in reply to How Index function works??

If you look at pp_index in pp.c, you will find that it calls fbm_instr, which is an implementation of the Boyer–Moore string search algorithm.

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Re^2: How Index function works??
by LanX (Saint) on Oct 24, 2011 at 08:22 UTC
    when I did speed tests comparing index and a simple m//, the regex (which uses Boyer-Moore) was (mostly) considerably faster.

    Since you are linking to v5.14.2 and I'm still using v5.10.0 I suppose that the implementation of index has changed.

    Cheers Rolf

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      According to perlreguts, the RE engine also uses fbm_index() to scan for the leftmost atom. There shouldn't be any reason why the performance of the two should differ by a large margin, and I would expect the regular expression to be a bit slower in the general case due to the setup. So I think it's either that your data somehow favours a branch in the RE engine that goes to fbm_index faster, or that the benchmark is not measuring what you want. But I also vaguely remember some thread about such a discrepancy on this site, but I can't find it is index faster than regexp for fixed text token?.

      git blame tells me nobody touched index since 2009, and that change was some refcounting change. The other changes were in 2006.

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      If you don't mind, could you post the benchmark code? I'm just curious to look it over.


      Dave

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        Hi davido

        well it was in June, below what I found on my disk:

        There is an "X" in the middle of a 52MB string of repeated alphabet letters ($pattern = join "","a".."z")

        Depending if you look for "Xabc..xyz" or "abc...xyzX" the different approaches show their strength.

        I did more tests which I can't find anymore strongly indicating that index doesn't use Boyer-Moore.

        Just vary the position of the "X". I'd be glad if you looked it over. :)

        use Time::HiRes qw[ time ];
 
 
my $pattern = join "","a".."z";
 
my $str= $pattern x 1E6 . "X" .$pattern x1E6;
$\="\n";
$|=1;
 
print "Length: ",length $str;
 
print "\n---End X";
 
$start=time;
print "Match: ", $str =~/${pattern}X/;
printf "\t took %.3f sec\n",time-$start;
 
$start=time;
print "Index: ", index $str , "${pattern}X";
printf "\t took %.3f sec\n",time-$start;
 
print "\n---Start X";
 
$start=time;
print "Match: ", $str =~/X${pattern}/;
printf "\t took %.3f sec\n",time-$start;
 
$start=time;
print "Index: ", index $str , "X${pattern}";
printf "\t took %.3f sec\n",time-$start;

        [download]

        RESULT:

        Length: 52000001
 
---End X
Match: 1
     took 0.021 sec
Index: 25999974
     took 0.263 sec
 
---Start X
Match: 1
     took 0.165 sec
Index: 26000000
     took 0.094 sec

        [download]

        Cheers Rolf

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      LanX How big was your file?? I used file sized of 300 MB where for a pattern it took less than second but with regex it took over 5mins and I had to stop the script... I am using perl v5.14
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        52 MB and as you can see from my test I'm cosntructing a case where Boyer-Moore must be slower than brute force.

        Cheers Rolf

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