/* Hash function */ #include "mairix.h" /* -------------------------------------------------------------------- lookup2.c, by Bob Jenkins, December 1996, Public Domain. hash(), hash2(), hash3, and mix() are externally useful functions. Routines to test the hash are included if SELF_TEST is defined. You can use this free for any purpose. It has no warranty. -------------------------------------------------------------------- */ #include #include #include #define hashsize(n) ((unsigned int)1<<(n)) #define hashmask(n) (hashsize(n)-1) /* -------------------------------------------------------------------- mix -- mix 3 32-bit values reversibly. For every delta with one or two bit set, and the deltas of all three high bits or all three low bits, whether the original value of a,b,c is almost all zero or is uniformly distributed, * If mix() is run forward or backward, at least 32 bits in a,b,c have at least 1/4 probability of changing. * If mix() is run forward, every bit of c will change between 1/3 and 2/3 of the time. (Well, 22/100 and 78/100 for some 2-bit deltas.) mix() was built out of 36 single-cycle latency instructions in a structure that could supported 2x parallelism, like so: a -= b; a -= c; x = (c>>13); b -= c; a ^= x; b -= a; x = (a<<8); c -= a; b ^= x; c -= b; x = (b>>13); ... Unfortunately, superscalar Pentiums and Sparcs can't take advantage of that parallelism. They've also turned some of those single-cycle latency instructions into multi-cycle latency instructions. Still, this is the fastest good hash I could find. There were about 2^^68 to choose from. I only looked at a billion or so. -------------------------------------------------------------------- */ #define mix(a,b,c) \ { \ a -= b; a -= c; a ^= (c>>13); \ b -= c; b -= a; b ^= (a<<8); \ c -= a; c -= b; c ^= (b>>13); \ a -= b; a -= c; a ^= (c>>12); \ b -= c; b -= a; b ^= (a<<16); \ c -= a; c -= b; c ^= (b>>5); \ a -= b; a -= c; a ^= (c>>3); \ b -= c; b -= a; b ^= (a<<10); \ c -= a; c -= b; c ^= (b>>15); \ } /* same, but slower, works on systems that might have 8 byte ub4's */ #define mix2(a,b,c) \ { \ a -= b; a -= c; a ^= (c>>13); \ b -= c; b -= a; b ^= (a<< 8); \ c -= a; c -= b; c ^= ((b&0xffffffff)>>13); \ a -= b; a -= c; a ^= ((c&0xffffffff)>>12); \ b -= c; b -= a; b = (b ^ (a<<16)) & 0xffffffff; \ c -= a; c -= b; c = (c ^ (b>> 5)) & 0xffffffff; \ a -= b; a -= c; a = (a ^ (c>> 3)) & 0xffffffff; \ b -= c; b -= a; b = (b ^ (a<<10)) & 0xffffffff; \ c -= a; c -= b; c = (c ^ (b>>15)) & 0xffffffff; \ } /* -------------------------------------------------------------------- hash() -- hash a variable-length key into a 32-bit value k : the key (the unaligned variable-length array of bytes) len : the length of the key, counting by bytes level : can be any 4-byte value Returns a 32-bit value. Every bit of the key affects every bit of the return value. Every 1-bit and 2-bit delta achieves avalanche. About 36+6len instructions. The best hash table sizes are powers of 2. There is no need to do mod a prime (mod is sooo slow!). If you need less than 32 bits, use a bitmask. For example, if you need only 10 bits, do h = (h & hashmask(10)); In which case, the hash table should have hashsize(10) elements. If you are hashing n strings (ub1 **)k, do it like this: for (i=0, h=0; i= 12) { a += (k[0] +((unsigned int)k[1]<<8) +((unsigned int)k[2]<<16) +((unsigned int)k[3]<<24)); b += (k[4] +((unsigned int)k[5]<<8) +((unsigned int)k[6]<<16) +((unsigned int)k[7]<<24)); c += (k[8] +((unsigned int)k[9]<<8) +((unsigned int)k[10]<<16)+((unsigned int)k[11]<<24)); mix(a,b,c); k += 12; len -= 12; } /*------------------------------------- handle the last 11 bytes */ c += length; switch(len) /* all the case statements fall through */ { case 11: c+=((unsigned int)k[10]<<24); case 10: c+=((unsigned int)k[9]<<16); case 9 : c+=((unsigned int)k[8]<<8); /* the first byte of c is reserved for the length */ case 8 : b+=((unsigned int)k[7]<<24); case 7 : b+=((unsigned int)k[6]<<16); case 6 : b+=((unsigned int)k[5]<<8); case 5 : b+=k[4]; case 4 : a+=((unsigned int)k[3]<<24); case 3 : a+=((unsigned int)k[2]<<16); case 2 : a+=((unsigned int)k[1]<<8); case 1 : a+=k[0]; /* case 0: nothing left to add */ } mix(a,b,c); /*-------------------------------------------- report the result */ return c; }