Struct HashMap 

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

A hash map implemented with linear probing and Robin Hood bucket stealing.

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. Many alternative algorithms are available on crates.io, such as the fnv crate.

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.

Relevant papers/articles:

1. Pedro Celis. “Robin Hood Hashing”
2. Emmanuel Goossaert. “Robin Hood hashing”
3. Emmanuel Goossaert. “Robin Hood hashing: backward shift deletion”

Examples

use rayon_hash::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);
}

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

use rayon_hash::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();

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

use rayon_hash::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 mut vikings = HashMap::new();

vikings.insert(Viking::new("Einar", "Norway"), 25);
vikings.insert(Viking::new("Olaf", "Denmark"), 24);
vikings.insert(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);
}

A HashMap with fixed list of elements can be initialized from an array:

use rayon_hash::HashMap;

fn main() {
    let timber_resources: HashMap<&str, i32> =
    [("Norway", 100),
     ("Denmark", 50),
     ("Iceland", 10)]
     .iter().cloned().collect();
    // use the values stored in map
}

Implementations

impl<K: Sync, V, S> HashMap<K, V, S>

pub fn par_keys(&self) -> ParKeys<'_, K, V>

impl<K, V: Sync, S> HashMap<K, V, S>

pub fn par_values(&self) -> ParValues<'_, K, V>

impl<K, V: Send, S> HashMap<K, V, S>

pub fn par_values_mut(&mut self) -> ParValuesMut<'_, K, V>

impl<K, V, S> HashMap<K, V, S>

where K: Eq + Hash + Sync, V: PartialEq + Sync, S: BuildHasher + Sync,

pub fn par_eq(&self, other: &Self) -> bool

impl<K: Hash + Eq, V> HashMap<K, V, RandomState>

pub fn new() -> HashMap<K, V, RandomState>

Creates an empty HashMap.

The hash map is initially created with a capacity of 0, so it will not allocate until it is first inserted into.

Examples

use rayon_hash::HashMap;
let mut map: HashMap<&str, i32> = HashMap::new();

pub fn with_capacity(capacity: usize) -> HashMap<K, V, RandomState>

Creates an empty HashMap with the specified capacity.

The hash map will be able to hold at least capacity elements without reallocating. If capacity is 0, the hash map will not allocate.

Examples

use rayon_hash::HashMap;
let mut map: HashMap<&str, i32> = HashMap::with_capacity(10);

impl<K, V, S> HashMap<K, V, S>

pub fn capacity(&self) -> usize

Returns the number of elements the map can hold without reallocating.

This number is a lower bound; the HashMap<K, V> might be able to hold more, but is guaranteed to be able to hold at least this many.

Examples

use rayon_hash::HashMap;
let map: HashMap<i32, i32> = HashMap::with_capacity(100);
assert!(map.capacity() >= 100);

pub fn keys(&self) -> Keys<'_, K, V>

An iterator visiting all keys in arbitrary order. The iterator element type is &'a K.

Examples

use rayon_hash::HashMap;

let mut map = HashMap::new();
map.insert("a", 1);
map.insert("b", 2);
map.insert("c", 3);

for key in map.keys() {
    println!("{}", key);
}

pub fn values(&self) -> Values<'_, K, V>

An iterator visiting all values in arbitrary order. The iterator element type is &'a V.

Examples

use rayon_hash::HashMap;

let mut map = HashMap::new();
map.insert("a", 1);
map.insert("b", 2);
map.insert("c", 3);

for val in map.values() {
    println!("{}", val);
}

pub fn values_mut(&mut self) -> ValuesMut<'_, K, V>

An iterator visiting all values mutably in arbitrary order. The iterator element type is &'a mut V.

Examples

use rayon_hash::HashMap;

let mut map = HashMap::new();

map.insert("a", 1);
map.insert("b", 2);
map.insert("c", 3);

for val in map.values_mut() {
    *val = *val + 10;
}

for val in map.values() {
    println!("{}", val);
}

pub fn iter(&self) -> Iter<'_, K, V>

An iterator visiting all key-value pairs in arbitrary order. The iterator element type is (&'a K, &'a V).

Examples

use rayon_hash::HashMap;

let mut map = HashMap::new();
map.insert("a", 1);
map.insert("b", 2);
map.insert("c", 3);

for (key, val) in map.iter() {
    println!("key: {} val: {}", key, val);
}

pub fn iter_mut(&mut self) -> IterMut<'_, K, V>

An iterator visiting all key-value pairs in arbitrary order, with mutable references to the values. The iterator element type is (&'a K, &'a mut V).

Examples

use rayon_hash::HashMap;

let mut map = HashMap::new();
map.insert("a", 1);
map.insert("b", 2);
map.insert("c", 3);

// Update all values
for (_, val) in map.iter_mut() {
    *val *= 2;
}

for (key, val) in &map {
    println!("key: {} val: {}", key, val);
}

pub fn len(&self) -> usize

Returns the number of elements in the map.

Examples

use rayon_hash::HashMap;

let mut a = HashMap::new();
assert_eq!(a.len(), 0);
a.insert(1, "a");
assert_eq!(a.len(), 1);

pub fn is_empty(&self) -> bool

Returns true if the map contains no elements.

Examples

use rayon_hash::HashMap;

let mut a = HashMap::new();
assert!(a.is_empty());
a.insert(1, "a");
assert!(!a.is_empty());

pub fn drain(&mut self) -> Drain<'_, K, V>

Clears the map, returning all key-value pairs as an iterator. Keeps the allocated memory for reuse.

Examples

use rayon_hash::HashMap;

let mut a = HashMap::new();
a.insert(1, "a");
a.insert(2, "b");

for (k, v) in a.drain().take(1) {
    assert!(k == 1 || k == 2);
    assert!(v == "a" || v == "b");
}

assert!(a.is_empty());

pub fn clear(&mut self)

Clears the map, removing all key-value pairs. Keeps the allocated memory for reuse.

Examples

use rayon_hash::HashMap;

let mut a = HashMap::new();
a.insert(1, "a");
a.clear();
assert!(a.is_empty());

impl<K, V, S> HashMap<K, V, S>

where K: Eq + Hash, S: BuildHasher,

pub fn with_hasher(hash_builder: S) -> HashMap<K, V, S>

Creates an empty HashMap which will use the given hash builder to hash keys.

The created map has the default initial capacity.

Warning: hash_builder is normally randomly generated, and is designed to allow HashMaps to be resistant to attacks that cause many collisions and very poor performance. Setting it manually using this function can expose a DoS attack vector.

Examples

use rayon_hash::HashMap;
use rayon_hash::hash_map::RandomState;

let s = RandomState::new();
let mut map = HashMap::with_hasher(s);
map.insert(1, 2);

pub fn with_capacity_and_hasher( capacity: usize, hash_builder: S, ) -> HashMap<K, V, S>

Creates an empty HashMap with the specified capacity, using hash_builder to hash the keys.

The hash map will be able to hold at least capacity elements without reallocating. If capacity is 0, the hash map will not allocate.

Warning: hash_builder is normally randomly generated, and is designed to allow HashMaps to be resistant to attacks that cause many collisions and very poor performance. Setting it manually using this function can expose a DoS attack vector.

Examples

use rayon_hash::HashMap;
use rayon_hash::hash_map::RandomState;

let s = RandomState::new();
let mut map = HashMap::with_capacity_and_hasher(10, s);
map.insert(1, 2);

pub fn hasher(&self) -> &S

Returns a reference to the map’s BuildHasher.

Examples

use rayon_hash::HashMap;
use rayon_hash::hash_map::RandomState;

let hasher = RandomState::new();
let map: HashMap<i32, i32> = HashMap::with_hasher(hasher);
let hasher: &RandomState = map.hasher();

pub fn reserve(&mut self, additional: usize)

Reserves capacity for at least additional more elements to be inserted in the HashMap. The collection may reserve more space to avoid frequent reallocations.

Panics

Panics if the new allocation size overflows usize.

Examples

use rayon_hash::HashMap;
let mut map: HashMap<&str, i32> = HashMap::new();
map.reserve(10);

pub fn shrink_to_fit(&mut self)

Shrinks the capacity of the map as much as possible. It will drop down as much as possible while maintaining the internal rules and possibly leaving some space in accordance with the resize policy.

Examples

use rayon_hash::HashMap;

let mut map: HashMap<i32, i32> = HashMap::with_capacity(100);
map.insert(1, 2);
map.insert(3, 4);
assert!(map.capacity() >= 100);
map.shrink_to_fit();
assert!(map.capacity() >= 2);

pub fn entry(&mut self, key: K) -> Entry<'_, K, V>

Gets the given key’s corresponding entry in the map for in-place manipulation.

Examples

use rayon_hash::HashMap;

let mut letters = HashMap::new();

for ch in "a short treatise on fungi".chars() {
    let counter = letters.entry(ch).or_insert(0);
    *counter += 1;
}

assert_eq!(letters[&'s'], 2);
assert_eq!(letters[&'t'], 3);
assert_eq!(letters[&'u'], 1);
assert_eq!(letters.get(&'y'), None);

pub fn get<Q>(&self, k: &Q) -> Option<&V>

where K: Borrow<Q>, Q: Hash + Eq + ?Sized,

Returns a reference to the value corresponding to the key.

The key may be any borrowed form of the map’s key type, but Hash and Eq on the borrowed form must match those for the key type.

Examples

use rayon_hash::HashMap;

let mut map = HashMap::new();
map.insert(1, "a");
assert_eq!(map.get(&1), Some(&"a"));
assert_eq!(map.get(&2), None);

pub fn contains_key<Q>(&self, k: &Q) -> bool

where K: Borrow<Q>, Q: Hash + Eq + ?Sized,

Returns true if the map contains a value for the specified key.

The key may be any borrowed form of the map’s key type, but Hash and Eq on the borrowed form must match those for the key type.

Examples

use rayon_hash::HashMap;

let mut map = HashMap::new();
map.insert(1, "a");
assert_eq!(map.contains_key(&1), true);
assert_eq!(map.contains_key(&2), false);

pub fn get_mut<Q>(&mut self, k: &Q) -> Option<&mut V>

where K: Borrow<Q>, Q: Hash + Eq + ?Sized,

Returns a mutable reference to the value corresponding to the key.

The key may be any borrowed form of the map’s key type, but Hash and Eq on the borrowed form must match those for the key type.

Examples

use rayon_hash::HashMap;

let mut map = HashMap::new();
map.insert(1, "a");
if let Some(x) = map.get_mut(&1) {
    *x = "b";
}
assert_eq!(map[&1], "b");

pub fn insert(&mut self, k: K, v: V) -> Option<V>

Inserts a key-value pair into the map.

If the map did not have this key present, None is returned.

If the map did have this key present, the value is updated, and the old value is returned. The key is not updated, though; this matters for types that can be == without being identical. See the module-level documentation for more.

Examples

use rayon_hash::HashMap;

let mut map = HashMap::new();
assert_eq!(map.insert(37, "a"), None);
assert_eq!(map.is_empty(), false);

map.insert(37, "b");
assert_eq!(map.insert(37, "c"), Some("b"));
assert_eq!(map[&37], "c");

pub fn remove<Q>(&mut self, k: &Q) -> Option<V>

where K: Borrow<Q>, Q: Hash + Eq + ?Sized,

Removes a key from the map, returning the value at the key if the key was previously in the map.

The key may be any borrowed form of the map’s key type, but Hash and Eq on the borrowed form must match those for the key type.

Examples

use rayon_hash::HashMap;

let mut map = HashMap::new();
map.insert(1, "a");
assert_eq!(map.remove(&1), Some("a"));
assert_eq!(map.remove(&1), None);

pub fn remove_entry<Q>(&mut self, k: &Q) -> Option<(K, V)>

where K: Borrow<Q>, Q: Hash + Eq + ?Sized,

Removes a key from the map, returning the stored key and value if the key was previously in the map.

The key may be any borrowed form of the map’s key type, but Hash and Eq on the borrowed form must match those for the key type.

Examples

use rayon_hash::HashMap;

let mut map = HashMap::new();
map.insert(1, "a");
assert_eq!(map.remove_entry(&1), Some((1, "a")));
assert_eq!(map.remove(&1), None);

pub fn retain<F>(&mut self, f: F)

where F: FnMut(&K, &mut V) -> bool,

Retains only the elements specified by the predicate.

In other words, remove all pairs (k, v) such that f(&k,&mut v) returns false.

Examples

use rayon_hash::HashMap;

let mut map: HashMap<i32, i32> = (0..8).map(|x|(x, x*10)).collect();
map.retain(|&k, _| k % 2 == 0);
assert_eq!(map.len(), 4);

Trait Implementations

impl<K: Clone, V: Clone, S: Clone> Clone for HashMap<K, V, S>

fn clone(&self) -> HashMap<K, V, S>

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<K, V, S> Debug for HashMap<K, V, S>

where K: Eq + Hash + Debug, V: Debug, S: BuildHasher,

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<K, V, S> Default for HashMap<K, V, S>

where K: Eq + Hash, S: BuildHasher + Default,

fn default() -> HashMap<K, V, S>

Creates an empty HashMap<K, V, S>, with the Default value for the hasher.

impl<'a, K, V, S> Extend<(&'a K, &'a V)> for HashMap<K, V, S>

where K: Eq + Hash + Copy, V: Copy, S: BuildHasher,

fn extend<T: IntoIterator<Item = (&'a K, &'a V)>>(&mut self, iter: T)

Extends a collection with the contents of an iterator.

fn extend_one(&mut self, item: A)

🔬This is a nightly-only experimental API. (extend_one)

Extends a collection with exactly one element.

fn extend_reserve(&mut self, additional: usize)

🔬This is a nightly-only experimental API. (extend_one)

Reserves capacity in a collection for the given number of additional elements.

impl<K, V, S> Extend<(K, V)> for HashMap<K, V, S>

where K: Eq + Hash, S: BuildHasher,

fn extend<T: IntoIterator<Item = (K, V)>>(&mut self, iter: T)

Extends a collection with the contents of an iterator.

fn extend_one(&mut self, item: A)

🔬This is a nightly-only experimental API. (extend_one)

Extends a collection with exactly one element.

fn extend_reserve(&mut self, additional: usize)

🔬This is a nightly-only experimental API. (extend_one)

Reserves capacity in a collection for the given number of additional elements.

impl<K, V, S> FromIterator<(K, V)> for HashMap<K, V, S>

where K: Eq + Hash, S: BuildHasher + Default,

fn from_iter<T: IntoIterator<Item = (K, V)>>(iter: T) -> HashMap<K, V, S>

Creates a value from an iterator.

impl<K, V, S> FromParallelIterator<(K, V)> for HashMap<K, V, S>

where K: Eq + Hash + Send, V: Send, S: BuildHasher + Default + Send,

Collect (key, value) pairs from a parallel iterator into a hashmap. If multiple pairs correspond to the same key, then the ones produced earlier in the parallel iterator will be overwritten, just as with a sequential iterator.

fn from_par_iter<P>(par_iter: P) -> Self

where P: IntoParallelIterator<Item = (K, V)>,

Creates an instance of the collection from the parallel iterator par_iter.

impl<K, Q, V, S> Index<&Q> for HashMap<K, V, S>

where K: Eq + Hash + Borrow<Q>, Q: Eq + Hash + ?Sized, S: BuildHasher,

fn index(&self, key: &Q) -> &V

Returns a reference to the value corresponding to the supplied key.

Panics

Panics if the key is not present in the HashMap.

type Output = V

The returned type after indexing.

impl<'a, K, V, S> IntoIterator for &'a HashMap<K, V, S>

type Item = (&'a K, &'a V)

The type of the elements being iterated over.

type IntoIter = Iter<'a, K, V>

Which kind of iterator are we turning this into?

fn into_iter(self) -> Iter<'a, K, V>

Creates an iterator from a value.

impl<'a, K, V, S> IntoIterator for &'a mut HashMap<K, V, S>

type Item = (&'a K, &'a mut V)

The type of the elements being iterated over.

type IntoIter = IterMut<'a, K, V>

Which kind of iterator are we turning this into?

fn into_iter(self) -> IterMut<'a, K, V>

Creates an iterator from a value.

impl<K, V, S> IntoIterator for HashMap<K, V, S>

fn into_iter(self) -> IntoIter<K, V>

Creates a consuming iterator, that is, one that moves each key-value pair out of the map in arbitrary order. The map cannot be used after calling this.

Examples

use rayon_hash::HashMap;

let mut map = HashMap::new();
map.insert("a", 1);
map.insert("b", 2);
map.insert("c", 3);

// Not possible with .iter()
let vec: Vec<(&str, i32)> = map.into_iter().collect();

type Item = (K, V)

The type of the elements being iterated over.

type IntoIter = IntoIter<K, V>

Which kind of iterator are we turning this into?

impl<'a, K: Sync, V: Sync, S> IntoParallelIterator for &'a HashMap<K, V, S>

type Item = (&'a K, &'a V)

The type of item that the parallel iterator will produce.

type Iter = ParIter<'a, K, V>

The parallel iterator type that will be created.

fn into_par_iter(self) -> Self::Iter

Converts self into a parallel iterator.

impl<'a, K: Sync, V: Send, S> IntoParallelIterator for &'a mut HashMap<K, V, S>

type Item = (&'a K, &'a mut V)

The type of item that the parallel iterator will produce.

type Iter = ParIterMut<'a, K, V>

The parallel iterator type that will be created.

fn into_par_iter(self) -> Self::Iter

Converts self into a parallel iterator.

impl<K: Send, V: Send, S> IntoParallelIterator for HashMap<K, V, S>

type Item = (K, V)

The type of item that the parallel iterator will produce.

type Iter = ParIntoIter<K, V>

The parallel iterator type that will be created.

fn into_par_iter(self) -> Self::Iter

Converts self into a parallel iterator.

impl<'a, K, V, S> ParallelExtend<(&'a K, &'a V)> for HashMap<K, V, S>

where K: Copy + Eq + Hash + Send + Sync, V: Copy + Send + Sync, S: BuildHasher + Send,

Extend a hash map with copied items from a parallel iterator.

fn par_extend<I>(&mut self, par_iter: I)

where I: IntoParallelIterator<Item = (&'a K, &'a V)>,

Extends an instance of the collection with the elements drawn from the parallel iterator par_iter.

impl<K, V, S> ParallelExtend<(K, V)> for HashMap<K, V, S>

where K: Eq + Hash + Send, V: Send, S: BuildHasher + Send,

Extend a hash map with items from a parallel iterator.

fn par_extend<I>(&mut self, par_iter: I)

where I: IntoParallelIterator<Item = (K, V)>,

Extends an instance of the collection with the elements drawn from the parallel iterator par_iter.

impl<K, V, S> PartialEq for HashMap<K, V, S>

where K: Eq + Hash, V: PartialEq, S: BuildHasher,

fn eq(&self, other: &HashMap<K, V, S>) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<K, V, S> Eq for HashMap<K, V, S>

where K: Eq + Hash, V: Eq, S: BuildHasher,