rustmax::nom::lib::std::hash

Trait Hash

1.0.0 · Source

pub trait Hash {
    // Required method
    fn hash<H>(&self, state: &mut H)
       where H: Hasher;

    // Provided method
    fn hash_slice<H>(data: &[Self], state: &mut H)
       where H: Hasher,
             Self: Sized { ... }
}

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)].

Violating this property is a logic error. The behavior resulting from a logic error is not specified, but users of the trait must ensure that such logic errors do not result in undefined behavior. This means that unsafe code must not rely on the correctness of these methods.

§Prefix collisions

Implementations of hash should ensure that the data they pass to the Hasher are prefix-free. That is, values which are not equal 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§

1.0.0 · Source

fn hash<H>(&self, state: &mut H)
where H: Hasher,

Feeds this value into the given Hasher.

§Examples

use std::hash::{DefaultHasher, Hash, Hasher};

let mut hasher = DefaultHasher::new();
7920.hash(&mut hasher);
println!("Hash is {:x}!", hasher.finish());

Provided Methods§

1.3.0 · Source

fn hash_slice<H>(data: &[Self], state: &mut H)
where H: Hasher, Self: Sized,

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::hash::{DefaultHasher, 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());

Dyn Compatibility§

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety".

Implementors§

1.29.0 · Source§

impl Hash for !

1.0.0 · Source§

Trait Hash Copy item path

§Implementing Hash

§Hash and Eq

§Prefix collisions

§Portability

Required Methods§

fn hash<H>(&self, state: &mut H)where H: Hasher,

§Examples

Provided Methods§

fn hash_slice<H>(data: &[Self], state: &mut H)where H: Hasher, Self: Sized,

§Examples

Dyn Compatibility§

Implementors§

impl Hash for !

impl Hash for ()

impl Hash for ATerm

impl Hash for rustmax::bindgen::Abi

impl Hash for syn::ty::Abi

impl Hash for Abstract

impl Hash for Access

impl Hash for AccessFlags

impl Hash for AccessKind

impl Hash for AccessMode

impl Hash for Alignment

impl Hash for And

impl Hash for AndAnd

impl Hash for AndEq

impl Hash for AngleBracketedGenericArguments

impl Hash for Ansi256Color

impl Hash for AnsiColor

impl Hash for AnyDelimiterCodec

impl Hash for AnyExtension

impl Hash for ArchiveOffset

impl Hash for Arm

impl Hash for As

impl Hash for AsciiChar

impl Hash for AssocConst

impl Hash for AssocType

impl Hash for Async

impl Hash for At

impl Hash for nix::fcntl::AtFlags

impl Hash for rustix::backend::fs::types::AtFlags

impl Hash for AttrStyle

impl Hash for syn::attr::Attribute

impl Hash for icu_locale_core::extensions::unicode::attribute::Attribute

impl Hash for Attributes

impl Hash for Authority

§Examples

impl Hash for Auto

impl Hash for Await

impl Hash for B0

impl Hash for B1

impl Hash for bstr::bstr::BStr

impl Hash for winnow::stream::bstr::BStr

impl Hash for BString

impl Hash for BareFnArg

impl Hash for BareVariadic

impl Hash for BarycentricWeightScale

impl Hash for BaudRate

impl Hash for Become

impl Hash for BidiClass

impl Hash for zerocopy::byteorder::BigEndian

impl Hash for byteorder_lite::BigEndian

impl Hash for num_bigint::bigint::BigInt

impl Hash for rustmax::num_bigint::BigInt

impl Hash for num_bigint::biguint::BigUint

impl Hash for rustmax::num_bigint::BigUint

impl Hash for BinOp

impl Hash for BindgenError

impl Hash for Block

impl Hash for BlockDescription

impl Hash for BlockIndex

impl Hash for BlockType

impl Hash for BoundLifetimes

impl Hash for syn::token::Box

impl Hash for Brace

impl Hash for Bracket

impl Hash for Break

impl Hash for BufferFormat

impl Hash for BuildMetadata

Trait Hash

§Implementing `Hash`

§`Hash` and `Eq`

fn hash<H>(&self, state: &mut H)
where H: Hasher,

fn hash_slice<H>(data: &[Self], state: &mut H)
where H: Hasher, Self: Sized,