NAME

       Bloom::Filter - Sample Perl Bloom filter implementation

DESCRIPTION

       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>.

SYNOPSIS

               use Bloom::Filter

               my $bf = Bloom::Filter->new( capacity => 10, error_rate => .001 );

               $bf->add( @keys );

               while ( <> ) {
                       chomp;
                       print "Found $_\n" if $bf->check( $_ );
               }

CONSTRUCTORS

       new %PARAMS
           Create a brand new instance.  Allowable params are "error_rate", "capacity".

       init
           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.

ACCESSORS

       capacity
           Returns the total capacity of the Bloom filter

       error_rate
           Returns the configured maximum error rate

       length
           Returns the length of the Bloom filter in bits

       key_count
           Returns the number of items currently stored in the filter

       on_bits
           Returns the number of 'on' bits in the filter

       salts
           Returns the list of salts used to create the hash functions

PUBLIC METHODS

       add @KEYS
           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.

       check @KEYS
           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.

INTERNAL METHODS

       _calculate_shortest_filter_length CAPACITY ERR_RATE
           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.

       _get_cells KEY
           Given a key, hashes it using the list of salts and returns an array of cell indexes
           corresponding to the key.

AUTHOR

       Maciej Ceglowski <maciej@ceglowski.com>

CHANGELOG

       Feb 2007 big speedup by Dmitriy Ryaboy <dmitriy.ryaboy@ask.com> (thanks!)

COPYRIGHT AND LICENSE

       (c) 2004 Maciej Ceglowski

       This is free software, distributed under version 2 of the GNU Public License (GPL).