Trait core::hash::BuildHasher 1.7.0[−][src]
pub trait BuildHasher {
type Hasher: Hasher;
fn build_hasher(&self) -> Self::Hasher;
fn hash_one<T: Hash>(&self, x: T) -> u64
where
Self: Sized,
{ ... }
}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::collections::hash_map::RandomState;
use std::hash::{BuildHasher, Hasher};
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());RunAssociated Types
Required methods
Provided methods
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
#![feature(build_hasher_simple_hash_one)]
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::collections::hash_map::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