Add FxSeededState & co

src/lib.rs

@@ -34,6 +34,8 @@ extern crate rand;
 #[cfg(feature = "rand")]
 mod random_state;
 
+mod seeded_state;
+
 use core::convert::TryInto;
 use core::default::Default;
 #[cfg(feature = "std")]
@@ -55,6 +57,8 @@ pub type FxHashSet<V> = HashSet<V, BuildHasherDefault<FxHasher>>;
 #[cfg(feature = "rand")]
 pub use random_state::{FxHashMapRand, FxHashSetRand, FxRandomState};
 
+pub use seeded_state::{FxHashMapSeed, FxHashSetSeed, FxSeededState};
+
 /// A speedy hash algorithm for use within rustc. The hashmap in liballoc
 /// by default uses SipHash which isn't quite as speedy as we want. In the
 /// compiler we're not really worried about DOS attempts, so we use a fast

src/seeded_state.rs

@@ -0,0 +1,56 @@
+use std::collections::{HashMap, HashSet};
+
+use crate::FxHasher;
+
+/// Type alias for a hashmap using the `fx` hash algorithm with [`FxSeededState`].
+pub type FxHashMapSeed<K, V> = HashMap<K, V, FxSeededState>;
+
+/// Type alias for a hashmap using the `fx` hash algorithm with [`FxSeededState`].
+pub type FxHashSetSeed<V> = HashSet<V, FxSeededState>;
+
+/// [`FxSetState`] 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);
+/// ```
+pub struct FxSeededState {
+    seed: usize,
+}
+
+impl FxSeededState {
+    /// Constructs a new `FxSeededState` that is initialized with a `seed`.
+    pub 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::{FxHashMapSeed, FxSeededState};
+
+    #[test]
+    fn different_states_are_different() {
+        let a = FxHashMapSeed::<&str, u32>::with_hasher(FxSeededState::with_seed(1));
+        let b = FxHashMapSeed::<&str, u32>::with_hasher(FxSeededState::with_seed(2));
+
+        assert_ne!(
+            a.hasher().build_hasher().hash,
+            b.hasher().build_hasher().hash
+        );
+    }
+}