runtime/lookupa.m

   1 /*
   2 --------------------------------------------------------------------
   3 lookupa.c, by Bob Jenkins, December 1996.  Same as lookup2.c
   4 Use this code however you wish.  Public Domain.  No warranty.
   5 Source is http://burtleburtle.net/bob/c/lookupa.c
   6 --------------------------------------------------------------------
   7 */
   8 #ifndef STANDARD
   9 #include "standard.h"
  10 #endif
  11 #ifndef LOOKUPA
  12 #include "lookupa.h"
  13 #endif
  14
  15 /*
  16 --------------------------------------------------------------------
  17 mix -- mix 3 32-bit values reversibly.
  18 For every delta with one or two bit set, and the deltas of all three
  19   high bits or all three low bits, whether the original value of a,b,c
  20   is almost all zero or is uniformly distributed,
  21 * If mix() is run forward or backward, at least 32 bits in a,b,c
  22   have at least 1/4 probability of changing.
  23 * If mix() is run forward, every bit of c will change between 1/3 and
  24   2/3 of the time.  (Well, 22/100 and 78/100 for some 2-bit deltas.)
  25 mix() was built out of 36 single-cycle latency instructions in a
  26   structure that could supported 2x parallelism, like so:
  27       a -= b;
  28       a -= c; x = (c>>13);
  29       b -= c; a ^= x;
  30       b -= a; x = (a<<8);
  31       c -= a; b ^= x;
  32       c -= b; x = (b>>13);
  33       ...
  34   Unfortunately, superscalar Pentiums and Sparcs can't take advantage
  35   of that parallelism.  They've also turned some of those single-cycle
  36   latency instructions into multi-cycle latency instructions.  Still,
  37   this is the fastest good hash I could find.  There were about 2^^68
  38   to choose from.  I only looked at a billion or so.
  39 --------------------------------------------------------------------
  40 */
  41 #define mix(a,b,c) \
  42 { \
  43   a -= b; a -= c; a ^= (c>>13); \
  44   b -= c; b -= a; b ^= (a<<8); \
  45   c -= a; c -= b; c ^= (b>>13); \
  46   a -= b; a -= c; a ^= (c>>12);  \
  47   b -= c; b -= a; b ^= (a<<16); \
  48   c -= a; c -= b; c ^= (b>>5); \
  49   a -= b; a -= c; a ^= (c>>3);  \
  50   b -= c; b -= a; b ^= (a<<10); \
  51   c -= a; c -= b; c ^= (b>>15); \
  52 }
  53
  54 /*
  55 --------------------------------------------------------------------
  56 lookup() -- hash a variable-length key into a 32-bit value
  57   k     : the key (the unaligned variable-length array of bytes)
  58   len   : the length of the key, counting by bytes
  59   level : can be any 4-byte value
  60 Returns a 32-bit value.  Every bit of the key affects every bit of
  61 the return value.  Every 1-bit and 2-bit delta achieves avalanche.
  62 About 6len+35 instructions.
  63
  64 The best hash table sizes are powers of 2.  There is no need to do
  65 mod a prime (mod is sooo slow!).  If you need less than 32 bits,
  66 use a bitmask.  For example, if you need only 10 bits, do
  67   h = (h & hashmask(10));
  68 In which case, the hash table should have hashsize(10) elements.
  69
  70 If you are hashing n strings (ub1 **)k, do it like this:
  71   for (i=0, h=0; i<n; ++i) h = lookup( k[i], len[i], h);
  72
  73 By Bob Jenkins, 1996.  bob_jenkins@burtleburtle.net.  You may use this
  74 code any way you wish, private, educational, or commercial.
  75
  76 See http://burtleburtle.net/bob/hash/evahash.html
  77 Use for hash table lookup, or anything where one collision in 2^32 is
  78 acceptable.  Do NOT use for cryptographic purposes.
  79 --------------------------------------------------------------------
  80 */
  81
  82 __private_extern__ ub4 lookup( k, length, level)
  83 register ub1 *k;        /* the key */
  84 register ub4  length;   /* the length of the key */
  85 register ub4  level;    /* the previous hash, or an arbitrary value */
  86 {
  87    register ub4 a,b,c,len;
  88
  89    /* Set up the internal state */
  90    len = length;
  91    a = b = 0x9e3779b9;  /* the golden ratio; an arbitrary value */
  92    c = level;           /* the previous hash value */
  93
  94    /*---------------------------------------- handle most of the key */
  95    while (len >= 12)
  96    {
  97       a += (k[0] +((ub4)k[1]<<8) +((ub4)k[2]<<16) +((ub4)k[3]<<24));
  98       b += (k[4] +((ub4)k[5]<<8) +((ub4)k[6]<<16) +((ub4)k[7]<<24));
  99       c += (k[8] +((ub4)k[9]<<8) +((ub4)k[10]<<16)+((ub4)k[11]<<24));
 100       mix(a,b,c);
 101       k += 12; len -= 12;
 102    }
 103
 104    /*------------------------------------- handle the last 11 bytes */
 105    c += length;
 106    switch(len)              /* all the case statements fall through */
 107    {
 108    case 11: c+=((ub4)k[10]<<24);
 109    case 10: c+=((ub4)k[9]<<16);
 110    case 9 : c+=((ub4)k[8]<<8);
 111       /* the first byte of c is reserved for the length */
 112    case 8 : b+=((ub4)k[7]<<24);
 113    case 7 : b+=((ub4)k[6]<<16);
 114    case 6 : b+=((ub4)k[5]<<8);
 115    case 5 : b+=k[4];
 116    case 4 : a+=((ub4)k[3]<<24);
 117    case 3 : a+=((ub4)k[2]<<16);
 118    case 2 : a+=((ub4)k[1]<<8);
 119    case 1 : a+=k[0];
 120      /* case 0: nothing left to add */
 121    }
 122    mix(a,b,c);
 123    /*-------------------------------------------- report the result */
 124    return c;
 125 }
 126
 127
 128 /*
 129 --------------------------------------------------------------------
 130 mixc -- mixc 8 4-bit values as quickly and thoroughly as possible.
 131 Repeating mix() three times achieves avalanche.
 132 Repeating mix() four times eliminates all funnels and all
 133   characteristics stronger than 2^{-11}.
 134 --------------------------------------------------------------------
 135 */
 136 #define mixc(a,b,c,d,e,f,g,h) \
 137 { \
 138    a^=b<<11; d+=a; b+=c; \
 139    b^=c>>2;  e+=b; c+=d; \
 140    c^=d<<8;  f+=c; d+=e; \
 141    d^=e>>16; g+=d; e+=f; \
 142    e^=f<<10; h+=e; f+=g; \
 143    f^=g>>4;  a+=f; g+=h; \
 144    g^=h<<8;  b+=g; h+=a; \
 145    h^=a>>9;  c+=h; a+=b; \
 146 }
 147
 148 /*
 149 --------------------------------------------------------------------
 150 checksum() -- hash a variable-length key into a 256-bit value
 151   k     : the key (the unaligned variable-length array of bytes)
 152   len   : the length of the key, counting by bytes
 153   state : an array of CHECKSTATE 4-byte values (256 bits)
 154 The state is the checksum.  Every bit of the key affects every bit of
 155 the state.  There are no funnels.  About 112+6.875len instructions.
 156
 157 If you are hashing n strings (ub1 **)k, do it like this:
 158   for (i=0; i<8; ++i) state[i] = 0x9e3779b9;
 159   for (i=0, h=0; i<n; ++i) checksum( k[i], len[i], state);
 160
 161 (c) Bob Jenkins, 1996.  bob_jenkins@burtleburtle.net.  You may use this
 162 code any way you wish, private, educational, or commercial, as long
 163 as this whole comment accompanies it.
 164
 165 See http://burtleburtle.net/bob/hash/evahash.html
 166 Use to detect changes between revisions of documents, assuming nobody
 167 is trying to cause collisions.  Do NOT use for cryptography.
 168 --------------------------------------------------------------------
 169 */
 170 __private_extern__ void  checksum( k, len, state)
 171 register ub1 *k;
 172 register ub4  len;
 173 register ub4 *state;
 174 {
 175    register ub4 a,b,c,d,e,f,g,h,length;
 176
 177    /* Use the length and level; add in the golden ratio. */
 178    length = len;
 179    a=state[0]; b=state[1]; c=state[2]; d=state[3];
 180    e=state[4]; f=state[5]; g=state[6]; h=state[7];
 181
 182    /*---------------------------------------- handle most of the key */
 183    while (len >= 32)
 184    {
 185       a += (k[0] +(k[1]<<8) +(k[2]<<16) +(k[3]<<24));
 186       b += (k[4] +(k[5]<<8) +(k[6]<<16) +(k[7]<<24));
 187       c += (k[8] +(k[9]<<8) +(k[10]<<16)+(k[11]<<24));
 188       d += (k[12]+(k[13]<<8)+(k[14]<<16)+(k[15]<<24));
 189       e += (k[16]+(k[17]<<8)+(k[18]<<16)+(k[19]<<24));
 190       f += (k[20]+(k[21]<<8)+(k[22]<<16)+(k[23]<<24));
 191       g += (k[24]+(k[25]<<8)+(k[26]<<16)+(k[27]<<24));
 192       h += (k[28]+(k[29]<<8)+(k[30]<<16)+(k[31]<<24));
 193       mixc(a,b,c,d,e,f,g,h);
 194       mixc(a,b,c,d,e,f,g,h);
 195       mixc(a,b,c,d,e,f,g,h);
 196       mixc(a,b,c,d,e,f,g,h);
 197       k += 32; len -= 32;
 198    }
 199
 200    /*------------------------------------- handle the last 31 bytes */
 201    h += length;
 202    switch(len)
 203    {
 204    case 31: h+=(k[30]<<24);
 205    case 30: h+=(k[29]<<16);
 206    case 29: h+=(k[28]<<8);
 207    case 28: g+=(k[27]<<24);
 208    case 27: g+=(k[26]<<16);
 209    case 26: g+=(k[25]<<8);
 210    case 25: g+=k[24];
 211    case 24: f+=(k[23]<<24);
 212    case 23: f+=(k[22]<<16);
 213    case 22: f+=(k[21]<<8);
 214    case 21: f+=k[20];
 215    case 20: e+=(k[19]<<24);
 216    case 19: e+=(k[18]<<16);
 217    case 18: e+=(k[17]<<8);
 218    case 17: e+=k[16];
 219    case 16: d+=(k[15]<<24);
 220    case 15: d+=(k[14]<<16);
 221    case 14: d+=(k[13]<<8);
 222    case 13: d+=k[12];
 223    case 12: c+=(k[11]<<24);
 224    case 11: c+=(k[10]<<16);
 225    case 10: c+=(k[9]<<8);
 226    case 9 : c+=k[8];
 227    case 8 : b+=(k[7]<<24);
 228    case 7 : b+=(k[6]<<16);
 229    case 6 : b+=(k[5]<<8);
 230    case 5 : b+=k[4];
 231    case 4 : a+=(k[3]<<24);
 232    case 3 : a+=(k[2]<<16);
 233    case 2 : a+=(k[1]<<8);
 234    case 1 : a+=k[0];
 235    }
 236    mixc(a,b,c,d,e,f,g,h);
 237    mixc(a,b,c,d,e,f,g,h);
 238    mixc(a,b,c,d,e,f,g,h);
 239    mixc(a,b,c,d,e,f,g,h);
 240
 241    /*-------------------------------------------- report the result */
 242    state[0]=a; state[1]=b; state[2]=c; state[3]=d;
 243    state[4]=e; state[5]=f; state[6]=g; state[7]=h;
 244 }