# [−][src]Crate bloom_filter_simple

bloom_filter_simple is a library that offers different implementations of a data structure for filtering elements. The data structure is based on the ideas presented by Burton Howard Bloom and is therefore known as bloom filter:

Burton H. Bloom. 1970. Space/time trade-offs in hash coding with allowable errors. Commun. ACM 13, 7 (July 1970), 422–426. DOI: https://doi.org/10.1145/362686.362692

# Overview

Basic description taken from Wikipedia:

"A Bloom filter is a space-efficient probabilistic data structure, conceived by Burton Howard Bloom in 1970, that is used to test whether an element is a member of a set. False positive matches are possible, but false negatives are not – in other words, a query returns either "possibly in set" or "definitely not in set". Elements can be added to the set, but not removed (though this can be addressed with the counting Bloom filter variant); the more items added, the larger the probability of false positives." ("Bloom filter". Definition, para. 1. In Wikipedia. Retrieved December 02, 2020, from https://en.wikipedia.org/wiki/Bloom_filter)

# Bloom Filter Implementations

The library offers two basic types of bloom filter implementations.

## Kirsch-Mitzenmacher Bloom Filter (KMBloomFilter)

This type of bloom filter uses two hashers to simulate an arbitrary number of additional hash functions.

The implementation is based on the work of Kirsch and Mitzenmacher [1]. In their work, they demonstrated that it is possible to apply simulated hash functions in a bloom filter effectively, i.e., without loss in the asymptotic false positive probability. Given two hash functions h_1(x) and h_2(x), an i-th additional hash function g_i(x) can be simulated as g_i(x) = h_1(x) + i * h_2(x).

[1] Kirsch A., Mitzenmacher M. (2006) Less Hashing, Same Performance: Building a Better Bloom Filter. In: Azar Y., Erlebach T. (eds) Algorithms – ESA 2006. ESA 2006. Lecture Notes in Computer Science, vol 4168. Springer, Berlin, Heidelberg. https://doi.org/10.1007/11841036_42

## Seeded Bloom Filter (SeededBloomFilter)

A bloom filter that uses a single Hasher that can be seeded to simulate an arbitrary number of hash functions. Internally, the implementation uses ahash::AHasher.

# Examples

In the following, you can find simple examples of how to initialize and use the different bloom filter types.

## Default Bloom Filter

The crate offers a default type for a KMBloomFilter that uses ahash::AHasher and Rust's std::collections::hash_map::DefaultHasher to simulate more hash functions. We compared different hash functions for use by KMBloomFilter, and this combination yielded the best results with respect to the filter's false positive probability.

We recommend using DefaultBloomFilter for quickly getting started.

```use bloom_filter_simple::{BloomFilter,DefaultBloomFilter};

fn main() {
// We plan on storing at most 10,000 elements
let desired_capacity = 10_000;
// The chance of a false positive increases with each inserted element.
// This parameter specifies that the chance should be less than 0.01% (0.0001)
// when the desired capacity has been reached. In other words, the chance
// that the bloom filter returns true when checking whether a novel element
// has been inserted before is less than 0.01% (0.0001).
let desired_fp_probability = 0.0001;

let mut filter = DefaultBloomFilter::new(desired_capacity, desired_fp_probability);

// You can insert any type implementing the Hash trait. The bloom filter does
// not store the inserted elements but only their hashes. Hence, there is no
// transfer of ownership required.
filter.insert(&5i32);
filter.insert(&"Some text");
filter.insert(&10_000usize);

// You can check whether a value has been inserted into the filter before.
assert_eq!(false, filter.contains(&3));
assert_eq!(true, filter.contains(&5));
assert_eq!(true, filter.contains(&"Some text"));
}```

## KMBloomFilter

Initialization and application of a KMBloomFilter.

```use bloom_filter_simple::{BloomFilter,KMBloomFilter};
use ahash::AHasher;
use std::collections::hash_map::DefaultHasher;

fn main() {
// We plan on storing at most 10,000 elements
let desired_capacity = 10_000;
// We want to assure that the chance of a false positive is less than 0.01% (0.0001)
// for up to desired_capacity elements.
let desired_fp_probability = 0.0001;

// We initialize a new KMBloomFilter by specifying the desired Hashers as type
// parameters. It is possible to use any type that implements Hasher + Default.
// Default is required to receive a new instance of a hasher after a value was
// hashed, because the Hasher trait does not provide an interface for resetting
// a hasher implementing it. This is required to receive the same hash value
// when inserting or checking the same element multiple times.
let mut filter: KMBloomFilter<AHasher, DefaultHasher> = KMBloomFilter::new(
desired_capacity,
desired_fp_probability
);

// You can insert any type implementing the Hash trait. The bloom filter does not
// store the inserted elements but only their hashes. Hence, there is no transfer
// of ownership required.
filter.insert(&5i32);
filter.insert(&"Some text");
filter.insert(&10_000usize);

// You can check whether a value has been inserted into the filter before.
assert_eq!(false, filter.contains(&3));
assert_eq!(true, filter.contains(&5));
assert_eq!(true, filter.contains(&"Some text"));
}```

## SeededBloomFilter

Initialization and application of a SeededBloomFilter.

```use bloom_filter_simple::{BloomFilter,SeededBloomFilter};

fn main() {
// We plan on storing at most 10,000 elements
let desired_capacity = 10_000;
// We want to assure that the chance of a false positive is less than 0.0001
// for up to desired_capacity elements.
let desired_fp_probability = 0.0001;

// A SeededBloomFilter uses a single seeded ahash::AHasher internally.
let mut filter = SeededBloomFilter::new(desired_capacity, desired_fp_probability);

// You can insert any type implementing the Hash trait. The bloom filter does
// not store the inserted elements but only their hashes. Hence, there is no
// transfer of ownership required.
filter.insert(&5i32);
filter.insert(&"Some text");
filter.insert(&10_000usize);

// You can check whether a value has been inserted into the filter before.
assert_eq!(false, filter.contains(&3));
assert_eq!(true, filter.contains(&5));
assert_eq!(true, filter.contains(&"Some text"));
}```

## Structs

 KMBloomFilter Bloom filter implementation using the improvements described by Kirsch and Mitzenmacher: SeededBloomFilter A bloom filter that uses a single Hasher that can be seeded to simulate an arbitrary number of hash functions.

## Traits

 BloomFilter This trait defines the basic functionality supported by the bloom filters in this library.

## Type Definitions

 DefaultBloomFilter A default implementation of KMBloomFilter using ahash::AHasher and collections::hash_map::DefaultHasher.