Crate hash32

32-bit hashing algorithms

Why?

Because 32-bit architectures are a thing (e.g. ARM Cortex-M) and you don’t want your hashing function to pull in a bunch of slow 64-bit compiler intrinsics (software implementations of 64-bit operations).

Relationship to core::hash

This crate extends core::hash::Hasher with a 32-bit version, hash32::Hasher.

The hash32::Hasher trait requires the hasher to perform only 32-bit operations when computing the hash. The trait method hash32::Hasher::finish32 returns the hasher’s result as a u32. The core::hash::Hasher::finish method zero-extends the hash32::Hasher::finish32 result to a u64.

Since hash32::Hasher extends core::hash::Hasher, the hash32::Hasher trait can be used with any type which implements the core::hash::Hash trait.

Hashers

This crate provides implementations of the following 32-bit hashing algorithms:

• FnvHasher Fowler-Noll-Vo 1a
• Murmur3Hasher MurmurHash3

Picking a hasher

• FnvHasher is faster and consumes less code space than Murmur3Hasher.
• Murmur3Hasher offers better collision resistance than FnvHasher.

Security

Hashers provided by this crate are not cryptographically secure, and must not be used for security purposes. Additionally, unlike std::hash::DefaultHasher the provided hash algorithms lack denial-of-service protection, and must only be used with trusted data.

Generic code

In generic code, the trait bound H: core::hash::Hasher accepts both 64-bit hashers such as std::hash::DefaultHasher; and 32-bit hashers such as the ones defined in this crate, FnvHasher, and Murmur3Hasher.

The trait bound H: hash32::Hasher is more restrictive as it only accepts 32-bit hashers.

MSRV

This crate is guaranteed to compile on latest stable Rust. It might compile on older versions but that may change in any new patch release.

Examples

use hash32::{FnvHasher, Hasher as _};

#[derive(Hash)]
struct Person {
    id: u32,
    name: &'static str,
    phone: u64,
}

let person1 = Person {
    id: 5,
    name: "Janet",
    phone: 555_666_7777,
};
let person2 = Person {
    id: 5,
    name: "Bob",
    phone: 555_666_7777,
};

assert!(calculate_hash(&person1) != calculate_hash(&person2));

fn calculate_hash<T: core::hash::Hash>(t: &T) -> u32 {
    let mut fnv: FnvHasher = Default::default();
    t.hash(&mut fnv);
    fnv.finish32()
}

Structs

FnvHasher

32-bit Fowler-Noll-Vo hasher

Murmur3Hasher

32-bit MurmurHash3 hasher

Traits

Hasher

An extension of core::hash::Hasher for 32-bit hashers.