Re: API for String substitution (excluding regex)

[Philipp Janda <siffiejoe@...>]

Am 03.11.2014 um 09:32 schröbte Marco Mastropaolo:
> If I remember correctly, the first thing gsub does is checking if the
> second argument is a pattern or not and, if not, it goes on with a direct
> replace. Thus the performance impact is minimal in that case -a linear scan
> over the second arg string.

AFAICS, only `string.find` does that (if the `plain` argument is falsish).

> This if I understood the question correctly, if not, sorry :)
> Il 03/nov/2014 06:09 "Rv Rv" <rvrv7575@yahoo.com> ha scritto:
>
> > The Lua API string.gsub will also take a pattern as an argument. I am
> > assuming that because of the assumption that a pattern may be present as an
> > argument, there will be additional processing that will be done every time
> > a string comparison is made.  If we know for sure that the argument is
> > going to be a string, is there way to avoid this additional performance
> > penalty. I was looking for an API that operates only on string like strcpy
> > in C , but could not find anything
> > Is there any faster way to do string substitution in Lua apart from using
> > string.gsub.

There has been a discussion about a similar `plain` option for 
`string.gsub` not long ago[1]. You could implement something like that 
in C quite easily (using Lua's `luaL_Buffer` API), but AFAIK noone has 
done so yet. You can try one of the real regex libraries[2] for Lua -- a 
regular expression compiled to a finite state machine could be faster 
than an interpreted Lua pattern. And of course it's always worth 
checking out LPeg[3] (although I doubt that it will be faster in this case).

Always assuming that string substitution is actually a bottleneck in 
your program ...


Philipp


  [1]: http://lua-users.org/lists/lua-l/2014-08/msg00584.html
  [2]: http://rrthomas.github.io/lrexlib/
  [3]: http://www.inf.puc-rio.br/~roberto/lpeg/lpeg.html