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Re: Re: Bloom::Filter Usage

by jreades (Friar)
on Apr 20, 2004 at 16:12 UTC ( #346691=note: print w/replies, xml ) Need Help??

in reply to Re: Bloom::Filter Usage
in thread Bloom::Filter Usage

This is where things get very strange for me

Here's the code I'm running as a test:

#!/usr/bin/perl use strict; use Digest::SHA1 qw(sha1); use Bloom::Filter; my $filter = Bloom::Filter->new(error_rate => 0.0001, capacity => 100) +; my @salts; # None of these work # Option 1: # push @salts, "msis"; # Option 2: # for my $salt (0..3) { # push @salts, sub { sha1($salt,$_[0]) }; # } # Option 3: # for my $salt (0..3) { # push @salts, sha1($salt,$_[0]); # } $filter->set_salts(@salts); $filter->add("Foo"); $filter->add("Bar"); $filter->add("Baz"); print STDOUT ($filter->check("Bar") ? "Found" : "Not Found"), "\n"; print STDOUT ($filter->check("Bim") ? "Found" : "Not Found"), "\n"; exit;
it looks like one of these should have worked, but when I run my test code I always get the following:

jreades@sort:~>> ./ Use of uninitialized value in numeric gt (>) at Bloom/ line 1 +26. Argument "" isn't numeric in numeric eq (==) at Bloom/ line 8 +6. Argument "@" isn't numeric in numeric eq (==) at Bloom/ line +86. Found Argument "" isn't numeric in numeric eq (==) at Bloom/ line 8 +6. Argument "" isn't numeric in numeric eq (==) at Bloom/ line 8 +6. Found

It's just possible that there's something wrong with the module itself, and I've emailed the author asking for any tips or tricks but I haven't heard back from him/her yet.

And yes, I do think that the approach is pretty neat -- when someone suggested it and I did some reading it leapt out as a very low-cost way to perform a high-cost operation. And, as you said, even taking a fairly high false-positive rate of 0.1% you still end up with a tiny fraction of your original search space.

I did notice the odd bias towards email addresses but figured it might not affect what I was trying to do. What algorithm would you suggest as an alternative for working with 12-digit numeric keys more efficiently?


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Re: Bloom::Filter Usage [patch]
by thpfft (Chaplain) on Apr 20, 2004 at 19:11 UTC

    On closer inspection, Bloom::Filter is a bit broken, unless I'm badly misreading it:

    * The add() method doesn't actually add anything, because the new() method doesn't initialise the $self->{contents} hashref. it needs to be changed so that either the add method assigns directly to $self->{contents}, or the new() method changes 'keys' to 'contents'. that's probably just a typo.

    * because $self->{contents} is not populated, _calculate_filter_length() always returns undef, so the default filter length is always used.

    * because $self->{contents} is not populated, build_filter doesn't build a filter anyway.

    * the use of == to compare bitstrings in check() is generating warnings, as you've seen. Its purpose is to test that ($a & $b) is the same as $a, ie that all the on bits in $a are also on in $b, and never mind the off bits. Someone better than me at pack(), ie anyone at all, will know how to test equivalence using the bitstrings themselves: meanwhile, you can make it work by testing with the unpacked versions.

    * And anyway, it's not incremental. Every time you add a new key, the whole filter is rebuilt by way of a loop on $self->{contents}. This makes it more or less useless for your purposes: you presumably want an iterative check($_) || add($_) mechanism that can be placed in the equivalent of a while(<HUGE_FILE>) loop. As it stands, Bloom::Filter will perform another loop across your whole dataset-so-far for each input line, which might slow things down a bit.

    You will need to roll your own, I think, unless the author can be persuaded to accommodate both approaches as well as fixing the bugs, but if you patch Bloom::Filter with this, at least your test script should work:

    --- Tue Apr 20 20:01:37 2004 +++ Tue Apr 20 20:04:34 2004 @@ -25,7 +25,7 @@ min_length => 20, filter_length => 20, %params, - keys => {} + contents => {} }, $class; } @@ -52,7 +52,7 @@ my $list = $self->{contents}; foreach my $add ( @addresses ) { # convert email to SHA1 hash if necessary - $add = sha1_base64( $add ) if $add =~ /@/o; + #$add = sha1_base64( $add ) if $add =~ /@/o; $list->{$add}++; } $self->{reindex_flag} = 1; @@ -83,7 +83,11 @@ # A match occurs if every bit we check is on foreach my $key ( @keys ) { my $mask = $self->_make_bitmask( $key ); - push @result, ($mask == ( $mask & $self->{filter} )); + #push @result, ($mask == ( $mask & $self->{filter} )); + + my $m = unpack("b*", $mask); + push @result, ($m == unpack("b*", ($mask & $self->{filter}))) +; + } return ( wantarray() ? @result : $result[0] ); }
      I am the module author...
      Sorry - looks like an update I thought I had made to CPAN before publishing the article never got through. There should be a v0.02 now on CPAN (same as the code in Listing 1 linked from the article) that has the bugs fixed.

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