# Crate fnv [−] [src]

An implementation of the Fowler–Noll–Vo hash function.

## About

The FNV hash function is a custom `Hasher` implementation that is more efficient for smaller hash keys.

The Rust FAQ 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.

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.

## Using FNV in a `HashMap`

The `FnvHashMap` type alias is the easiest way to use the standard library’s `HashMap` with FNV.

    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.

    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);

## Structs

FnvHasher | An implementation of the Fowler–Noll–Vo hash function.

## Type Definitions

FnvBuildHasher | A builder for default FNV hashers.
FnvHashMap | A `HashMap` using a default FNV hasher.
FnvHashSet | A `HashSet` using a default FNV hasher.