Bloom::Filter - Sample Perl Bloom filter implementation
A Bloom filter is a probabilistic algorithm for doing existence tests in less memory than a full list of keys would require. The tradeoff to using Bloom filters is a certain configurable risk of false positives. This module implements a simple Bloom filter with configurable capacity and false positive rate. Bloom filters were first described in a 1970 paper by Burton Bloom, see http://portal.acm.org/citation.cfm?id=362692&dl=ACM&coll=portal.
use Bloom::Filter my $bf = Bloom::Filter->new( capacity => 10, error_rate => .001 ); $bf->add( @keys ); while ( <> ) { chomp; print "Found $_\n" if $bf->check( $_ ); }
Create a brand new instance. Allowable params are error_rate
, capacity
.
Calculates the best number of hash functions and optimum filter length, creates some random salts, and generates a blank bit vector. Called automatically by constructor.
Returns the total capacity of the Bloom filter
Returns the configured maximum error rate
Returns the length of the Bloom filter in bits
Returns the number of items currently stored in the filter
Returns the number of 'on' bits in the filter
Returns the list of salts used to create the hash functions
Adds the list of keys to the filter. Will fail, return undef
and complain if the number of keys in the filter exceeds the configured capacity.
Checks the provided key list against the Bloom filter, and returns a list of equivalent length, with true or false values depending on whether there was a match.
Given a desired error rate and maximum capacity, returns the optimum combination of vector length (in bits) and number of hash functions to use in building the filter, where "optimum" means shortest vector length.
Given a key, hashes it using the list of salts and returns an array of cell indexes corresponding to the key.
Maciej Ceglowski <maciej@ceglowski.com>
Feb 2007 big speedup by Dmitriy Ryaboy <dmitriy.ryaboy@ask.com> (thanks!)
(c) 2004 Maciej Ceglowski
This is free software, distributed under version 2 of the GNU Public License (GPL).