2 * Copyright 2012 Facebook, Inc.
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
8 * http://www.apache.org/licenses/LICENSE-2.0
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
17 #ifndef FOLLY_BASE_HASH_H_
18 #define FOLLY_BASE_HASH_H_
26 * Various hashing functions.
29 namespace folly { namespace hash {
31 // This is a general-purpose way to create a single hash from multiple
32 // hashable objects. It relies on std::hash<T> being available for all
33 // relevant types and combines those hashes in an order-dependent way
34 // to yield a new hash.
36 // Never used, but gcc demands it.
37 inline size_t hash_combine() {
41 // This is the Hash128to64 function from Google's cityhash (available
42 // under the MIT License). We use it to reduce multiple 64 bit hashes
43 // into a single hash.
44 inline size_t hash_128_to_64(const size_t upper, const size_t lower) {
45 // Murmur-inspired hashing.
46 const size_t kMul = 0x9ddfea08eb382d69ULL;
47 size_t a = (lower ^ upper) * kMul;
49 size_t b = (upper ^ a) * kMul;
55 template <typename T, typename... Ts>
56 size_t hash_combine(const T& t, const Ts&... ts) {
57 size_t seed = std::hash<T>()(t);
58 if (sizeof...(ts) == 0) {
61 size_t remainder = hash_combine(ts...);
62 return hash_128_to_64(seed, remainder);
65 //////////////////////////////////////////////////////////////////////
68 * Thomas Wang 64 bit mix hash function
71 inline uint64_t twang_mix64(uint64_t key) {
72 key = (~key) + (key << 21); // key *= (1 << 21) - 1; key -= 1;
73 key = key ^ (key >> 24);
74 key = key + (key << 3) + (key << 8); // key *= 1 + (1 << 3) + (1 << 8)
75 key = key ^ (key >> 14);
76 key = key + (key << 2) + (key << 4); // key *= 1 + (1 << 2) + (1 << 4)
77 key = key ^ (key >> 28);
78 key = key + (key << 31); // key *= 1 + (1 << 31)
83 * Inverse of twang_mix64
85 * Note that twang_unmix64 is significantly slower than twang_mix64.
88 inline uint64_t twang_unmix64(uint64_t key) {
89 // See the comments in jenkins_rev_unmix32 for an explanation as to how this
91 key *= 4611686016279904257U;
92 key ^= (key >> 28) ^ (key >> 56);
93 key *= 14933078535860113213U;
94 key ^= (key >> 14) ^ (key >> 28) ^ (key >> 42) ^ (key >> 56);
95 key *= 15244667743933553977U;
96 key ^= (key >> 24) ^ (key >> 48);
97 key = (key + 1) * 9223367638806167551U;
102 * Thomas Wang downscaling hash function
105 inline uint32_t twang_32from64(uint64_t key) {
106 key = (~key) + (key << 18);
107 key = key ^ (key >> 31);
109 key = key ^ (key >> 11);
110 key = key + (key << 6);
111 key = key ^ (key >> 22);
112 return (uint32_t) key;
116 * Robert Jenkins' reversible 32 bit mix hash function
119 inline uint32_t jenkins_rev_mix32(uint32_t key) {
120 key += (key << 12); // key *= (1 + (1 << 12))
122 key += (key << 4); // key *= (1 + (1 << 4))
124 key += (key << 10); // key *= (1 + (1 << 10))
126 // key *= (1 + (1 << 7)) * (1 + (1 << 12))
133 * Inverse of jenkins_rev_mix32
135 * Note that jenkinks_rev_unmix32 is significantly slower than
139 inline uint32_t jenkins_rev_unmix32(uint32_t key) {
140 // These are the modular multiplicative inverses (in Z_2^32) of the
141 // multiplication factors in jenkins_rev_mix32, in reverse order. They were
142 // computed using the Extended Euclidean algorithm, see
143 // http://en.wikipedia.org/wiki/Modular_multiplicative_inverse
146 // The inverse of a ^= (a >> n) is
148 // for (int i = n; i < 32; i += n) {
152 (key >> 2) ^ (key >> 4) ^ (key >> 6) ^ (key >> 8) ^
153 (key >> 10) ^ (key >> 12) ^ (key >> 14) ^ (key >> 16) ^
154 (key >> 18) ^ (key >> 20) ^ (key >> 22) ^ (key >> 24) ^
155 (key >> 26) ^ (key >> 28) ^ (key >> 30);
157 key ^= (key >> 9) ^ (key >> 18) ^ (key >> 27);
165 * Fowler / Noll / Vo (FNV) Hash
166 * http://www.isthe.com/chongo/tech/comp/fnv/
169 const uint32_t FNV_32_HASH_START = 216613626UL;
170 const uint64_t FNV_64_HASH_START = 14695981039346656037ULL;
172 inline uint32_t fnv32(const char* s,
173 uint32_t hash = FNV_32_HASH_START) {
175 hash += (hash << 1) + (hash << 4) + (hash << 7) +
176 (hash << 8) + (hash << 24);
182 inline uint32_t fnv32_buf(const void* buf,
184 uint32_t hash = FNV_32_HASH_START) {
185 const char* char_buf = reinterpret_cast<const char*>(buf);
187 for (int i = 0; i < n; ++i) {
188 hash += (hash << 1) + (hash << 4) + (hash << 7) +
189 (hash << 8) + (hash << 24);
196 inline uint32_t fnv32(const std::string& str,
197 uint64_t hash = FNV_32_HASH_START) {
198 return fnv32_buf(str.data(), str.size(), hash);
201 inline uint64_t fnv64(const char* s,
202 uint64_t hash = FNV_64_HASH_START) {
204 hash += (hash << 1) + (hash << 4) + (hash << 5) + (hash << 7) +
205 (hash << 8) + (hash << 40);
211 inline uint64_t fnv64_buf(const void* buf,
213 uint64_t hash = FNV_64_HASH_START) {
214 const char* char_buf = reinterpret_cast<const char*>(buf);
216 for (int i = 0; i < n; ++i) {
217 hash += (hash << 1) + (hash << 4) + (hash << 5) + (hash << 7) +
218 (hash << 8) + (hash << 40);
224 inline uint64_t fnv64(const std::string& str,
225 uint64_t hash = FNV_64_HASH_START) {
226 return fnv64_buf(str.data(), str.size(), hash);
230 * Paul Hsieh: http://www.azillionmonkeys.com/qed/hash.html
233 #define get16bits(d) (*((const uint16_t*) (d)))
235 inline uint32_t hsieh_hash32_buf(const void* buf, int len) {
236 const char* s = reinterpret_cast<const char*>(buf);
241 if (len <= 0 || buf == 0) {
249 for (;len > 0; len--) {
250 hash += get16bits (s);
251 tmp = (get16bits (s+2) << 11) ^ hash;
252 hash = (hash << 16) ^ tmp;
253 s += 2*sizeof (uint16_t);
257 /* Handle end cases */
260 hash += get16bits(s);
262 hash ^= s[sizeof (uint16_t)] << 18;
266 hash += get16bits(s);
276 /* Force "avalanching" of final 127 bits */
289 inline uint32_t hsieh_hash32(const char* s) {
290 return hsieh_hash32_buf(s, std::strlen(s));
293 inline uint32_t hsieh_hash32_str(const std::string& str) {
294 return hsieh_hash32_buf(str.data(), str.size());
297 //////////////////////////////////////////////////////////////////////
304 template<> struct hasher<int32_t> {
305 size_t operator()(int32_t key) const {
306 return hash::jenkins_rev_mix32(uint32_t(key));
310 template<> struct hasher<uint32_t> {
311 size_t operator()(uint32_t key) const {
312 return hash::jenkins_rev_mix32(key);
316 template<> struct hasher<int64_t> {
317 size_t operator()(int64_t key) const {
318 return hash::twang_mix64(uint64_t(key));
322 template<> struct hasher<uint64_t> {
323 size_t operator()(uint64_t key) const {
324 return hash::twang_mix64(key);
330 // Custom hash functions.
332 // Hash function for pairs. Requires default hash functions for both
333 // items in the pair.
334 template <typename T1, typename T2>
335 class hash<std::pair<T1, T2> > {
337 size_t operator()(const std::pair<T1, T2>& x) const {
338 return folly::hash::hash_combine(x.first, x.second);