doc/examples/rculfhash/jhash.h

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