Hi holger At ESUG Jan showed that he is working on a kind of PetitParser compiler. Le 16/8/15 17:58, Holger Freyther a écrit :
Hi,
once again I am not sure if this is the right list. The first parser I wrote using PetitParser was a SIP (and then MGCP) parser. I have recently ported[1] the code to Pharo and with Pharo it is very tempting to Use BlockClosure>>#bench to get an idea of the speed.
I have two performance âissuesâ and wonder if others hand similar issues with PetitParser and if there is a general approach to this.
1.) Combining two PPCharsetPredicates does not combine the âclassificationâ table it had. One could create a PPPredicateObjectParser subclass that is special casing >>#/ to build a combined classification table.
2.) When blindly following a BNF enumeration of "A or B or C or D or E or CatchAllâ and each âA, Bâ follow common pattern (e.g. token COLON value) one pays a high cost in the backtracking and constructing the PPFailure for each failed case.
In my SIPGrammar I have action parsers for To ==>.. From ==> and would like to keep that. At the same time I would be happy if the token in front of the colon is only consumed once and then delegated to the right parser and if that one failed use the âcatch allâ one.
I donât know which abstraction would be needed to allow creating optimized PetitParsers for such grammars.
sorry for the long mail, long details and context is below. What thierry will tell you is that if you do not need to compose your grammars and the grammar is not evolving and you need speed then a SmaCC approache with LGR, LALR or something else is probably faster.
Stef
kind regards holger
Full details:
1.) CharSetPredicate
| aParser bParser combinedParser aTime bTime cTime |
aParser := #digit asParser. bParser := #letter asParser. combinedParser := aParser / bParser.
aTime := [ aParser parse: 'b'] bench. bTime := [ bParser parse: 'b'] bench. cTime := [ combinedParser parse: 'b'] bench. { aTime. bTime. cTime }
cTime is bounded by the time execution time of of the slowest of these parsers + overhead (e.g. PPFailure creation).
e.g.
#('559,000 per second.' '1,010,000 per second.' '429,000 per second.')
With a proof of concept PPPredicateCharSetParser
#('1,330,000 per second.' '1,550,000 per second.' '1,580,000 per second.â)
The noise is pretty string here but what is important is that bParser and the combinedParser are in the same ballpark.
2.) Choice Parser
The BNF grammar of the parser is roughly:
Request = Request-Line *( message-header ) CRLF [ message-body ]
message-header = (Accept ⦠/ To / From / Via / extension-header) CRLF
Alert-Info = "Alert-Info" HCOLON alert-param *(COMMA alert-param) Accept = "Accept" HCOLON [ accept-range *(COMMA accept-range) ]
So there can be several lines of âmessage-headerâ. And each method header starts with a token/word, a colon and then the parameter. âextension-headerâ is kind of a catch all if no other rule matched. E.g. if a client sends a To which is wrongly encoded it would end up with the extension-header.
I transferred the above naively to PetitParser and end up with something like parsing ~500 messages a second. The main cost appears to come from the choice parser that needs to create a PetitFailure all the time. E.g. if you have a line like this:
âFrom: âHolger Freytherâ <sip:323234@foo.de>â
The choice parser will start with the âAcceptâ rule, parse the token (âFromâ and then create a PPFailure, then ⦠rules, then âToâ, parse the token.. So we have parsing the same token more than once and creating PPFailures all the time. I ended up creating a custom parser that will peek the token, have a horrible chain of token = âXYZâ ifTrue and then dispatch to the other rule.
It would be nice if PetitParser could be taught to only parse the token once and then delegate to the param rule. E.g. a PPAnyOfParser that allows to specify the token to match, the parser to continue with and a fallback parser?
[1] http://smalltalkhub.com/#!/~osmocom/SIP http://smalltalkhub.com/#!/~osmocom/MGCP