src/hash.c

   1 /*
   2  * The contents of this file are public domain.
   3  *
   4  * Based on: lookup3.c, by Bob Jenkins, May 2006, Public Domain.
   5  */
   6
   7 #include "system.h"
   8 #include "hash.h"
   9
  10 /*
  11  * A simple version of Bob Jenkins' lookup3.c hash.
  12  *
  13  * It is supposed to give same results as hashlittle() on little-endian
  14  * and hashbig() on big-endian machines.
  15  *
  16  * Speed seems comparable to Jenkins' optimized version (~ -10%).
  17  * Actual difference varies as it depends on cpu/compiler/libc details.
  18  */
  19
  20 /* rotate uint32 */
  21 #define rot(x,k) (((x)<<(k)) | ((x)>>(32-(k))))
  22
  23 /* mix 3 32-bit values reversibly */
  24 #define mix(a, b, c) do { \
  25         a -= c;  a ^= rot(c, 4);  c += b; \
  26         b -= a;  b ^= rot(a, 6);  a += c; \
  27         c -= b;  c ^= rot(b, 8);  b += a; \
  28         a -= c;  a ^= rot(c,16);  c += b; \
  29         b -= a;  b ^= rot(a,19);  a += c; \
  30         c -= b;  c ^= rot(b, 4);  b += a; \
  31 } while (0)
  32
  33 /* final mixing of 3 32-bit values (a,b,c) into c */
  34 #define final(a, b, c) do { \
  35         c ^= b; c -= rot(b,14); \
  36         a ^= c; a -= rot(c,11); \
  37         b ^= a; b -= rot(a,25); \
  38         c ^= b; c -= rot(b,16); \
  39         a ^= c; a -= rot(c, 4); \
  40         b ^= a; b -= rot(a,14); \
  41         c ^= b; c -= rot(b,24); \
  42 } while (0)
  43
  44 /*
  45  * GCC does not know how to optimize short variable-length copies.
  46  * Its faster to do dumb inlined copy than call out to libc.
  47  */
  48 static inline void simple_memcpy(void *dst_, const void *src_, size_t len)
  49 {
  50         const uint8_t *src = src_;
  51         uint8_t *dst = dst_;
  52         while (len--)
  53                 *dst++ = *src++;
  54 }
  55
  56 /* short version - let compiler worry about memory access */
  57 uint32_t lookup3_hash(const void *data, size_t len)
  58 {
  59         uint32_t a, b, c;
  60         uint32_t buf[3];
  61         const uint8_t *p = data;
  62
  63         a = b = c = 0xdeadbeef + len;
  64         if (len == 0)
  65                 goto done;
  66
  67         while (len > 12) {
  68                 memcpy(buf, p, 12);
  69                 a += buf[0];
  70                 b += buf[1];
  71                 c += buf[2];
  72                 mix(a, b, c);
  73                 p += 12;
  74                 len -= 12;
  75         }
  76
  77         buf[0] = buf[1] = buf[2] = 0;
  78         simple_memcpy(buf, p, len);
  79         a += buf[0];
  80         b += buf[1];
  81         c += buf[2];
  82         final(a, b, c);
  83 done:
  84         return c;
  85 }
  86
  87
  88 /*
  89  * Reversible integer hash function by Thomas Wang.
  90  */
  91
  92 uint32_t hash32(uint32_t v)
  93 {
  94         v = ~v + (v << 15);
  95         v = v ^ (v >> 12);
  96         v = v + (v << 2);
  97         v = v ^ (v >> 4);
  98         v = v * 2057;
  99         v = v ^ (v >> 16);
 100         return v;
 101 }
 102