1use crate::HashFn;
2use std::convert::TryInto;
3
4/// Implements the hash function from the rustc-hash crate.
5#[derive(Eq, PartialEq)]
6pub struct FxHashFn;
7
8impl 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