crates/rustc-hash/src/seeded_state.rs

use crate::FxHasher;

/// Type alias for a hashmap using the `fx` hash algorithm with [`FxSeededState`].
#[cfg(feature = "std")]
pub type FxHashMapSeed<K, V> = std::collections::HashMap<K, V, FxSeededState>;

/// Type alias for a hashmap using the `fx` hash algorithm with [`FxSeededState`].
#[cfg(feature = "std")]
pub type FxHashSetSeed<V> = std::collections::HashSet<V, FxSeededState>;

/// [`FxSeededState`] is an alternative state for `HashMap` types, allowing to use [`FxHasher`] with a set seed.
///
/// ```
/// # use std::collections::HashMap;
/// use rustc_hash::FxSeededState;
///
/// let mut map = HashMap::with_hasher(FxSeededState::with_seed(12));
/// map.insert(15, 610);
/// assert_eq!(map[&15], 610);
/// ```
#[derive(Clone)]
pub struct FxSeededState {
    seed: usize,
}

impl FxSeededState {
    /// Constructs a new `FxSeededState` that is initialized with a `seed`.
    pub const fn with_seed(seed: usize) -> FxSeededState {
        Self { seed }
    }
}

impl core::hash::BuildHasher for FxSeededState {
    type Hasher = FxHasher;

    fn build_hasher(&self) -> Self::Hasher {
        FxHasher::with_seed(self.seed)
    }
}

#[cfg(test)]
mod tests {
    use core::hash::BuildHasher;

    use crate::FxSeededState;

    #[test]
    fn cloned_seeded_states_are_equal() {
        let seed = 2;
        let a = FxSeededState::with_seed(seed);
        let b = a.clone();

        assert_eq!(a.seed, b.seed);
        assert_eq!(a.seed, seed);

        assert_eq!(a.build_hasher().hash, b.build_hasher().hash);
    }

    #[test]
    fn same_seed_produces_same_hasher() {
        let seed = 1;
        let a = FxSeededState::with_seed(seed);
        let b = FxSeededState::with_seed(seed);

        // The hashers should be the same, as they have the same seed.
        assert_eq!(a.build_hasher().hash, b.build_hasher().hash);
    }

    #[test]
    fn different_states_are_different() {
        let a = FxSeededState::with_seed(1);
        let b = FxSeededState::with_seed(2);

        assert_ne!(a.build_hasher().hash, b.build_hasher().hash);
    }
}