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