https://github.com/servo/rust-fnv

Fowler–Noll–Vo hash function
https://github.com/servo/rust-fnv

Last synced: 4 months ago
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Fowler–Noll–Vo hash function

• Host: GitHub
• URL: https://github.com/servo/rust-fnv
• Owner: servo
• License: apache-2.0
• Created: 2015-04-06T18:51:46.000Z (almost 10 years ago)
• Default Branch: main
• Last Pushed: 2024-03-20T08:58:38.000Z (11 months ago)
• Last Synced: 2024-11-01T13:07:22.255Z (4 months ago)
• Language: Rust
• Homepage: https://doc.servo.org/fnv/
• Size: 53.7 KB
• Stars: 337
• Watchers: 24
• Forks: 28
• Open Issues: 6
• Metadata Files:
  • Readme: README.md
  • License: LICENSE-APACHE

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README

        
# rust-fnv

An implementation of the [Fowler–Noll–Vo hash function][chongo].

### [Read the documentation](https://doc.servo.org/fnv/)

## About

The FNV hash function is a custom `Hasher` implementation that is more

efficient for smaller hash keys.

[The Rust Standard Library documentation][docs] states that while the default

`Hasher` implementation, SipHash, is good in many cases, it is notably slower

than other algorithms with short keys, such as when you have a map of integers

to other values.  In cases like these, [FNV is demonstrably faster][graphs].

Its disadvantages are that it performs badly on larger inputs, and

provides no protection against collision attacks, where a malicious user

can craft specific keys designed to slow a hasher down. Thus, it is

important to profile your program to ensure that you are using small hash

keys, and be certain that your program could not be exposed to malicious

inputs (including being a networked server).

The Rust compiler itself uses FNV, as it is not worried about

denial-of-service attacks, and can assume that its inputs are going to be

small—a perfect use case for FNV.

## Usage

To include this crate in your program, add the following to your `Cargo.toml`:

```toml

[dependencies]

fnv = "1.0.3"

```

## Using FNV in a HashMap

The `FnvHashMap` type alias is the easiest way to use the standard library’s

`HashMap` with FNV.

```rust

use fnv::FnvHashMap;

let mut map = FnvHashMap::default();

map.insert(1, "one");

map.insert(2, "two");

map = FnvHashMap::with_capacity_and_hasher(10, Default::default());

map.insert(1, "one");

map.insert(2, "two");

```

Note, the standard library’s `HashMap::new` and `HashMap::with_capacity`

are only implemented for the `RandomState` hasher, so using `Default` to

get the hasher is the next best option.

## Using FNV in a HashSet

Similarly, `FnvHashSet` is a type alias for the standard library’s `HashSet`

with FNV.

```rust

use fnv::FnvHashSet;

let mut set = FnvHashSet::default();

set.insert(1);

set.insert(2);

set = FnvHashSet::with_capacity_and_hasher(10, Default::default());

set.insert(1);

set.insert(2);

```

[chongo]: http://www.isthe.com/chongo/tech/comp/fnv/index.html

[docs]: https://doc.rust-lang.org/std/collections/struct.HashMap.html

[graphs]: https://cglab.ca/~abeinges/blah/hash-rs/