6

HashMap in std::collections::hash_map - Rust

 2 years ago
source link: https://doc.rust-lang.org/stable/std/collections/hash_map/struct.HashMap.html
Go to the source link to view the article. You can view the picture content, updated content and better typesetting reading experience. If the link is broken, please click the button below to view the snapshot at that time.

Struct std::collections::hash_map::HashMap1.0.0[−][src]

pub struct HashMap<K, V, S = RandomState> { /* fields omitted */ }

A hash map implemented with quadratic probing and SIMD lookup.

By default, HashMap uses a hashing algorithm selected to provide resistance against HashDoS attacks. The algorithm is randomly seeded, and a reasonable best-effort is made to generate this seed from a high quality, secure source of randomness provided by the host without blocking the program. Because of this, the randomness of the seed depends on the output quality of the system’s random number generator when the seed is created. In particular, seeds generated when the system’s entropy pool is abnormally low such as during system boot may be of a lower quality.

The default hashing algorithm is currently SipHash 1-3, though this is subject to change at any point in the future. While its performance is very competitive for medium sized keys, other hashing algorithms will outperform it for small keys such as integers as well as large keys such as long strings, though those algorithms will typically not protect against attacks such as HashDoS.

The hashing algorithm can be replaced on a per-HashMap basis using the default, with_hasher, and with_capacity_and_hasher methods. There are many alternative hashing algorithms available on crates.io.

It is required that the keys implement the Eq and Hash traits, although this can frequently be achieved by using #[derive(PartialEq, Eq, Hash)]. If you implement these yourself, 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 be equal.

It is a logic error for a key to be modified in such a way that the key’s hash, as determined by the Hash trait, or its equality, as determined by the Eq trait, changes while it is in the map. This is normally only possible through Cell, RefCell, global state, I/O, or unsafe code. The behavior resulting from such a logic error is not specified, but will not result in undefined behavior. This could include panics, incorrect results, aborts, memory leaks, and non-termination.

The hash table implementation is a Rust port of Google’s SwissTable. The original C++ version of SwissTable can be found here, and this CppCon talk gives an overview of how the algorithm works.

Examples

use std::collections::HashMap;

// Type inference lets us omit an explicit type signature (which
// would be `HashMap<String, String>` in this example).
let mut book_reviews = HashMap::new();

// Review some books.
book_reviews.insert(
    "Adventures of Huckleberry Finn".to_string(),
    "My favorite book.".to_string(),
);
book_reviews.insert(
    "Grimms' Fairy Tales".to_string(),
    "Masterpiece.".to_string(),
);
book_reviews.insert(
    "Pride and Prejudice".to_string(),
    "Very enjoyable.".to_string(),
);
book_reviews.insert(
    "The Adventures of Sherlock Holmes".to_string(),
    "Eye lyked it alot.".to_string(),
);

// Check for a specific one.
// When collections store owned values (String), they can still be
// queried using references (&str).
if !book_reviews.contains_key("Les Misérables") {
    println!("We've got {} reviews, but Les Misérables ain't one.",
             book_reviews.len());
}

// oops, this review has a lot of spelling mistakes, let's delete it.
book_reviews.remove("The Adventures of Sherlock Holmes");

// Look up the values associated with some keys.
let to_find = ["Pride and Prejudice", "Alice's Adventure in Wonderland"];
for &book in &to_find {
    match book_reviews.get(book) {
        Some(review) => println!("{}: {}", book, review),
        None => println!("{} is unreviewed.", book)
    }
}

// Look up the value for a key (will panic if the key is not found).
println!("Review for Jane: {}", book_reviews["Pride and Prejudice"]);

// Iterate over everything.
for (book, review) in &book_reviews {
    println!("{}: \"{}\"", book, review);
}
Run

A HashMap with a known list of items can be initialized from an array:

use std::collections::HashMap;

let solar_distance = HashMap::from([
    ("Mercury", 0.4),
    ("Venus", 0.7),
    ("Earth", 1.0),
    ("Mars", 1.5),
]);
Run

HashMap implements an Entry API, which allows for complex methods of getting, setting, updating and removing keys and their values:

use std::collections::HashMap;

// type inference lets us omit an explicit type signature (which
// would be `HashMap<&str, u8>` in this example).
let mut player_stats = HashMap::new();

fn random_stat_buff() -> u8 {
    // could actually return some random value here - let's just return
    // some fixed value for now
    42
}

// insert a key only if it doesn't already exist
player_stats.entry("health").or_insert(100);

// insert a key using a function that provides a new value only if it
// doesn't already exist
player_stats.entry("defence").or_insert_with(random_stat_buff);

// update a key, guarding against the key possibly not being set
let stat = player_stats.entry("attack").or_insert(100);
*stat += random_stat_buff();
Run

The easiest way to use HashMap with a custom key type is to derive Eq and Hash. We must also derive PartialEq.

use std::collections::HashMap;

#[derive(Hash, Eq, PartialEq, Debug)]
struct Viking {
    name: String,
    country: String,
}

impl Viking {
    /// Creates a new Viking.
    fn new(name: &str, country: &str) -> Viking {
        Viking { name: name.to_string(), country: country.to_string() }
    }
}

// Use a HashMap to store the vikings' health points.
let vikings = HashMap::from([
    (Viking::new("Einar", "Norway"), 25),
    (Viking::new("Olaf", "Denmark"), 24),
    (Viking::new("Harald", "Iceland"), 12),
]);

// Use derived implementation to print the status of the vikings.
for (viking, health) in &vikings {
    println!("{:?} has {} hp", viking, health);
}
Run

report

Recommend

About Joyk


Aggregate valuable and interesting links.
Joyk means Joy of geeK