rustHow to use a custom hash function with a Rust HashMap?

Using a custom hash function with a Rust HashMap is easy. You can use the HashMap::with_hasher method to create a HashMap with a custom Hasher implementation.

use std::collections::HashMap;
use std::hash::{BuildHasher, Hasher};

struct MyHasher;

impl Hasher for MyHasher {
    fn finish(&self) -> u64 {
        0
    }

    fn write(&mut self, _bytes: &[u8]) {
        // Do nothing
    }
}

impl BuildHasher for MyHasher {
    type Hasher = MyHasher;

    fn build_hasher(&self) -> Self::Hasher {
        MyHasher
    }
}

let mut map: HashMap<&str, i32, MyHasher> = HashMap::with_hasher(MyHasher);
map.insert("foo", 42);

assert_eq!(map.get("foo"), Some(&42));

The code above creates a HashMap with a custom Hasher implementation called MyHasher. The MyHasher struct implements the Hasher and BuildHasher traits, and the write and finish methods. The write method does nothing, and the finish method always returns 0.

The HashMap::with_hasher method is then used to create a HashMap with the MyHasher hasher. The map.insert method is then used to insert a key-value pair into the HashMap. Finally, the map.get method is used to retrieve the value associated with the key.

Helpful links

• HashMap documentation
• Hasher trait documentation
• BuildHasher trait documentation

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