Trait core::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: Hash>(&self, x: T) -> u64
where 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
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());
RunRequired Associated Types§
Required Methods§
Provided Methods§
1.71.0 · sourcefn hash_one<T: Hash>(&self, x: T) -> u64
fn hash_one<T: Hash>(&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))
);
Run