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use crate::HashFn;
use std::convert::TryInto;
/// Implements the hash function from the rustc-hash crate.
#[derive(Eq, PartialEq)]
pub struct FxHashFn;
impl HashFn for FxHashFn {
// This function is marked as #[inline] because that allows LLVM to know the
// actual size of `bytes` and thus eliminate all unneeded branches below.
#[inline]
fn hash(mut bytes: &[u8]) -> u32 {
let mut hash_value = 0;
while bytes.len() >= 8 {
hash_value = add_to_hash(hash_value, read_u64(bytes));
bytes = &bytes[8..];
}
if bytes.len() >= 4 {
hash_value = add_to_hash(
hash_value,
u32::from_le_bytes(bytes[..4].try_into().unwrap()) as u64,
);
bytes = &bytes[4..];
}
if bytes.len() >= 2 {
hash_value = add_to_hash(
hash_value,
u16::from_le_bytes(bytes[..2].try_into().unwrap()) as u64,
);
bytes = &bytes[2..];
}
if bytes.len() >= 1 {
hash_value = add_to_hash(hash_value, bytes[0] as u64);
}
return hash_value as u32;
#[inline]
fn add_to_hash(current_hash: u64, value: u64) -> u64 {
use std::ops::BitXor;
current_hash
.rotate_left(5)
.bitxor(value)
// This constant is part of FxHash's definition:
// https://github.com/rust-lang/rustc-hash/blob/5e09ea0a1/src/lib.rs#L67
.wrapping_mul(0x517cc1b727220a95)
}
#[inline]
fn read_u64(bytes: &[u8]) -> u64 {
u64::from_le_bytes(bytes[..8].try_into().unwrap())
}
}
}
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