| 1 | use crate::HashFn; |
|---|---|
| 2 | use std::convert::TryInto; |
| 3 | |
| 4 | /// Implements the hash function from the rustc-hash crate. |
| 5 | #[derive(Eq, PartialEq)] |
| 6 | pub struct FxHashFn; |
| 7 | |
| 8 | impl HashFn for FxHashFn { |
| 9 | // This function is marked as #[inline] because that allows LLVM to know the |
| 10 | // actual size of `bytes` and thus eliminate all unneeded branches below. |
| 11 | #[inline] |
| 12 | fn hash(mut bytes: &[u8]) -> u32 { |
| 13 | let mut hash_value = 0; |
| 14 | |
| 15 | while bytes.len() >= 8 { |
| 16 | hash_value = add_to_hash(hash_value, read_u64(bytes)); |
| 17 | bytes = &bytes[8..]; |
| 18 | } |
| 19 | |
| 20 | if bytes.len() >= 4 { |
| 21 | hash_value = add_to_hash( |
| 22 | hash_value, |
| 23 | u32::from_le_bytes(bytes[..4].try_into().unwrap()) as u64, |
| 24 | ); |
| 25 | bytes = &bytes[4..]; |
| 26 | } |
| 27 | |
| 28 | if bytes.len() >= 2 { |
| 29 | hash_value = add_to_hash( |
| 30 | hash_value, |
| 31 | u16::from_le_bytes(bytes[..2].try_into().unwrap()) as u64, |
| 32 | ); |
| 33 | bytes = &bytes[2..]; |
| 34 | } |
| 35 | |
| 36 | if bytes.len() >= 1 { |
| 37 | hash_value = add_to_hash(hash_value, bytes[0] as u64); |
| 38 | } |
| 39 | |
| 40 | return hash_value as u32; |
| 41 | |
| 42 | #[inline] |
| 43 | fn add_to_hash(current_hash: u64, value: u64) -> u64 { |
| 44 | use std::ops::BitXor; |
| 45 | current_hash |
| 46 | .rotate_left(5) |
| 47 | .bitxor(value) |
| 48 | // This constant is part of FxHash's definition: |
| 49 | // https://github.com/rust-lang/rustc-hash/blob/5e09ea0a1/src/lib.rs#L67 |
| 50 | .wrapping_mul(0x517cc1b727220a95) |
| 51 | } |
| 52 | |
| 53 | #[inline] |
| 54 | fn read_u64(bytes: &[u8]) -> u64 { |
| 55 | u64::from_le_bytes(bytes[..8].try_into().unwrap()) |
| 56 | } |
| 57 | } |
| 58 | } |
| 59 |