city.cc
Go to the documentation of this file.
1 // Copyright 2018 The Abseil Authors.
2 //
3 // Licensed under the Apache License, Version 2.0 (the "License");
4 // you may not use this file except in compliance with the License.
5 // You may obtain a copy of the License at
6 //
7 // https://www.apache.org/licenses/LICENSE-2.0
8 //
9 // Unless required by applicable law or agreed to in writing, software
10 // distributed under the License is distributed on an "AS IS" BASIS,
11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 // See the License for the specific language governing permissions and
13 // limitations under the License.
14 //
15 // This file provides CityHash64() and related functions.
16 //
17 // It's probably possible to create even faster hash functions by
18 // writing a program that systematically explores some of the space of
19 // possible hash functions, by using SIMD instructions, or by
20 // compromising on hash quality.
21 
23 
24 #include <string.h> // for memcpy and memset
25 #include <algorithm>
26 
27 #include "absl/base/config.h"
30 #include "absl/base/optimization.h"
31 
32 namespace absl {
33 namespace hash_internal {
34 
35 #ifdef ABSL_IS_BIG_ENDIAN
36 #define uint32_in_expected_order(x) (absl::gbswap_32(x))
37 #define uint64_in_expected_order(x) (absl::gbswap_64(x))
38 #else
39 #define uint32_in_expected_order(x) (x)
40 #define uint64_in_expected_order(x) (x)
41 #endif
42 
43 static uint64_t Fetch64(const char *p) {
44  return uint64_in_expected_order(ABSL_INTERNAL_UNALIGNED_LOAD64(p));
45 }
46 
47 static uint32_t Fetch32(const char *p) {
48  return uint32_in_expected_order(ABSL_INTERNAL_UNALIGNED_LOAD32(p));
49 }
50 
51 // Some primes between 2^63 and 2^64 for various uses.
52 static const uint64_t k0 = 0xc3a5c85c97cb3127ULL;
53 static const uint64_t k1 = 0xb492b66fbe98f273ULL;
54 static const uint64_t k2 = 0x9ae16a3b2f90404fULL;
55 
56 // Magic numbers for 32-bit hashing. Copied from Murmur3.
57 static const uint32_t c1 = 0xcc9e2d51;
58 static const uint32_t c2 = 0x1b873593;
59 
60 // A 32-bit to 32-bit integer hash copied from Murmur3.
61 static uint32_t fmix(uint32_t h) {
62  h ^= h >> 16;
63  h *= 0x85ebca6b;
64  h ^= h >> 13;
65  h *= 0xc2b2ae35;
66  h ^= h >> 16;
67  return h;
68 }
69 
70 static uint32_t Rotate32(uint32_t val, int shift) {
71  // Avoid shifting by 32: doing so yields an undefined result.
72  return shift == 0 ? val : ((val >> shift) | (val << (32 - shift)));
73 }
74 
75 #undef PERMUTE3
76 #define PERMUTE3(a, b, c) \
77  do { \
78  std::swap(a, b); \
79  std::swap(a, c); \
80  } while (0)
81 
82 static uint32_t Mur(uint32_t a, uint32_t h) {
83  // Helper from Murmur3 for combining two 32-bit values.
84  a *= c1;
85  a = Rotate32(a, 17);
86  a *= c2;
87  h ^= a;
88  h = Rotate32(h, 19);
89  return h * 5 + 0xe6546b64;
90 }
91 
92 static uint32_t Hash32Len13to24(const char *s, size_t len) {
93  uint32_t a = Fetch32(s - 4 + (len >> 1));
94  uint32_t b = Fetch32(s + 4);
95  uint32_t c = Fetch32(s + len - 8);
96  uint32_t d = Fetch32(s + (len >> 1));
97  uint32_t e = Fetch32(s);
98  uint32_t f = Fetch32(s + len - 4);
99  uint32_t h = len;
100 
101  return fmix(Mur(f, Mur(e, Mur(d, Mur(c, Mur(b, Mur(a, h)))))));
102 }
103 
104 static uint32_t Hash32Len0to4(const char *s, size_t len) {
105  uint32_t b = 0;
106  uint32_t c = 9;
107  for (size_t i = 0; i < len; i++) {
108  signed char v = s[i];
109  b = b * c1 + v;
110  c ^= b;
111  }
112  return fmix(Mur(b, Mur(len, c)));
113 }
114 
115 static uint32_t Hash32Len5to12(const char *s, size_t len) {
116  uint32_t a = len, b = len * 5, c = 9, d = b;
117  a += Fetch32(s);
118  b += Fetch32(s + len - 4);
119  c += Fetch32(s + ((len >> 1) & 4));
120  return fmix(Mur(c, Mur(b, Mur(a, d))));
121 }
122 
123 uint32_t CityHash32(const char *s, size_t len) {
124  if (len <= 24) {
125  return len <= 12
126  ? (len <= 4 ? Hash32Len0to4(s, len) : Hash32Len5to12(s, len))
127  : Hash32Len13to24(s, len);
128  }
129 
130  // len > 24
131  uint32_t h = len, g = c1 * len, f = g;
132 
133  uint32_t a0 = Rotate32(Fetch32(s + len - 4) * c1, 17) * c2;
134  uint32_t a1 = Rotate32(Fetch32(s + len - 8) * c1, 17) * c2;
135  uint32_t a2 = Rotate32(Fetch32(s + len - 16) * c1, 17) * c2;
136  uint32_t a3 = Rotate32(Fetch32(s + len - 12) * c1, 17) * c2;
137  uint32_t a4 = Rotate32(Fetch32(s + len - 20) * c1, 17) * c2;
138  h ^= a0;
139  h = Rotate32(h, 19);
140  h = h * 5 + 0xe6546b64;
141  h ^= a2;
142  h = Rotate32(h, 19);
143  h = h * 5 + 0xe6546b64;
144  g ^= a1;
145  g = Rotate32(g, 19);
146  g = g * 5 + 0xe6546b64;
147  g ^= a3;
148  g = Rotate32(g, 19);
149  g = g * 5 + 0xe6546b64;
150  f += a4;
151  f = Rotate32(f, 19);
152  f = f * 5 + 0xe6546b64;
153  size_t iters = (len - 1) / 20;
154  do {
155  uint32_t b0 = Rotate32(Fetch32(s) * c1, 17) * c2;
156  uint32_t b1 = Fetch32(s + 4);
157  uint32_t b2 = Rotate32(Fetch32(s + 8) * c1, 17) * c2;
158  uint32_t b3 = Rotate32(Fetch32(s + 12) * c1, 17) * c2;
159  uint32_t b4 = Fetch32(s + 16);
160  h ^= b0;
161  h = Rotate32(h, 18);
162  h = h * 5 + 0xe6546b64;
163  f += b1;
164  f = Rotate32(f, 19);
165  f = f * c1;
166  g += b2;
167  g = Rotate32(g, 18);
168  g = g * 5 + 0xe6546b64;
169  h ^= b3 + b1;
170  h = Rotate32(h, 19);
171  h = h * 5 + 0xe6546b64;
172  g ^= b4;
173  g = absl::gbswap_32(g) * 5;
174  h += b4 * 5;
175  h = absl::gbswap_32(h);
176  f += b0;
177  PERMUTE3(f, h, g);
178  s += 20;
179  } while (--iters != 0);
180  g = Rotate32(g, 11) * c1;
181  g = Rotate32(g, 17) * c1;
182  f = Rotate32(f, 11) * c1;
183  f = Rotate32(f, 17) * c1;
184  h = Rotate32(h + g, 19);
185  h = h * 5 + 0xe6546b64;
186  h = Rotate32(h, 17) * c1;
187  h = Rotate32(h + f, 19);
188  h = h * 5 + 0xe6546b64;
189  h = Rotate32(h, 17) * c1;
190  return h;
191 }
192 
193 // Bitwise right rotate. Normally this will compile to a single
194 // instruction, especially if the shift is a manifest constant.
195 static uint64_t Rotate(uint64_t val, int shift) {
196  // Avoid shifting by 64: doing so yields an undefined result.
197  return shift == 0 ? val : ((val >> shift) | (val << (64 - shift)));
198 }
199 
200 static uint64_t ShiftMix(uint64_t val) { return val ^ (val >> 47); }
201 
202 static uint64_t HashLen16(uint64_t u, uint64_t v) {
203  return Hash128to64(uint128(u, v));
204 }
205 
206 static uint64_t HashLen16(uint64_t u, uint64_t v, uint64_t mul) {
207  // Murmur-inspired hashing.
208  uint64_t a = (u ^ v) * mul;
209  a ^= (a >> 47);
210  uint64_t b = (v ^ a) * mul;
211  b ^= (b >> 47);
212  b *= mul;
213  return b;
214 }
215 
216 static uint64_t HashLen0to16(const char *s, size_t len) {
217  if (len >= 8) {
218  uint64_t mul = k2 + len * 2;
219  uint64_t a = Fetch64(s) + k2;
220  uint64_t b = Fetch64(s + len - 8);
221  uint64_t c = Rotate(b, 37) * mul + a;
222  uint64_t d = (Rotate(a, 25) + b) * mul;
223  return HashLen16(c, d, mul);
224  }
225  if (len >= 4) {
226  uint64_t mul = k2 + len * 2;
227  uint64_t a = Fetch32(s);
228  return HashLen16(len + (a << 3), Fetch32(s + len - 4), mul);
229  }
230  if (len > 0) {
231  uint8_t a = s[0];
232  uint8_t b = s[len >> 1];
233  uint8_t c = s[len - 1];
234  uint32_t y = static_cast<uint32_t>(a) + (static_cast<uint32_t>(b) << 8);
235  uint32_t z = len + (static_cast<uint32_t>(c) << 2);
236  return ShiftMix(y * k2 ^ z * k0) * k2;
237  }
238  return k2;
239 }
240 
241 // This probably works well for 16-byte strings as well, but it may be overkill
242 // in that case.
243 static uint64_t HashLen17to32(const char *s, size_t len) {
244  uint64_t mul = k2 + len * 2;
245  uint64_t a = Fetch64(s) * k1;
246  uint64_t b = Fetch64(s + 8);
247  uint64_t c = Fetch64(s + len - 8) * mul;
248  uint64_t d = Fetch64(s + len - 16) * k2;
249  return HashLen16(Rotate(a + b, 43) + Rotate(c, 30) + d,
250  a + Rotate(b + k2, 18) + c, mul);
251 }
252 
253 // Return a 16-byte hash for 48 bytes. Quick and dirty.
254 // Callers do best to use "random-looking" values for a and b.
255 static std::pair<uint64_t, uint64_t> WeakHashLen32WithSeeds(uint64_t w, uint64_t x,
256  uint64_t y, uint64_t z,
257  uint64_t a, uint64_t b) {
258  a += w;
259  b = Rotate(b + a + z, 21);
260  uint64_t c = a;
261  a += x;
262  a += y;
263  b += Rotate(a, 44);
264  return std::make_pair(a + z, b + c);
265 }
266 
267 // Return a 16-byte hash for s[0] ... s[31], a, and b. Quick and dirty.
268 static std::pair<uint64_t, uint64_t> WeakHashLen32WithSeeds(const char *s, uint64_t a,
269  uint64_t b) {
270  return WeakHashLen32WithSeeds(Fetch64(s), Fetch64(s + 8), Fetch64(s + 16),
271  Fetch64(s + 24), a, b);
272 }
273 
274 // Return an 8-byte hash for 33 to 64 bytes.
275 static uint64_t HashLen33to64(const char *s, size_t len) {
276  uint64_t mul = k2 + len * 2;
277  uint64_t a = Fetch64(s) * k2;
278  uint64_t b = Fetch64(s + 8);
279  uint64_t c = Fetch64(s + len - 24);
280  uint64_t d = Fetch64(s + len - 32);
281  uint64_t e = Fetch64(s + 16) * k2;
282  uint64_t f = Fetch64(s + 24) * 9;
283  uint64_t g = Fetch64(s + len - 8);
284  uint64_t h = Fetch64(s + len - 16) * mul;
285  uint64_t u = Rotate(a + g, 43) + (Rotate(b, 30) + c) * 9;
286  uint64_t v = ((a + g) ^ d) + f + 1;
287  uint64_t w = absl::gbswap_64((u + v) * mul) + h;
288  uint64_t x = Rotate(e + f, 42) + c;
289  uint64_t y = (absl::gbswap_64((v + w) * mul) + g) * mul;
290  uint64_t z = e + f + c;
291  a = absl::gbswap_64((x + z) * mul + y) + b;
292  b = ShiftMix((z + a) * mul + d + h) * mul;
293  return b + x;
294 }
295 
296 uint64_t CityHash64(const char *s, size_t len) {
297  if (len <= 32) {
298  if (len <= 16) {
299  return HashLen0to16(s, len);
300  } else {
301  return HashLen17to32(s, len);
302  }
303  } else if (len <= 64) {
304  return HashLen33to64(s, len);
305  }
306 
307  // For strings over 64 bytes we hash the end first, and then as we
308  // loop we keep 56 bytes of state: v, w, x, y, and z.
309  uint64_t x = Fetch64(s + len - 40);
310  uint64_t y = Fetch64(s + len - 16) + Fetch64(s + len - 56);
311  uint64_t z = HashLen16(Fetch64(s + len - 48) + len, Fetch64(s + len - 24));
312  std::pair<uint64_t, uint64_t> v = WeakHashLen32WithSeeds(s + len - 64, len, z);
313  std::pair<uint64_t, uint64_t> w = WeakHashLen32WithSeeds(s + len - 32, y + k1, x);
314  x = x * k1 + Fetch64(s);
315 
316  // Decrease len to the nearest multiple of 64, and operate on 64-byte chunks.
317  len = (len - 1) & ~static_cast<size_t>(63);
318  do {
319  x = Rotate(x + y + v.first + Fetch64(s + 8), 37) * k1;
320  y = Rotate(y + v.second + Fetch64(s + 48), 42) * k1;
321  x ^= w.second;
322  y += v.first + Fetch64(s + 40);
323  z = Rotate(z + w.first, 33) * k1;
324  v = WeakHashLen32WithSeeds(s, v.second * k1, x + w.first);
325  w = WeakHashLen32WithSeeds(s + 32, z + w.second, y + Fetch64(s + 16));
326  std::swap(z, x);
327  s += 64;
328  len -= 64;
329  } while (len != 0);
330  return HashLen16(HashLen16(v.first, w.first) + ShiftMix(y) * k1 + z,
331  HashLen16(v.second, w.second) + x);
332 }
333 
334 uint64_t CityHash64WithSeed(const char *s, size_t len, uint64_t seed) {
335  return CityHash64WithSeeds(s, len, k2, seed);
336 }
337 
338 uint64_t CityHash64WithSeeds(const char *s, size_t len, uint64_t seed0,
339  uint64_t seed1) {
340  return HashLen16(CityHash64(s, len) - seed0, seed1);
341 }
342 
343 } // namespace hash_internal
344 } // namespace absl
int v
Definition: variant_test.cc:81
static uint32_t fmix(uint32_t h)
Definition: city.cc:61
uint64_t CityHash64WithSeed(const char *s, size_t len, uint64_t seed)
Definition: city.cc:334
uint32_t CityHash32(const char *s, size_t len)
Definition: city.cc:123
static uint64_t Fetch64(const char *p)
Definition: city.cc:43
static const uint64_t k1
Definition: city.cc:53
static uint64_t HashLen16(uint64_t u, uint64_t v)
Definition: city.cc:202
static const uint64_t k0
Definition: city.cc:52
uint64_t gbswap_64(uint64_t host_int)
Definition: endian.h:72
#define PERMUTE3(a, b, c)
Definition: city.cc:76
static uint32_t Hash32Len5to12(const char *s, size_t len)
Definition: city.cc:115
static uint32_t Hash32Len0to4(const char *s, size_t len)
Definition: city.cc:104
T::first_type a1
std::pair< uint64_t, uint64_t > uint128
Definition: city.h:55
Definition: algorithm.h:29
#define uint64_in_expected_order(x)
Definition: city.cc:40
static uint32_t Hash32Len13to24(const char *s, size_t len)
Definition: city.cc:92
uint64_t CityHash64(const char *s, size_t len)
Definition: city.cc:296
T::second_type b2
uint32_t gbswap_32(uint32_t host_int)
Definition: endian.h:96
T::second_type b1
uint64_t CityHash64WithSeeds(const char *s, size_t len, uint64_t seed0, uint64_t seed1)
Definition: city.cc:338
static std::pair< uint64_t, uint64_t > WeakHashLen32WithSeeds(uint64_t w, uint64_t x, uint64_t y, uint64_t z, uint64_t a, uint64_t b)
Definition: city.cc:255
static const uint32_t c1
Definition: city.cc:57
static uint64_t Rotate(uint64_t val, int shift)
Definition: city.cc:195
static uint32_t Mur(uint32_t a, uint32_t h)
Definition: city.cc:82
static uint64_t HashLen33to64(const char *s, size_t len)
Definition: city.cc:275
void swap(absl::InlinedVector< T, N, A > &a, absl::InlinedVector< T, N, A > &b) noexcept(noexcept(a.swap(b)))
T::first_type a2
static const uint32_t c2
Definition: city.cc:58
static uint32_t Fetch32(const char *p)
Definition: city.cc:47
uint64_t Hash128to64(const uint128 &x)
Definition: city.h:77
#define uint32_in_expected_order(x)
Definition: city.cc:39
static uint64_t ShiftMix(uint64_t val)
Definition: city.cc:200
static const uint64_t k2
Definition: city.cc:54
uint64_t b
Definition: layout_test.cc:50
static uint64_t HashLen17to32(const char *s, size_t len)
Definition: city.cc:243
static uint64_t HashLen0to16(const char *s, size_t len)
Definition: city.cc:216
static uint32_t Rotate32(uint32_t val, int shift)
Definition: city.cc:70


abseil_cpp
Author(s):
autogenerated on Mon Feb 28 2022 21:31:17