pub trait Hash {
fn hash<H>(&self, state: &mut H)
where
H: Hasher;
fn hash_slice<H>(data: &[Self], state: &mut H)
where
H: Hasher,
{ ... }
}Expand description
A hashable type.
Types implementing Hash are able to be hashed with an instance of
Hasher.
Implementing Hash
You can derive Hash with #[derive(Hash)] if all fields implement Hash.
The resulting hash will be the combination of the values from calling
hash on each field.
#[derive(Hash)]
struct Rustacean {
name: String,
country: String,
}If you need more control over how a value is hashed, you can of course
implement the Hash trait yourself:
use std::hash::{Hash, Hasher};
struct Person {
id: u32,
name: String,
phone: u64,
}
impl Hash for Person {
fn hash<H: Hasher>(&self, state: &mut H) {
self.id.hash(state);
self.phone.hash(state);
}
}Hash and Eq
When implementing both Hash and Eq, it is important that the following
property holds:
k1 == k2 -> hash(k1) == hash(k2)In other words, if two keys are equal, their hashes must also be equal.
HashMap and HashSet both rely on this behavior.
Thankfully, you won’t need to worry about upholding this property when
deriving both Eq and Hash with #[derive(PartialEq, Eq, Hash)].
Prefix collisions
Implementations of hash should ensure that the data they
pass to the Hasher are prefix-free. That is,
unequal values should cause two different sequences of values to be written,
and neither of the two sequences should be a prefix of the other.
For example, the standard implementation of Hash for &str passes an extra
0xFF byte to the Hasher so that the values ("ab", "c") and ("a", "bc") hash differently.
Portability
Due to differences in endianness and type sizes, data fed by Hash to a Hasher
should not be considered portable across platforms. Additionally the data passed by most
standard library types should not be considered stable between compiler versions.
This means tests shouldn’t probe hard-coded hash values or data fed to a Hasher and
instead should check consistency with Eq.
Serialization formats intended to be portable between platforms or compiler versions should
either avoid encoding hashes or only rely on Hash and Hasher implementations that
provide additional guarantees.
Required methods
Provided methods
1.3.0 · sourcefn hash_slice<H>(data: &[Self], state: &mut H) where
H: Hasher,
fn hash_slice<H>(data: &[Self], state: &mut H) where
H: Hasher,
Feeds a slice of this type into the given Hasher.
This method is meant as a convenience, but its implementation is
also explicitly left unspecified. It isn’t guaranteed to be
equivalent to repeated calls of hash and implementations of
Hash should keep that in mind and call hash themselves
if the slice isn’t treated as a whole unit in the PartialEq
implementation.
For example, a VecDeque implementation might naïvely call
as_slices and then hash_slice on each slice, but this
is wrong since the two slices can change with a call to
make_contiguous without affecting the PartialEq
result. Since these slices aren’t treated as singular
units, and instead part of a larger deque, this method cannot
be used.
Examples
use std::collections::hash_map::DefaultHasher;
use std::hash::{Hash, Hasher};
let mut hasher = DefaultHasher::new();
let numbers = [6, 28, 496, 8128];
Hash::hash_slice(&numbers, &mut hasher);
println!("Hash is {:x}!", hasher.finish());Implementations on Foreign Types
impl<T, const LANES: usize> Hash for Simd<T, LANES> where
T: SimdElement + Hash,
LaneCount<LANES>: SupportedLaneCount,
impl<T, const LANES: usize> Hash for Simd<T, LANES> where
T: SimdElement + Hash,
LaneCount<LANES>: SupportedLaneCount,
The hash of an array is the same as that of the corresponding slice,
as required by the Borrow implementation.
#![feature(build_hasher_simple_hash_one)]
use std::hash::BuildHasher;
let b = std::collections::hash_map::RandomState::new();
let a: [u8; 3] = [0xa8, 0x3c, 0x09];
let s: &[u8] = &[0xa8, 0x3c, 0x09];
assert_eq!(b.hash_one(a), b.hash_one(s));Writes the contents of the BitSlice, in semantic bit order, into a hasher.
Implementors
The hash of a vector is the same as that of the corresponding slice,
as required by the core::borrow::Borrow implementation.
#![feature(build_hasher_simple_hash_one)]
use std::hash::BuildHasher;
let b = std::collections::hash_map::RandomState::new();
let v: Vec<u8> = vec![0xa8, 0x3c, 0x09];
let s: &[u8] = &[0xa8, 0x3c, 0x09];
assert_eq!(b.hash_one(v), b.hash_one(s));