1//! The foldhash implementation optimized for speed.
2
3use core::hash::{BuildHasher, Hasher};
4
5use crate::seed::{gen_per_hasher_seed, GlobalSeed, SharedSeed};
6use crate::{folded_multiply, hash_bytes_long, hash_bytes_medium, rotate_right, ARBITRARY3};
7
8/// A [`Hasher`] instance implementing foldhash, optimized for speed.
9///
10/// While you can create one directly with [`FoldHasher::with_seed`], you
11/// most likely want to use [`RandomState`], [`SeedableRandomState`] or
12/// [`FixedState`] to create [`FoldHasher`]s.
13#[derive(Clone)]
14pub struct FoldHasher {
15 accumulator: u64,
16 sponge: u128,
17 sponge_len: u8,
18 fold_seed: u64,
19 expand_seed: u64,
20 expand_seed2: u64,
21 expand_seed3: u64,
22}
23
24impl FoldHasher {
25 /// Initializes this [`FoldHasher`] with the given per-hasher seed and
26 /// [`SharedSeed`].
27 #[inline]
28 pub fn with_seed(per_hasher_seed: u64, shared_seed: &SharedSeed) -> FoldHasher {
29 FoldHasher {
30 accumulator: per_hasher_seed,
31 sponge: 0,
32 sponge_len: 0,
33 fold_seed: shared_seed.seeds[0],
34 expand_seed: shared_seed.seeds[1],
35 expand_seed2: shared_seed.seeds[2],
36 expand_seed3: shared_seed.seeds[3],
37 }
38 }
39
40 #[inline(always)]
41 fn write_num<T: Into<u128>>(&mut self, x: T) {
42 let bits: usize = 8 * core::mem::size_of::<T>();
43 if self.sponge_len as usize + bits > 128 {
44 let lo = self.sponge as u64;
45 let hi = (self.sponge >> 64) as u64;
46 self.accumulator = folded_multiply(lo ^ self.accumulator, hi ^ self.fold_seed);
47 self.sponge = x.into();
48 self.sponge_len = bits as u8;
49 } else {
50 self.sponge |= x.into() << self.sponge_len;
51 self.sponge_len += bits as u8;
52 }
53 }
54}
55
56impl Hasher for FoldHasher {
57 #[inline(always)]
58 fn write(&mut self, bytes: &[u8]) {
59 // We perform overlapping reads in the byte hash which could lead to
60 // trivial length-extension attacks. These should be defeated by
61 // adding a length-dependent rotation on our unpredictable seed
62 // which costs only a single cycle (or none if executed with
63 // instruction-level parallelism).
64 let len = bytes.len();
65 let base_seed = rotate_right(self.accumulator, len as u32);
66 if len <= 16 {
67 let mut s0 = base_seed;
68 let mut s1 = self.expand_seed;
69 // XOR the input into s0, s1, then multiply and fold.
70 if len >= 8 {
71 s0 ^= u64::from_ne_bytes(bytes[0..8].try_into().unwrap());
72 s1 ^= u64::from_ne_bytes(bytes[len - 8..].try_into().unwrap());
73 } else if len >= 4 {
74 s0 ^= u32::from_ne_bytes(bytes[0..4].try_into().unwrap()) as u64;
75 s1 ^= u32::from_ne_bytes(bytes[len - 4..].try_into().unwrap()) as u64;
76 } else if len > 0 {
77 let lo = bytes[0];
78 let mid = bytes[len / 2];
79 let hi = bytes[len - 1];
80 s0 ^= lo as u64;
81 s1 ^= ((hi as u64) << 8) | mid as u64;
82 }
83 self.accumulator = folded_multiply(s0, s1);
84 } else if len < 256 {
85 self.accumulator = hash_bytes_medium(
86 bytes,
87 base_seed,
88 base_seed.wrapping_add(self.expand_seed),
89 self.fold_seed,
90 );
91 } else {
92 self.accumulator = hash_bytes_long(
93 bytes,
94 base_seed,
95 base_seed.wrapping_add(self.expand_seed),
96 base_seed.wrapping_add(self.expand_seed2),
97 base_seed.wrapping_add(self.expand_seed3),
98 self.fold_seed,
99 );
100 }
101 }
102
103 #[inline(always)]
104 fn write_u8(&mut self, i: u8) {
105 self.write_num(i);
106 }
107
108 #[inline(always)]
109 fn write_u16(&mut self, i: u16) {
110 self.write_num(i);
111 }
112
113 #[inline(always)]
114 fn write_u32(&mut self, i: u32) {
115 self.write_num(i);
116 }
117
118 #[inline(always)]
119 fn write_u64(&mut self, i: u64) {
120 self.write_num(i);
121 }
122
123 #[inline(always)]
124 fn write_u128(&mut self, i: u128) {
125 let lo = i as u64;
126 let hi = (i >> 64) as u64;
127 self.accumulator = folded_multiply(lo ^ self.accumulator, hi ^ self.fold_seed);
128 }
129
130 #[inline(always)]
131 fn write_usize(&mut self, i: usize) {
132 // u128 doesn't implement From<usize>.
133 #[cfg(target_pointer_width = "32")]
134 self.write_num(i as u32);
135 #[cfg(target_pointer_width = "64")]
136 self.write_num(i as u64);
137 }
138
139 #[inline(always)]
140 fn finish(&self) -> u64 {
141 if self.sponge_len > 0 {
142 let lo = self.sponge as u64;
143 let hi = (self.sponge >> 64) as u64;
144 folded_multiply(lo ^ self.accumulator, hi ^ self.fold_seed)
145 } else {
146 self.accumulator
147 }
148 }
149}
150
151/// A [`BuildHasher`] for [`fast::FoldHasher`](FoldHasher) that is randomly initialized.
152#[derive(Copy, Clone, Debug)]
153pub struct RandomState {
154 per_hasher_seed: u64,
155 global_seed: GlobalSeed,
156}
157
158impl Default for RandomState {
159 #[inline(always)]
160 fn default() -> Self {
161 Self {
162 per_hasher_seed: gen_per_hasher_seed(),
163 global_seed: GlobalSeed::new(),
164 }
165 }
166}
167
168impl BuildHasher for RandomState {
169 type Hasher = FoldHasher;
170
171 #[inline(always)]
172 fn build_hasher(&self) -> FoldHasher {
173 FoldHasher::with_seed(self.per_hasher_seed, self.global_seed.get())
174 }
175}
176
177/// A [`BuildHasher`] for [`fast::FoldHasher`](FoldHasher) that is randomly
178/// initialized by default, but can also be initialized with a specific seed.
179///
180/// This can be useful for e.g. testing, but the downside is that this type
181/// has a size of 16 bytes rather than the 8 bytes [`RandomState`] is.
182#[derive(Copy, Clone, Debug)]
183pub struct SeedableRandomState {
184 per_hasher_seed: u64,
185 shared_seed: &'static SharedSeed,
186}
187
188impl Default for SeedableRandomState {
189 #[inline(always)]
190 fn default() -> Self {
191 Self::random()
192 }
193}
194
195impl SeedableRandomState {
196 /// Generates a random [`SeedableRandomState`], similar to [`RandomState`].
197 #[inline(always)]
198 pub fn random() -> Self {
199 Self {
200 per_hasher_seed: gen_per_hasher_seed(),
201 shared_seed: SharedSeed::global_random(),
202 }
203 }
204
205 /// Generates a fixed [`SeedableRandomState`], similar to [`FixedState`].
206 #[inline(always)]
207 pub fn fixed() -> Self {
208 Self {
209 per_hasher_seed: ARBITRARY3,
210 shared_seed: SharedSeed::global_fixed(),
211 }
212 }
213
214 /// Generates a [`SeedableRandomState`] with the given per-hasher seed
215 /// and [`SharedSeed`].
216 #[inline(always)]
217 pub fn with_seed(per_hasher_seed: u64, shared_seed: &'static SharedSeed) -> Self {
218 // XOR with ARBITRARY3 such that with_seed(0) matches default.
219 Self {
220 per_hasher_seed: per_hasher_seed ^ ARBITRARY3,
221 shared_seed,
222 }
223 }
224}
225
226impl BuildHasher for SeedableRandomState {
227 type Hasher = FoldHasher;
228
229 #[inline(always)]
230 fn build_hasher(&self) -> FoldHasher {
231 FoldHasher::with_seed(self.per_hasher_seed, self.shared_seed)
232 }
233}
234
235/// A [`BuildHasher`] for [`fast::FoldHasher`](FoldHasher) that always has the same fixed seed.
236///
237/// Not recommended unless you absolutely need determinism.
238#[derive(Copy, Clone, Debug)]
239pub struct FixedState {
240 per_hasher_seed: u64,
241}
242
243impl FixedState {
244 /// Creates a [`FixedState`] with the given per-hasher-seed.
245 #[inline(always)]
246 pub const fn with_seed(per_hasher_seed: u64) -> Self {
247 // XOR with ARBITRARY3 such that with_seed(0) matches default.
248 Self {
249 per_hasher_seed: per_hasher_seed ^ ARBITRARY3,
250 }
251 }
252}
253
254impl Default for FixedState {
255 #[inline(always)]
256 fn default() -> Self {
257 Self {
258 per_hasher_seed: ARBITRARY3,
259 }
260 }
261}
262
263impl BuildHasher for FixedState {
264 type Hasher = FoldHasher;
265
266 #[inline(always)]
267 fn build_hasher(&self) -> FoldHasher {
268 FoldHasher::with_seed(self.per_hasher_seed, SharedSeed::global_fixed())
269 }
270}
271