Use Rust’s `HashMap` when values need to be retrieved by key rather than by position. It is the standard choice for most key-based lookups; use `Vec` for a sequence of values instead. section 4.4 section 4.9
Two requirements shape the implementation:
- Keys must implement `Eq` and `Hash`. For a compound key, `#[derive(Hash, Eq, PartialEq, Debug)]` generates the needed implementations and adds useful debug output. `Eq` also requires `PartialEq`. section 4.4.2
- The default Siphash-1-3 hasher is adequate for most cases, but may be a poor fit for very small or very large keys. A custom hasher must satisfy `BuildHasher`, `Hasher`, and `Default` requirements. section 4.4 section 4.4.1
`BuildHasher::build_hasher()` creates a hasher. The required `Hasher` methods are `write()`, which accepts a byte slice, and `finish()`, which returns a `u64` hash. Existing crates can supply these implementations instead of requiring a new hash function. section 4.4.1
Listing 4.9 Code listing for using HashMap with MetroHash
use metrohash::MetroBuildHasher;
use std::collections::HashMap;
let mut map = HashMap::<String, String,
MetroBuildHasher>::default(); #1
map.insert("hello?".into(), "Hello!".into()); #2
println!("{:?}", map.get("hello?")); #3
This listing constructs a `HashMap` with `MetroBuildHasher`, inserts a pair, and retrieves its value by key. section 4.4.1
Listing 4.10 Code listing for a compound key type
#[derive(Hash, Eq, PartialEq, Debug)]
struct CompoundKey {
name: String,
value: i32,
}
This compound key demonstrates deriving the traits needed to use a struct as a map key. section 4.4.2
Start with `HashMap` and its default hasher for key-based lookup. Change the hasher when the key characteristics justify it; choose another collection only when its behavior addresses a concrete need. section 4.4 section 4.3.3
