1//! This is a copy of the `rustc_hash` crate, adapted to work as a module.
2//!
3//! If in the future it becomes more reasonable to add dependencies to
4//! `proc_macro`, this module should be removed and replaced with a dependency
5//! on the `rustc_hash` crate.
6
7use std::collections::HashMap;
8use std::hash::{BuildHasherDefault, Hasher};
9use std::ops::BitXor;
10
11/// Type alias for a hashmap using the `fx` hash algorithm.
12pub(super) type FxHashMap<K, V> = HashMap<K, V, BuildHasherDefault<FxHasher>>;
13
14/// A speedy hash algorithm for use within rustc. The hashmap in alloc by
15/// default uses SipHash which isn't quite as speedy as we want. In the compiler
16/// we're not really worried about DOS attempts, so we use a fast
17/// non-cryptographic hash.
18///
19/// This is the same as the algorithm used by Firefox -- which is a homespun
20/// one not based on any widely-known algorithm -- though modified to produce
21/// 64-bit hash values instead of 32-bit hash values. It consistently
22/// out-performs an FNV-based hash within rustc itself -- the collision rate is
23/// similar or slightly worse than FNV, but the speed of the hash function
24/// itself is much higher because it works on up to 8 bytes at a time.
25#[derive(Default)]
26pub(super) struct FxHasher {
27 hash: usize,
28}
29
30#[cfg(target_pointer_width = "32")]
31const K: usize = 0x9e3779b9;
32#[cfg(target_pointer_width = "64")]
33const K: usize = 0x517cc1b727220a95;
34
35impl FxHasher {
36 #[inline]
37 fn add_to_hash(&mut self, i: usize) {
38 self.hash = self.hash.rotate_left(5).bitxor(i).wrapping_mul(K);
39 }
40}
41
42impl Hasher for FxHasher {
43 #[inline]
44 fn write(&mut self, mut bytes: &[u8]) {
45 #[cfg(target_pointer_width = "32")]
46 let read_usize = |bytes: &[u8]| u32::from_ne_bytes(bytes[..4].try_into().unwrap());
47 #[cfg(target_pointer_width = "64")]
48 let read_usize = |bytes: &[u8]| u64::from_ne_bytes(bytes[..8].try_into().unwrap());
49
50 let mut hash = FxHasher { hash: self.hash };
51 assert!(size_of::<usize>() <= 8);
52 while bytes.len() >= size_of::<usize>() {
53 hash.add_to_hash(read_usize(bytes) as usize);
54 bytes = &bytes[size_of::<usize>()..];
55 }
56 if (size_of::<usize>() > 4) && (bytes.len() >= 4) {
57 hash.add_to_hash(u32::from_ne_bytes(bytes[..4].try_into().unwrap()) as usize);
58 bytes = &bytes[4..];
59 }
60 if (size_of::<usize>() > 2) && bytes.len() >= 2 {
61 hash.add_to_hash(u16::from_ne_bytes(bytes[..2].try_into().unwrap()) as usize);
62 bytes = &bytes[2..];
63 }
64 if (size_of::<usize>() > 1) && !bytes.is_empty() {
65 hash.add_to_hash(bytes[0] as usize);
66 }
67 self.hash = hash.hash;
68 }
69
70 #[inline]
71 fn write_u8(&mut self, i: u8) {
72 self.add_to_hash(i as usize);
73 }
74
75 #[inline]
76 fn write_u16(&mut self, i: u16) {
77 self.add_to_hash(i as usize);
78 }
79
80 #[inline]
81 fn write_u32(&mut self, i: u32) {
82 self.add_to_hash(i as usize);
83 }
84
85 #[cfg(target_pointer_width = "32")]
86 #[inline]
87 fn write_u64(&mut self, i: u64) {
88 self.add_to_hash(i as usize);
89 self.add_to_hash((i >> 32) as usize);
90 }
91
92 #[cfg(target_pointer_width = "64")]
93 #[inline]
94 fn write_u64(&mut self, i: u64) {
95 self.add_to_hash(i as usize);
96 }
97
98 #[inline]
99 fn write_usize(&mut self, i: usize) {
100 self.add_to_hash(i);
101 }
102
103 #[inline]
104 fn finish(&self) -> u64 {
105 self.hash as u64
106 }
107}
108