673989df4ca42f14802944d39792cad98aa54368
[urcu.git] / doc / examples / rculfhash / jhash.h
1 #ifndef _JHASH_H
2 #define _JHASH_H
3
4 /*
5 * jhash.h
6 *
7 * Example hash function.
8 *
9 * Copyright 2009-2012 - Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
10 *
11 * THIS MATERIAL IS PROVIDED AS IS, WITH ABSOLUTELY NO WARRANTY EXPRESSED
12 * OR IMPLIED. ANY USE IS AT YOUR OWN RISK.
13 *
14 * Permission is hereby granted to use or copy this program for any
15 * purpose, provided the above notices are retained on all copies.
16 * Permission to modify the code and to distribute modified code is
17 * granted, provided the above notices are retained, and a notice that
18 * the code was modified is included with the above copyright notice.
19 */
20
21 /*
22 * Hash function
23 * Source: http://burtleburtle.net/bob/c/lookup3.c
24 * Originally Public Domain
25 */
26
27 #define rot(x, k) (((x) << (k)) | ((x) >> (32 - (k))))
28
29 #define mix(a, b, c) \
30 do { \
31 a -= c; a ^= rot(c, 4); c += b; \
32 b -= a; b ^= rot(a, 6); a += c; \
33 c -= b; c ^= rot(b, 8); b += a; \
34 a -= c; a ^= rot(c, 16); c += b; \
35 b -= a; b ^= rot(a, 19); a += c; \
36 c -= b; c ^= rot(b, 4); b += a; \
37 } while (0)
38
39 #define final(a, b, c) \
40 { \
41 c ^= b; c -= rot(b, 14); \
42 a ^= c; a -= rot(c, 11); \
43 b ^= a; b -= rot(a, 25); \
44 c ^= b; c -= rot(b, 16); \
45 a ^= c; a -= rot(c, 4); \
46 b ^= a; b -= rot(a, 14); \
47 c ^= b; c -= rot(b, 24); \
48 }
49
50 #if (BYTE_ORDER == LITTLE_ENDIAN)
51 #define HASH_LITTLE_ENDIAN 1
52 #else
53 #define HASH_LITTLE_ENDIAN 0
54 #endif
55
56 /*
57 *
58 * hashlittle() -- hash a variable-length key into a 32-bit value
59 * k : the key (the unaligned variable-length array of bytes)
60 * length : the length of the key, counting by bytes
61 * initval : can be any 4-byte value
62 * Returns a 32-bit value. Every bit of the key affects every bit of
63 * the return value. Two keys differing by one or two bits will have
64 * totally different hash values.
65 *
66 * The best hash table sizes are powers of 2. There is no need to do
67 * mod a prime (mod is sooo slow!). If you need less than 32 bits,
68 * use a bitmask. For example, if you need only 10 bits, do
69 * h = (h & hashmask(10));
70 * In which case, the hash table should have hashsize(10) elements.
71 *
72 * If you are hashing n strings (uint8_t **)k, do it like this:
73 * for (i = 0, h = 0; i < n; ++i) h = hashlittle(k[i], len[i], h);
74 *
75 * By Bob Jenkins, 2006. bob_jenkins@burtleburtle.net. You may use this
76 * code any way you wish, private, educational, or commercial. It's free.
77 *
78 * Use for hash table lookup, or anything where one collision in 2^^32 is
79 * acceptable. Do NOT use for cryptographic purposes.
80 */
81 static
82 uint32_t hashlittle(const void *key, size_t length, uint32_t initval)
83 {
84 uint32_t a, b, c; /* internal state */
85 union {
86 const void *ptr;
87 size_t i;
88 } u;
89
90 /* Set up the internal state */
91 a = b = c = 0xdeadbeef + ((uint32_t)length) + initval;
92
93 u.ptr = key;
94 if (HASH_LITTLE_ENDIAN && ((u.i & 0x3) == 0)) {
95 const uint32_t *k = (const uint32_t *) key; /* read 32-bit chunks */
96
97 /*------ all but last block: aligned reads and affect 32 bits of (a,b,c) */
98 while (length > 12) {
99 a += k[0];
100 b += k[1];
101 c += k[2];
102 mix(a, b, c);
103 length -= 12;
104 k += 3;
105 }
106
107 /*----------------------------- handle the last (probably partial) block */
108 /*
109 * "k[2]&0xffffff" actually reads beyond the end of the string, but
110 * then masks off the part it's not allowed to read. Because the
111 * string is aligned, the masked-off tail is in the same word as the
112 * rest of the string. Every machine with memory protection I've seen
113 * does it on word boundaries, so is OK with this. But VALGRIND will
114 * still catch it and complain. The masking trick does make the hash
115 * noticably faster for short strings (like English words).
116 */
117 #ifndef VALGRIND
118
119 switch (length) {
120 case 12: c+=k[2]; b+=k[1]; a+=k[0]; break;
121 case 11: c+=k[2]&0xffffff; b+=k[1]; a+=k[0]; break;
122 case 10: c+=k[2]&0xffff; b+=k[1]; a+=k[0]; break;
123 case 9 : c+=k[2]&0xff; b+=k[1]; a+=k[0]; break;
124 case 8 : b+=k[1]; a+=k[0]; break;
125 case 7 : b+=k[1]&0xffffff; a+=k[0]; break;
126 case 6 : b+=k[1]&0xffff; a+=k[0]; break;
127 case 5 : b+=k[1]&0xff; a+=k[0]; break;
128 case 4 : a+=k[0]; break;
129 case 3 : a+=k[0]&0xffffff; break;
130 case 2 : a+=k[0]&0xffff; break;
131 case 1 : a+=k[0]&0xff; break;
132 case 0 : return c; /* zero length strings require no mixing */
133 }
134
135 #else /* make valgrind happy */
136 {
137 const uint8_t *k8;
138
139 k8 = (const uint8_t *) k;
140 switch (length) {
141 case 12: c+=k[2]; b+=k[1]; a+=k[0]; break;
142 case 11: c+=((uint32_t) k8[10])<<16; /* fall through */
143 case 10: c+=((uint32_t) k8[9])<<8; /* fall through */
144 case 9 : c+=k8[8]; /* fall through */
145 case 8 : b+=k[1]; a+=k[0]; break;
146 case 7 : b+=((uint32_t) k8[6])<<16; /* fall through */
147 case 6 : b+=((uint32_t) k8[5])<<8; /* fall through */
148 case 5 : b+=k8[4]; /* fall through */
149 case 4 : a+=k[0]; break;
150 case 3 : a+=((uint32_t) k8[2])<<16; /* fall through */
151 case 2 : a+=((uint32_t) k8[1])<<8; /* fall through */
152 case 1 : a+=k8[0]; break;
153 case 0 : return c;
154 }
155 }
156 #endif /* !valgrind */
157
158 } else if (HASH_LITTLE_ENDIAN && ((u.i & 0x1) == 0)) {
159 const uint16_t *k = (const uint16_t *) key; /* read 16-bit chunks */
160 const uint8_t *k8;
161
162 /*--------------- all but last block: aligned reads and different mixing */
163 while (length > 12)
164 {
165 a += k[0] + (((uint32_t) k[1])<<16);
166 b += k[2] + (((uint32_t) k[3])<<16);
167 c += k[4] + (((uint32_t) k[5])<<16);
168 mix(a, b, c);
169 length -= 12;
170 k += 6;
171 }
172
173 /*----------------------------- handle the last (probably partial) block */
174 k8 = (const uint8_t *) k;
175 switch(length)
176 {
177 case 12: c+=k[4]+(((uint32_t) k[5])<<16);
178 b+=k[2]+(((uint32_t) k[3])<<16);
179 a+=k[0]+(((uint32_t) k[1])<<16);
180 break;
181 case 11: c+=((uint32_t) k8[10])<<16; /* fall through */
182 case 10: c+=k[4];
183 b+=k[2]+(((uint32_t) k[3])<<16);
184 a+=k[0]+(((uint32_t) k[1])<<16);
185 break;
186 case 9 : c+=k8[8]; /* fall through */
187 case 8 : b+=k[2]+(((uint32_t) k[3])<<16);
188 a+=k[0]+(((uint32_t) k[1])<<16);
189 break;
190 case 7 : b+=((uint32_t) k8[6])<<16; /* fall through */
191 case 6 : b+=k[2];
192 a+=k[0]+(((uint32_t) k[1])<<16);
193 break;
194 case 5 : b+=k8[4]; /* fall through */
195 case 4 : a+=k[0]+(((uint32_t) k[1])<<16);
196 break;
197 case 3 : a+=((uint32_t) k8[2])<<16; /* fall through */
198 case 2 : a+=k[0];
199 break;
200 case 1 : a+=k8[0];
201 break;
202 case 0 : return c; /* zero length requires no mixing */
203 }
204
205 } else { /* need to read the key one byte at a time */
206 const uint8_t *k = (const uint8_t *)key;
207
208 /*--------------- all but the last block: affect some 32 bits of (a, b, c) */
209 while (length > 12) {
210 a += k[0];
211 a += ((uint32_t) k[1])<<8;
212 a += ((uint32_t) k[2])<<16;
213 a += ((uint32_t) k[3])<<24;
214 b += k[4];
215 b += ((uint32_t) k[5])<<8;
216 b += ((uint32_t) k[6])<<16;
217 b += ((uint32_t) k[7])<<24;
218 c += k[8];
219 c += ((uint32_t) k[9])<<8;
220 c += ((uint32_t) k[10])<<16;
221 c += ((uint32_t) k[11])<<24;
222 mix(a,b,c);
223 length -= 12;
224 k += 12;
225 }
226
227 /*-------------------------------- last block: affect all 32 bits of (c) */
228 switch (length) { /* all the case statements fall through */
229 case 12: c+=((uint32_t) k[11])<<24;
230 case 11: c+=((uint32_t) k[10])<<16;
231 case 10: c+=((uint32_t) k[9])<<8;
232 case 9 : c+=k[8];
233 case 8 : b+=((uint32_t) k[7])<<24;
234 case 7 : b+=((uint32_t) k[6])<<16;
235 case 6 : b+=((uint32_t) k[5])<<8;
236 case 5 : b+=k[4];
237 case 4 : a+=((uint32_t) k[3])<<24;
238 case 3 : a+=((uint32_t) k[2])<<16;
239 case 2 : a+=((uint32_t) k[1])<<8;
240 case 1 : a+=k[0];
241 break;
242 case 0 : return c;
243 }
244 }
245
246 final(a, b, c);
247 return c;
248 }
249
250 static inline
251 uint32_t jhash(const void *key, size_t length, uint32_t seed)
252 {
253 return hashlittle(key, length, seed);
254 }
255
256 #endif /* _JHASH_H */
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