Deterministic HashSet #276
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Vrixyz
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Sep 26, 2024
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| const K: u32 = 0x9e3779b9; | ||
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| /// This is the same as FxHasher, but with the guarantee that the internal hash is | ||
| /// an u32 instead of something that depends on the platform. | ||
| pub struct FxHasher32 { | ||
| hash: u32, | ||
| } | ||
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| impl Default for FxHasher32 { | ||
| #[inline] | ||
| fn default() -> FxHasher32 { | ||
| FxHasher32 { hash: 0 } | ||
| } | ||
| } | ||
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| impl FxHasher32 { | ||
| #[inline] | ||
| fn add_to_hash(&mut self, i: u32) { | ||
| use std::ops::BitXor; | ||
| self.hash = self.hash.rotate_left(5).bitxor(i).wrapping_mul(K); | ||
| } | ||
| } | ||
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| impl std::hash::Hasher for FxHasher32 { | ||
| #[inline] | ||
| fn write(&mut self, mut bytes: &[u8]) { | ||
| let read_u32 = |bytes: &[u8]| u32::from_ne_bytes(bytes[..4].try_into().unwrap()); | ||
| let mut hash = FxHasher32 { hash: self.hash }; | ||
| assert!(size_of::<u32>() <= 8); | ||
| while bytes.len() >= size_of::<u32>() { | ||
| hash.add_to_hash(read_u32(bytes)); | ||
| bytes = &bytes[size_of::<u32>()..]; | ||
| } | ||
| if (size_of::<u32>() > 4) && (bytes.len() >= 4) { | ||
| hash.add_to_hash(u32::from_ne_bytes(bytes[..4].try_into().unwrap())); | ||
| bytes = &bytes[4..]; | ||
| } | ||
| if (size_of::<u32>() > 2) && bytes.len() >= 2 { | ||
| hash.add_to_hash(u16::from_ne_bytes(bytes[..2].try_into().unwrap()) as u32); | ||
| bytes = &bytes[2..]; | ||
| } | ||
| if (size_of::<u32>() > 1) && !bytes.is_empty() { | ||
| hash.add_to_hash(bytes[0] as u32); | ||
| } | ||
| self.hash = hash.hash; | ||
| } | ||
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| #[inline] | ||
| fn write_u8(&mut self, i: u8) { | ||
| self.add_to_hash(i as u32); | ||
| } | ||
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| #[inline] | ||
| fn write_u16(&mut self, i: u16) { | ||
| self.add_to_hash(i as u32); | ||
| } | ||
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| #[inline] | ||
| fn write_u32(&mut self, i: u32) { | ||
| self.add_to_hash(i); | ||
| } | ||
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| #[inline] | ||
| fn write_u64(&mut self, i: u64) { | ||
| self.add_to_hash(i as u32); | ||
| self.add_to_hash((i >> 32) as u32); | ||
| } | ||
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| #[inline] | ||
| fn write_usize(&mut self, i: usize) { | ||
| self.add_to_hash(i as u32); | ||
| } | ||
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| #[inline] | ||
| fn finish(&self) -> u64 { | ||
| self.hash as u64 | ||
| } | ||
| } |
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This is all copied from hashmap , maybe worth splitting it out in another module and reuse ?
Vrixyz
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utils::HashSet#269Note that I'm not very comfortable with those hashers and determinism in general, I'd be more comfortable with some unit tests around it.