1 // Modified by Russ Cox to add "namespace re2". 2 // Also threw away all but hashword and hashword2. 3 // http://burtleburtle.net/bob/c/lookup3.c 4 5 /* 6 ------------------------------------------------------------------------------- 7 lookup3.c, by Bob Jenkins, May 2006, Public Domain. 8 9 These are functions for producing 32-bit hashes for hash table lookup. 10 hashword(), hashlittle(), hashlittle2(), hashbig(), mix(), and final() 11 are externally useful functions. Routines to test the hash are included 12 if SELF_TEST is defined. You can use this free for any purpose. It's in 13 the public domain. It has no warranty. 14 15 You probably want to use hashlittle(). hashlittle() and hashbig() 16 hash byte arrays. hashlittle() is is faster than hashbig() on 17 little-endian machines. Intel and AMD are little-endian machines. 18 On second thought, you probably want hashlittle2(), which is identical to 19 hashlittle() except it returns two 32-bit hashes for the price of one. 20 You could implement hashbig2() if you wanted but I haven't bothered here. 21 22 If you want to find a hash of, say, exactly 7 integers, do 23 a = i1; b = i2; c = i3; 24 mix(a,b,c); 25 a += i4; b += i5; c += i6; 26 mix(a,b,c); 27 a += i7; 28 final(a,b,c); 29 then use c as the hash value. If you have a variable length array of 30 4-byte integers to hash, use hashword(). If you have a byte array (like 31 a character string), use hashlittle(). If you have several byte arrays, or 32 a mix of things, see the comments above hashlittle(). 33 34 Why is this so big? I read 12 bytes at a time into 3 4-byte integers, 35 then mix those integers. This is fast (you can do a lot more thorough 36 mixing with 12*3 instructions on 3 integers than you can with 3 instructions 37 on 1 byte), but shoehorning those bytes into integers efficiently is messy. 38 ------------------------------------------------------------------------------- 39 */ 40 41 #include "util/util.h" 42 43 #define rot(x,k) (((x)<<(k)) | ((x)>>(32-(k)))) 44 45 /* 46 ------------------------------------------------------------------------------- 47 mix -- mix 3 32-bit values reversibly. 48 49 This is reversible, so any information in (a,b,c) before mix() is 50 still in (a,b,c) after mix(). 51 52 If four pairs of (a,b,c) inputs are run through mix(), or through 53 mix() in reverse, there are at least 32 bits of the output that 54 are sometimes the same for one pair and different for another pair. 55 This was tested for: 56 * pairs that differed by one bit, by two bits, in any combination 57 of top bits of (a,b,c), or in any combination of bottom bits of 58 (a,b,c). 59 * "differ" is defined as +, -, ^, or ~^. For + and -, I transformed 60 the output delta to a Gray code (a^(a>>1)) so a string of 1's (as 61 is commonly produced by subtraction) look like a single 1-bit 62 difference. 63 * the base values were pseudorandom, all zero but one bit set, or 64 all zero plus a counter that starts at zero. 65 66 Some k values for my "a-=c; a^=rot(c,k); c+=b;" arrangement that 67 satisfy this are 68 4 6 8 16 19 4 69 9 15 3 18 27 15 70 14 9 3 7 17 3 71 Well, "9 15 3 18 27 15" didn't quite get 32 bits diffing 72 for "differ" defined as + with a one-bit base and a two-bit delta. I 73 used http://burtleburtle.net/bob/hash/avalanche.html to choose 74 the operations, constants, and arrangements of the variables. 75 76 This does not achieve avalanche. There are input bits of (a,b,c) 77 that fail to affect some output bits of (a,b,c), especially of a. The 78 most thoroughly mixed value is c, but it doesn't really even achieve 79 avalanche in c. 80 81 This allows some parallelism. Read-after-writes are good at doubling 82 the number of bits affected, so the goal of mixing pulls in the opposite 83 direction as the goal of parallelism. I did what I could. Rotates 84 seem to cost as much as shifts on every machine I could lay my hands 85 on, and rotates are much kinder to the top and bottom bits, so I used 86 rotates. 87 ------------------------------------------------------------------------------- 88 */ 89 #define mix(a,b,c) \ 90 { \ 91 a -= c; a ^= rot(c, 4); c += b; \ 92 b -= a; b ^= rot(a, 6); a += c; \ 93 c -= b; c ^= rot(b, 8); b += a; \ 94 a -= c; a ^= rot(c,16); c += b; \ 95 b -= a; b ^= rot(a,19); a += c; \ 96 c -= b; c ^= rot(b, 4); b += a; \ 97 } 98 99 /* 100 ------------------------------------------------------------------------------- 101 final -- final mixing of 3 32-bit values (a,b,c) into c 102 103 Pairs of (a,b,c) values differing in only a few bits will usually 104 produce values of c that look totally different. This was tested for 105 * pairs that differed by one bit, by two bits, in any combination 106 of top bits of (a,b,c), or in any combination of bottom bits of 107 (a,b,c). 108 * "differ" is defined as +, -, ^, or ~^. For + and -, I transformed 109 the output delta to a Gray code (a^(a>>1)) so a string of 1's (as 110 is commonly produced by subtraction) look like a single 1-bit 111 difference. 112 * the base values were pseudorandom, all zero but one bit set, or 113 all zero plus a counter that starts at zero. 114 115 These constants passed: 116 14 11 25 16 4 14 24 117 12 14 25 16 4 14 24 118 and these came close: 119 4 8 15 26 3 22 24 120 10 8 15 26 3 22 24 121 11 8 15 26 3 22 24 122 ------------------------------------------------------------------------------- 123 */ 124 #define final(a,b,c) \ 125 { \ 126 c ^= b; c -= rot(b,14); \ 127 a ^= c; a -= rot(c,11); \ 128 b ^= a; b -= rot(a,25); \ 129 c ^= b; c -= rot(b,16); \ 130 a ^= c; a -= rot(c,4); \ 131 b ^= a; b -= rot(a,14); \ 132 c ^= b; c -= rot(b,24); \ 133 } 134 135 namespace re2 { 136 137 /* 138 -------------------------------------------------------------------- 139 This works on all machines. To be useful, it requires 140 -- that the key be an array of uint32_t's, and 141 -- that the length be the number of uint32_t's in the key 142 143 The function hashword() is identical to hashlittle() on little-endian 144 machines, and identical to hashbig() on big-endian machines, 145 except that the length has to be measured in uint32_ts rather than in 146 bytes. hashlittle() is more complicated than hashword() only because 147 hashlittle() has to dance around fitting the key bytes into registers. 148 -------------------------------------------------------------------- 149 */ 150 uint32 hashword( 151 const uint32 *k, /* the key, an array of uint32_t values */ 152 size_t length, /* the length of the key, in uint32_ts */ 153 uint32 initval) /* the previous hash, or an arbitrary value */ 154 { 155 uint32_t a,b,c; 156 157 /* Set up the internal state */ 158 a = b = c = 0xdeadbeef + (((uint32_t)length)<<2) + initval; 159 160 /*------------------------------------------------- handle most of the key */ 161 while (length > 3) 162 { 163 a += k[0]; 164 b += k[1]; 165 c += k[2]; 166 mix(a,b,c); 167 length -= 3; 168 k += 3; 169 } 170 171 /*------------------------------------------- handle the last 3 uint32_t's */ 172 switch(length) /* all the case statements fall through */ 173 { 174 case 3 : c+=k[2]; 175 case 2 : b+=k[1]; 176 case 1 : a+=k[0]; 177 final(a,b,c); 178 case 0: /* case 0: nothing left to add */ 179 break; 180 } 181 /*------------------------------------------------------ report the result */ 182 return c; 183 } 184 185 186 /* 187 -------------------------------------------------------------------- 188 hashword2() -- same as hashword(), but take two seeds and return two 189 32-bit values. pc and pb must both be nonnull, and *pc and *pb must 190 both be initialized with seeds. If you pass in (*pb)==0, the output 191 (*pc) will be the same as the return value from hashword(). 192 -------------------------------------------------------------------- 193 */ 194 void hashword2 ( 195 const uint32 *k, /* the key, an array of uint32_t values */ 196 size_t length, /* the length of the key, in uint32_ts */ 197 uint32 *pc, /* IN: seed OUT: primary hash value */ 198 uint32 *pb) /* IN: more seed OUT: secondary hash value */ 199 { 200 uint32_t a,b,c; 201 202 /* Set up the internal state */ 203 a = b = c = 0xdeadbeef + ((uint32_t)(length<<2)) + *pc; 204 c += *pb; 205 206 /*------------------------------------------------- handle most of the key */ 207 while (length > 3) 208 { 209 a += k[0]; 210 b += k[1]; 211 c += k[2]; 212 mix(a,b,c); 213 length -= 3; 214 k += 3; 215 } 216 217 /*------------------------------------------- handle the last 3 uint32_t's */ 218 switch(length) /* all the case statements fall through */ 219 { 220 case 3 : c+=k[2]; 221 case 2 : b+=k[1]; 222 case 1 : a+=k[0]; 223 final(a,b,c); 224 case 0: /* case 0: nothing left to add */ 225 break; 226 } 227 /*------------------------------------------------------ report the result */ 228 *pc=c; *pb=b; 229 } 230 231 } // namespace re2 232
