Optimization anyone


#1

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??

regards,
Horacio


#2

“H” == Horacio S. removed_email_address@domain.invalid writes:

H> Regexp.new("#{key1}.#{key2}|#{key2}.#{key1}")

vs

H> r1 = Regexp.new("#{key1}.#{key2}")
H> r2 = Regexp.new("#{key2}.
#{key1}")

H> Is this expected when using regular expressions??

yes, ruby has some optimizations. For example with the regexp
/abc.*def/

svg% ruby -rjj -e ‘/abc.*def/.dump’
Regexp /abc.*def/
0 exactn “abc” (3)
1 anychar_repeat
2 exactn “def” (3)
3 end
must : abc
optimize : exactn
svg%

It call the regexp engine (which is slow) only when it has found the
substring “abc” in the string

Now if you use /abc.*def|def.*abc/ you break this optimization

svg% ruby -rjj -e ‘/abc.*def|def.*abc/.dump’
Regexp /abc.*def|def.*abc/
0 on_failure_jump ==> 5
1 exactn “abc” (3)
2 anychar_repeat
3 exactn “def” (3)
4 jump ==> 8
5 exactn “def” (3)
6 anychar_repeat
7 exactn “abc” (3)
8 end
svg%

it must call the stupid (:-)) regexp engine for each line

Guy Decoux


#3

Horacio S. wrote:

r = Regexp.new("#{key1}\.*#{key2}|#{key2}\.*#{key1}")

code it takes forever.
I simply put the or statement outside the regular expresion and the

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

#4

ts removed_email_address@domain.invalid writes:

yes, ruby has some optimizations. For example with the regexp /abc.*def/

Do you know how oniguruma does that, per chance?


#5

“C” == Christian N. removed_email_address@domain.invalid writes:

C> Do you know how oniguruma does that, per chance?

You can compile oniguruma with debugging options

moulon% ./ruby -e ‘/abc.*def/’
list:814d418
string:814d3e8abc
qualifier:814d478{0,-1}
anychar:814d448
string:814d4d8def
optimize: EXACT_BM
anchor: []
sub anchor: []

exact: [abc]: length: 3
code length: 11
[exact3:abc] [anychar*-peek-next:d] [exact3:def] [end]
moulon%

Guy Decoux


#6

There’s a whole section in Mastering Regular Expressions that goes into
the
differences between regexp engines.

Summary: it makes a big difference how you setup your patterns!

On 11/22/05, Robert K. removed_email_address@domain.invalid wrote:

key2 = value[1]
more than 300 elements and running this code can take between 50 to

fast:
multiple pages you check.

‘There was an owl lived in an oak.
The more he heard, the less he spoke.
The less he spoke, the more he heard.’

Christian L.
removed_email_address@domain.invalid