---
name: "Bloom::Filter"
section: "3pm"
platform: "linux"
package: "libbloom-filter-perl"
version: "1.2-5"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/Bloom::Filter"
---

# Bloom::Filter(3pm)

Sample Perl Bloom filter implementation

## 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

Originally written by Maciej Ceglowski \<maciej@ceglowski.com\>. Currently maintained by Grzegorz Rożniecki \<xaerxess@gmail.com\>.

## CONTRIBUTORS

Dmitriy Ryaboy \<dmitriy.ryaboy@ask.com\> (big speedup in February 2007, thanks!)

## COPYRIGHT AND LICENSE

(c) 2004 Maciej Ceglowski

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