std::hash

Trait BuildHasher

1.7.0 · Source

pub trait BuildHasher {
    type Hasher: Hasher;

    // Required method
    fn build_hasher(&self) -> Self::Hasher;

    // Provided method
    fn hash_one<T>(&self, x: T) -> u64
       where T: Hash,
             Self: Sized,
             Self::Hasher: Hasher { ... }
}

Expand description

A trait for creating instances of Hasher.

A BuildHasher is typically used (e.g., by HashMap) to create Hashers for each key such that they are hashed independently of one another, since Hashers contain state.

For each instance of BuildHasher, the Hashers created by build_hasher should be identical. That is, if the same stream of bytes is fed into each hasher, the same output will also be generated.

§Examples

use std::hash::{BuildHasher, Hasher, RandomState};

let s = RandomState::new();
let mut hasher_1 = s.build_hasher();
let mut hasher_2 = s.build_hasher();

hasher_1.write_u32(8128);
hasher_2.write_u32(8128);

assert_eq!(hasher_1.finish(), hasher_2.finish());

Required Associated Types§

1.7.0 · Source

type Hasher: Hasher

Type of the hasher that will be created.

Required Methods§

1.7.0 · Source

fn build_hasher(&self) -> Self::Hasher

Creates a new hasher.

Each call to build_hasher on the same instance should produce identical Hashers.

§Examples

use std::hash::{BuildHasher, RandomState};

let s = RandomState::new();
let new_s = s.build_hasher();

Provided Methods§

1.71.0 · Source

fn hash_one<T>(&self, x: T) -> u64
where T: Hash, Self: Sized, Self::Hasher: Hasher,

Calculates the hash of a single value.

This is intended as a convenience for code which consumes hashes, such as the implementation of a hash table or in unit tests that check whether a custom Hash implementation behaves as expected.

This must not be used in any code which creates hashes, such as in an implementation of Hash. The way to create a combined hash of multiple values is to call Hash::hash multiple times using the same Hasher, not to call this method repeatedly and combine the results.

§Example

use std::cmp::{max, min};
use std::hash::{BuildHasher, Hash, Hasher};
struct OrderAmbivalentPair<T: Ord>(T, T);
impl<T: Ord + Hash> Hash for OrderAmbivalentPair<T> {
    fn hash<H: Hasher>(&self, hasher: &mut H) {
        min(&self.0, &self.1).hash(hasher);
        max(&self.0, &self.1).hash(hasher);
    }
}

// Then later, in a `#[test]` for the type...
let bh = std::hash::RandomState::new();
assert_eq!(
    bh.hash_one(OrderAmbivalentPair(1, 2)),
    bh.hash_one(OrderAmbivalentPair(2, 1))
);
assert_eq!(
    bh.hash_one(OrderAmbivalentPair(10, 2)),
    bh.hash_one(&OrderAmbivalentPair(2, 10))
);