> > if <unusual condition> then > <Handle unusual case> > goto Continue; > end if; > <bunch of else conditions> > end if; > <normal processing>; > end loop; > > i.E. A style guide or a RM I am not aware of? I usually want the loop head to be re-executed, so that if it gets modified during maintenance (to a for-loop or a while-loop), it won't be necessary to restructure the code. Besides, in most of these cases, the loop actually is a while-loop, walking a list or other data structure. Randy. ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Hierarchical States Machines 2004-04-28 19:57 ` Robert I. Eachus 2004-04-29 3:00 ` Randy Brukardt @ 2004-04-29 12:10 ` Wojtek Narczynski 1 sibling, 0 replies; 12+ messages in thread From: Wojtek Narczynski @ 2004-04-29 12:10 UTC (permalink / raw) Hello, > But goto has been kept in the language primarily because it is _necessary_ to > create maintainable finite state machines, push-down automata, and the like. In the absence of "last call optimisation". Regards, Wojtek ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Hierarchical States Machines 2004-04-28 18:48 Hierarchical States Machines Fabien 2004-04-28 19:39 ` Marius Amado Alves 2004-04-28 19:57 ` Robert I. Eachus @ 2004-04-29 3:58 ` Steve 2004-04-29 5:14 ` Robert I. Eachus 2004-04-29 15:41 ` Marius Amado Alves 2 siblings, 2 replies; 12+ messages in thread From: Steve @ 2004-04-29 3:58 UTC (permalink / raw) I don't have anything against using goto's for state machines... but... I usually set up an enumeration for each state: type my_states is ( start_state, initializing, etc, end_state ); And then use a loop with an enclosed case statement: state := start_state; loop case state is when start_state => ... state := initializing; when initializing => when etc => when end_state => end case; end loop; In my opinion it makes things easier to follow since the entry to a new state always happens from the same place. Steve (The Duck) "Fabien" <fab_lio@yahoo.fr> wrote in message news:fed2d60.0404281048.41d6c549@posting.google.com... > Hi everyone !!! > > Pretty new to ADA dev., i am currently trying to implement a > hierarchical state machine for my application. So far, i must confess > that i do not really get on well with it. I am thus looking for some > people who might have tried to do it and who could give me some useful > tips . > > Fabien > > pS : I don't look for a ready-to-use solution, just some help !!!!! ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Hierarchical States Machines 2004-04-29 3:58 ` Steve @ 2004-04-29 5:14 ` Robert I. Eachus 2004-04-29 6:36 ` tmoran 2004-04-29 15:41 ` Marius Amado Alves 1 sibling, 1 reply; 12+ messages in thread From: Robert I. Eachus @ 2004-04-29 5:14 UTC (permalink / raw) Steve wrote: > I don't have anything against using goto's for state machines... but... > > I usually set up an enumeration for each state... > > In my opinion it makes things easier to follow since the entry > to a new state always happens from the same place. Exactly. My point is that this: loop case State is when... end case; -- post processing end loop; ...is the real underlying structure, and you should use gotos when needed to preserve it. Randy mentioned one such case continue. (His code looked exactly like a FSM to me, even if he wants to call it something else.) Other typical patterns you run into are where you need multiple exits from the loop, or you need to return to the beginning while skipping the post-processing. For example in the Ada parser driver I wrote for state machines created by LALR1 on Multics, there were some "merged" state entries where several states action tables were almost identical. So LALR1 generated table entries that said essentially: if the next token is X do Y, otherwise go to the state table for state Z. You could distort the engine all out of shape to implement this, or put in a goto to just before the loop. Someone who had been taught the "gotos are evil" religion reorganized the code with a couple of added state variables to demonstrate the goto wasn't necessary. His code was only twenty lines longer than my code, and ran 40% slower. (Register pressure from those state variables push things that should have been in registers onto the stack.) Although he talked about looking at the generated code to see why it was so much slower, I think he realized that his version was the spaghetti. -- Robert I. Eachus "The terrorist enemy holds no territory, defends no population, is unconstrained by rules of warfare, and respects no law of morality. Such an enemy cannot be deterred, contained, appeased or negotiated with. It can only be destroyed--and that, ladies and gentlemen, is the business at hand." -- Dick Cheney ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Hierarchical States Machines 2004-04-29 5:14 ` Robert I. Eachus @ 2004-04-29 6:36 ` tmoran 2004-04-29 16:36 ` Robert I. Eachus 0 siblings, 1 reply; 12+ messages in thread From: tmoran @ 2004-04-29 6:36 UTC (permalink / raw) >and ran 40% slower. (Register pressure from those state variables push Do you mean the parser state transitions ran 40% slower, or the whole compiler, including IO, lexer, symbol tables, code generation, etc was slowed by 40%? ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Hierarchical States Machines 2004-04-29 6:36 ` tmoran @ 2004-04-29 16:36 ` Robert I. Eachus 0 siblings, 0 replies; 12+ messages in thread From: Robert I. Eachus @ 2004-04-29 16:36 UTC (permalink / raw) tmoran@acm.org wrote: >>and ran 40% slower. (Register pressure from those state variables push > > Do you mean the parser state transitions ran 40% slower, or the whole > compiler, including IO, lexer, symbol tables, code generation, etc > was slowed by 40%? Sorry, the first pass of the compiler, which included the scanner (lexer) and created the symbol table and AST. The second pass did the semantic analysis and 'decorated' the parse tree, and did all the code-generator independent optimizations. As I remember it, the running times were roughly 1-3-3. So it would have been 1.4-3-3, for a 5 to 6% overall slowdown. Even so it was pretty huge, and it shocked me. By the point this occurred we had done a lot of optimization of the LALR1 engine and structure so that (mostly for error correcting reasons) successive reduce operations on the parse stack were deferred, then all called at once after two successful succeeding shifts, or a successful shift and (pending) reduce. That sounds complicated, and it was. But it made for wonderful syntax error correction without increasing the parse table size. (Well we had about a half dozen added states to allow "panic mode" if none of the single or double token fixes worked.) We were initially worried about the changes for the error correction support slowing down performance, but since it grouped the actions that resulted in subroutine calls, it actually sped things up significantly by eliminating register spills. When we looked at the generated code to try and explain that 40% number, adding the two flags for the non-goto solution increased the number of active variables from 6 to 8 in the key loops--and we had six available registers once you eliminated the stack pointer and program counter. Worse the access pattern basically rotated through the variables. We thought about fixing that for both the Ada and Multics PL/1 compilers, but a little bit of monitoring indicated that this code was about the only thing around that hit this 'misfeature'. The wonderful thing about being in a compiler group, and using Multics, was that we could slip in performance counters like this on the production machine, and see whether or not we should actually fix anything. Technically our group supported the DPS-6 line not DPS-8M, but we were in Billerica, MA, Multics development was at CISL in Cambridge, MA, and GCOS-3/8 support was in Phoenix, AZ. So I could submit a performance counter proposal like that to CISL, and we and they would put it into the "development" version of the compiler within a few days. My favorite story about doing that was that we had a debate about using static links or a display to manage up-level references in the Ada/SIL compiler. We had a ferocious call-graph analyzer that merged stack frames whenever possible, and of course, all variables in library packages (and in the main program if it was not called recursively) were handled directly. So we put in a trigger that added the static pointers if necessary, and sent e-mail to a list of compiler people. Almost two years later, I finally got one of those messages--for an ACVC test. We decided we would take that code out and use a dynamic stack walk if it was really necessary. I don't think that change ever happened--it was of course, very low priority. -- Robert I. Eachus "The terrorist enemy holds no territory, defends no population, is unconstrained by rules of warfare, and respects no law of morality. Such an enemy cannot be deterred, contained, appeased or negotiated with. It can only be destroyed--and that, ladies and gentlemen, is the business at hand." -- Dick Cheney ^ permalink raw reply [flat|nested] 12+ messages in thread
* Re: Hierarchical States Machines 2004-04-29 3:58 ` Steve 2004-04-29 5:14 ` Robert I. Eachus @ 2004-04-29 15:41 ` Marius Amado Alves 1 sibling, 0 replies; 12+ messages in thread From: Marius Amado Alves @ 2004-04-29 15:41 UTC (permalink / raw) To: comp.lang.ada On Thursday 29 April 2004 04:58, Steve wrote: > I don't have anything against using goto's for state machines... but... > > I usually set up an enumeration for each state: > > type my_states is ( start_state, initializing, etc, end_state );... That's how I usually do it too. See http://www.liacc.up.pt/~maa/files/Ada_Automaton/ for an Ada tokenizer written this way. Programs Ada_Automaton_Test and Ada_Counter are example applications of the package. Ada_Counter counts the significant semicolons in a unit. ^ permalink raw reply [flat|nested] 12+ messages in thread
end of thread, other threads:[~2004-04-29 20:37 UTC | newest] Thread overview: 12+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2004-04-28 18:48 Hierarchical States Machines Fabien 2004-04-28 19:39 ` Marius Amado Alves 2004-04-28 19:57 ` Robert I. Eachus 2004-04-29 3:00 ` Randy Brukardt 2004-04-29 7:25 ` Martin Krischik 2004-04-29 20:37 ` Randy Brukardt 2004-04-29 12:10 ` Wojtek Narczynski 2004-04-29 3:58 ` Steve 2004-04-29 5:14 ` Robert I. Eachus 2004-04-29 6:36 ` tmoran 2004-04-29 16:36 ` Robert I. Eachus 2004-04-29 15:41 ` Marius Amado Alves
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