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; /* fall through */
 230                 case 11: c+=((uint32_t) k[10])<<16; /* fall through */
 231                 case 10: c+=((uint32_t) k[9])<<8; /* fall through */
 232                 case 9 : c+=k[8]; /* fall through */
 233                 case 8 : b+=((uint32_t) k[7])<<24; /* fall through */
 234                 case 7 : b+=((uint32_t) k[6])<<16; /* fall through */
 235                 case 6 : b+=((uint32_t) k[5])<<8; /* fall through */
 236                 case 5 : b+=k[4]; /* fall through */
 237                 case 4 : a+=((uint32_t) k[3])<<24; /* fall through */
 238                 case 3 : a+=((uint32_t) k[2])<<16; /* fall through */
 239                 case 2 : a+=((uint32_t) k[1])<<8; /* fall through */
 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 */