Parsing Compile

Hello

I use Parser since 2 or 2 years in a data acquisition program and it works fine. I'm currently developing the new release and I need to parse several expression with dynamic variables up to 20 times per second.

I'd like to modify the parser as to pre-compile the expression to avoid useless re-parsing every time I need to evaluate the expressions (up to 128 expressions, 20 times per second means too much time ..)

Can you give some clues on how to do that ?

Thanks for sharing this excellent piece of code

Thierry

Hiya. Can you link to what you're talking about so people don't have to go digging for it?

I presume the question refers to this:

http://www.gammon.com.au/forum/?id=4649

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You may well like to do this, but the parser is specifically written as something that evaluates at run-time. To "modify" it is to completely rewrite it.

You could conceivably (and with considerable effort) rewrite to pre-compile functions into pseudo-code (P-code) and then evaluate them as a separate operation.

However this goes well beyond the scope of a simple modification.

Can you give examples of the sorts of expressions you are evaluating? Perhaps hard-coded C functions can do it, if you are only varying parameters to the function.

Hello Nick

The application I'm working on is called Genesis. It's an OPC Client used to acquire data on process machines (mainly PLCs and DCS). Genesis allows the user to compute derived data from acquired data.
As such, any expression are added by the user. I already add some standard function as a scaling function but users are free to build up any kind of expression.

That where your parser is shining. It works fine for the purpose. With current version, the minimum rate of acquisition is 1 second. Several users ask me to reduce this limit. Now (in the release 3.0 still in preparation) it goes down to 50 ms (and I try to go to 25 ms). At such speed, I need to gain time everywhere as Genesis acquires data, compute derived data, stored all these data in a SQL data, displays the data in List and in Chart. A quite busy processor at 50 ms....

Thierry
www.tgmdev.be

Hello Nick

I wrote a test application to check an extended parser that fits my needs for Genesis: obtaining symbols value from the caller (and not from the parser) and using pre-compiled expression.
I got the very first result.
* Time for evaluating 10 expressions: about 100 ms
* Time for evaluation pre-compiled expressions: about 5 ms

I extend your parser to get a list of tokens and, using the same function hierarchy, I basically replace GetToken by a simple read of the token list.

I still need to work on it but it looks promising.

Thierry
www.tgmdev.be

That sounds like a good start. If you pre-parse the string into tokens that you have removed most of the string-matching, so the execution should be somewhat faster.