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Massively improve high bit benchmarks with some bit mixing

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This commit is contained in:
Gwilym Inzani 2023-05-10 16:14:04 +01:00
parent 82855ac66b
commit 626e4da8e4
1 changed files with 16 additions and 9 deletions
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25
agb-hashmap/src/lib.rs
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@ -477,7 +477,12 @@ where
{
let mut hasher = self.hasher.build_hasher();
key.hash(&mut hasher);
hasher.finish().into()
let result = hasher.finish();
// we want to allow truncation here since we're reducing 64 bits to 32
#[allow(clippy::cast_possible_truncation)]
let reduced = (result as u32) ^ ((result >> 32) as u32);
HashType::bit_mix(reduced)
}
}
@ -899,14 +904,6 @@ const fn number_before_resize(capacity: usize) -> usize {
#[derive(Default, Clone, Copy, PartialEq, Eq)]
pub(crate) struct HashType(u32);
impl From<u64> for HashType {
fn from(value: u64) -> Self {
// we explicitly want to allow truncation
#[allow(clippy::cast_possible_truncation)]
Self(value as u32)
}
}
impl From<usize> for HashType {
fn from(value: usize) -> Self {
// we explicitly want to allow truncation
@ -916,6 +913,16 @@ impl From<usize> for HashType {
}
impl HashType {
// 32 bit mix function from here: https://gist.github.com/badboy/6267743
fn bit_mix(key: u32) -> Self {
let key = !key.wrapping_add(key << 15);
let key = key ^ key.rotate_right(12);
let key = key.wrapping_add(key << 2);
let key = key ^ key.rotate_right(4);
let key = key.wrapping_add(key << 3).wrapping_add(key << 11);
Self(key ^ key.rotate_right(16))
}
pub(crate) fn fast_mod(self, len: usize) -> usize {
debug_assert!(len.is_power_of_two(), "Length must be a power of 2");
(self.0 as usize) & (len - 1)