From: Robert Klemme <bob.news@...>
Date: 2005-11-22T20:17:24+09:00
Subject: Re: Optimization anyone

Horacio Sanson wrote:
> I have this little script that takes a list of keyword sets, each set
> has only two keywords and for each one of them the script creates a
> regular expression like this:
>
> Regexp.new("#{key1}\.*#{key2}|#{key2}\.*#{key1}")
>
> then I match it to a string that contains a long text fetched from a
> web page.
>
> a more complete pseudo-code
>
> #########################################
> long_text = get_web_page(url)
>
> keyword_hash = load_keyword_array_from_database
>
> keyword_hash.each_pair { |id, value|
>
>     key1 = value[0]
>     key2 = value[1]
>
>     r = Regexp.new("#{key1}\.*#{key2}|#{key2}\.*#{key1}")
>     return id if long_text =~ r
> }
>
> return -1
> ###########################################
>
>
> Now this code works perfect, the problem is that the keyword_hash has
> more than 300 elements and running this code can take between 50 to
> 120 seconds. Since I am processing more than 1000 pages with this
> code it takes forever.
>
>
> I solved this problem by replacing the regular expression match to
>
>    r1 =  Regexp.new("#{key1}\.*#{key2}")
>    r2 =  Regexp.new("#{key2}\.*#{key1}")
>
>    return id if long_text =~ r1 or long_text =~ r2
>
>
> I simply put the or statement outside the regular expresion and the
> speedup was from 50~120sec to 0.40 secs per page.
>
>
> using the Benchmark class and running some test I got
>
> normal:   0   0
>     27.688000          0.015000         27.703000         ( 27.765000
> )
> fast:
>     0.469000             0.000000        0.484000       (0.954000)
>
>
> the speed difference is totally diferent.
>
> Is this expected when using regular expressions??

On obvious optimization is to create all regexps during
load_keyword_array_from_database() and not during iteration of the hash.
That way you just have to do it once and can reuse those regexps with
multiple pages you check.

Another possible optimization is to take your approach of splitting the
regexps a bit further and create two regexps - one for each keyword - and
return the id if both match.  This works only correctly if (i) keywords
don't overlap or (ii) you can use \b to ensure matching on word
boundaries.

Kind regards

    robert