rustc_hash/
seeded_state.rs

1use crate::FxHasher;
2
3/// Type alias for a hashmap using the `fx` hash algorithm with [`FxSeededState`].
4#[cfg(feature = "std")]
5pub type FxHashMapSeed<K, V> = std::collections::HashMap<K, V, FxSeededState>;
6
7/// Type alias for a hashmap using the `fx` hash algorithm with [`FxSeededState`].
8#[cfg(feature = "std")]
9pub type FxHashSetSeed<V> = std::collections::HashSet<V, FxSeededState>;
10
11/// [`FxSeededState`] is an alternative state for `HashMap` types, allowing to use [`FxHasher`] with a set seed.
12///
13/// ```
14/// # use std::collections::HashMap;
15/// use rustc_hash::FxSeededState;
16///
17/// let mut map = HashMap::with_hasher(FxSeededState::with_seed(12));
18/// map.insert(15, 610);
19/// assert_eq!(map[&15], 610);
20/// ```
21#[derive(Clone)]
22pub struct FxSeededState {
23    seed: usize,
24}
25
26impl FxSeededState {
27    /// Constructs a new `FxSeededState` that is initialized with a `seed`.
28    pub const fn with_seed(seed: usize) -> FxSeededState {
29        Self { seed }
30    }
31}
32
33impl core::hash::BuildHasher for FxSeededState {
34    type Hasher = FxHasher;
35
36    fn build_hasher(&self) -> Self::Hasher {
37        FxHasher::with_seed(self.seed)
38    }
39}
40
41#[cfg(test)]
42mod tests {
43    use core::hash::BuildHasher;
44
45    use crate::FxSeededState;
46
47    #[test]
48    fn cloned_seeded_states_are_equal() {
49        let seed = 2;
50        let a = FxSeededState::with_seed(seed);
51        let b = a.clone();
52
53        assert_eq!(a.seed, b.seed);
54        assert_eq!(a.seed, seed);
55
56        assert_eq!(a.build_hasher().hash, b.build_hasher().hash);
57    }
58
59    #[test]
60    fn same_seed_produces_same_hasher() {
61        let seed = 1;
62        let a = FxSeededState::with_seed(seed);
63        let b = FxSeededState::with_seed(seed);
64
65        // The hashers should be the same, as they have the same seed.
66        assert_eq!(a.build_hasher().hash, b.build_hasher().hash);
67    }
68
69    #[test]
70    fn different_states_are_different() {
71        let a = FxSeededState::with_seed(1);
72        let b = FxSeededState::with_seed(2);
73
74        assert_ne!(a.build_hasher().hash, b.build_hasher().hash);
75    }
76}
rustc_hash/seeded_state.rs

rustc_hash/
seeded_state.rs
