Trait std::hash::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
Hasher
s for each key such that they are hashed independently of one
another, since Hasher
s contain state.
For each instance of BuildHasher
, the Hasher
s 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
Required Associated Types§
Required Methods§
1.7.0 · sourcefn build_hasher(&self) -> Self::Hasher
fn build_hasher(&self) -> Self::Hasher
Provided Methods§
1.71.0 · sourcefn hash_one<T>(&self, x: T) -> u64
fn hash_one<T>(&self, x: T) -> u64
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))
);