OXYGENHUB
# 7f617ee9b3ad oxygen / crates / oxy-core / src / fasthash.rs
3.6 KiB · Rust · last changed Faster state loading: seeded fast hashing for the file graphs, no unused predecessor map, inline successor sets, lines moved rather than copied; OXY_TIMINGS=1 prints snapshot phase timings. Clean status at 10 000 files 0.96 s to 0.52 s, memory 657 to 239 MB 1 month ago source blame history raw
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//! A fast hasher for the in-memory graphs (spec §25): their keys begin with
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//! BLAKE3 hashes, so the bytes are already uniform and SipHash's cost buys
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//! nothing. A per-process random seed keeps bucket placement unpredictable,
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//! so keys chosen by someone else cannot be aimed at one bucket.
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use std::hash::{BuildHasher, Hasher};
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use std::sync::OnceLock;
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fn seed() -> u64 {
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    static SEED: OnceLock<u64> = OnceLock::new();
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    *SEED.get_or_init(rand::random::<u64>)
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}
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/// Builds [`FastHasher`]s with the process seed.
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#[derive(Clone, Copy, Debug)]
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pub struct FastState(u64);
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impl Default for FastState {
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    fn default() -> FastState {
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        FastState(seed())
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    }
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}
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impl BuildHasher for FastState {
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    type Hasher = FastHasher;
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    fn build_hasher(&self) -> FastHasher {
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        FastHasher(self.0)
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    }
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}
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/// Folds each 8-byte word into the state with a multiply and rotate, then
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/// finishes with SplitMix64's mixer so every input bit reaches every output bit.
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pub struct FastHasher(u64);
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impl FastHasher {
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    #[inline]
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    fn mix(&mut self, word: u64) {
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        self.0 = (self.0 ^ word)
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            .wrapping_mul(0x9e37_79b9_7f4a_7c15)
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            .rotate_left(31);
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    }
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}
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impl Hasher for FastHasher {
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    #[inline]
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    fn write(&mut self, bytes: &[u8]) {
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        let mut chunks = bytes.chunks_exact(8);
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        for c in &mut chunks {
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            self.mix(u64::from_le_bytes(c.try_into().unwrap()));
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        }
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        let rest = chunks.remainder();
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        if !rest.is_empty() {
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            let mut b = [0u8; 8];
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            b[..rest.len()].copy_from_slice(rest);
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            self.mix(u64::from_le_bytes(b) ^ ((rest.len() as u64) << 59));
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        }
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    }
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    #[inline]
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    fn write_u8(&mut self, n: u8) {
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        self.mix(n as u64 | 0x100);
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    }
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    #[inline]
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    fn write_u32(&mut self, n: u32) {
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        self.mix(n as u64 | 0x1_0000_0000);
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    }
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    #[inline]
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    fn write_u64(&mut self, n: u64) {
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        self.mix(n);
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    }
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    #[inline]
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    fn write_usize(&mut self, n: usize) {
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        self.mix(n as u64);
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    }
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    #[inline]
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    fn finish(&self) -> u64 {
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        let mut z = self.0;
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        z = (z ^ (z >> 30)).wrapping_mul(0xbf58_476d_1ce4_e5b9);
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        z = (z ^ (z >> 27)).wrapping_mul(0x94d0_49bb_1331_11eb);
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        z ^ (z >> 31)
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    }
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}
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pub type FastMap<K, V> = std::collections::HashMap<K, V, FastState>;
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pub type FastSet<K> = std::collections::HashSet<K, FastState>;
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#[cfg(test)]
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mod tests {
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    use super::*;
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    use std::hash::Hash;
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    fn h<T: Hash + ?Sized>(state: &FastState, v: &T) -> u64 {
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        state.hash_one(v)
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    }
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    #[test]
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    fn distinct_keys_spread_and_equal_keys_agree() {
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        let s = FastState(42);
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        let a = ([7u8; 32], 1u32);
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        assert_eq!(h(&s, &a), h(&s, &a));
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        assert_ne!(h(&s, &a), h(&s, &([7u8; 32], 2u32)), "the index matters");
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        assert_ne!(
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            h(&s, &[1u8, 2, 3][..]),
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            h(&s, &[1u8, 2, 3, 0][..]),
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            "length matters"
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        );
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        // Consecutive indices of one change land in different buckets of a small table.
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        let buckets: std::collections::HashSet<u64> =
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            (0..64u32).map(|i| h(&s, &([9u8; 32], i)) & 63).collect();
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        assert!(buckets.len() > 32, "{} of 64 buckets used", buckets.len());
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        assert_ne!(
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            h(&FastState(1), &a),
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            h(&FastState(2), &a),
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            "the seed matters"
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        );
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    }
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    #[test]
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    fn maps_work() {
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        let mut m: FastMap<([u8; 32], u32), u32> = FastMap::default();
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        for i in 0..10_000u32 {
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            m.insert(([(i % 251) as u8; 32], i), i);
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        }
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        assert_eq!(m.len(), 10_000);
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        assert_eq!(m[&([77u8; 32], 77)], 77);
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    }
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}