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Diffstat (limited to 'ext/tdb/lookup3.c')
| -rw-r--r-- | ext/tdb/lookup3.c | 429 |
1 files changed, 429 insertions, 0 deletions
diff --git a/ext/tdb/lookup3.c b/ext/tdb/lookup3.c new file mode 100644 index 0000000..23a9088 --- /dev/null +++ b/ext/tdb/lookup3.c @@ -0,0 +1,429 @@ +#include "rbtdb.h" + +/* + * lookup3 implementation copied from tdb.git + * (commit 3258cf3f11bf7c68a2e69e1808c4551cc899725a), + * as that tdb distribution isn't commonly available yet (as of 2010.11.29) + */ +#ifndef HAVE_TDB_JENKINS_HASH + +#ifndef WORDS_BIGENDIAN +# define HASH_LITTLE_ENDIAN 1 +# define HASH_BIG_ENDIAN 0 +#else +# define HASH_LITTLE_ENDIAN 0 +# define HASH_BIG_ENDIAN 1 +#endif + +/* +------------------------------------------------------------------------------- +lookup3.c, by Bob Jenkins, May 2006, Public Domain. + +These are functions for producing 32-bit hashes for hash table lookup. +hash_word(), hashlittle(), hashlittle2(), hashbig(), mix(), and final() +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's in +the public domain. It has no warranty. + +You probably want to use hashlittle(). hashlittle() and hashbig() +hash byte arrays. hashlittle() is is faster than hashbig() on +little-endian machines. Intel and AMD are little-endian machines. +On second thought, you probably want hashlittle2(), which is identical to +hashlittle() except it returns two 32-bit hashes for the price of one. +You could implement hashbig2() if you wanted but I haven't bothered here. + +If you want to find a hash of, say, exactly 7 integers, do + a = i1; b = i2; c = i3; + mix(a,b,c); + a += i4; b += i5; c += i6; + mix(a,b,c); + a += i7; + final(a,b,c); +then use c as the hash value. If you have a variable length array of +4-byte integers to hash, use hash_word(). If you have a byte array (like +a character string), use hashlittle(). If you have several byte arrays, or +a mix of things, see the comments above hashlittle(). + +Why is this so big? I read 12 bytes at a time into 3 4-byte integers, +then mix those integers. This is fast (you can do a lot more thorough +mixing with 12*3 instructions on 3 integers than you can with 3 instructions +on 1 byte), but shoehorning those bytes into integers efficiently is messy. +*/ + +#define hashsize(n) ((uint32_t)1<<(n)) +#define hashmask(n) (hashsize(n)-1) +#define rot(x,k) (((x)<<(k)) | ((x)>>(32-(k)))) + +/* +------------------------------------------------------------------------------- +mix -- mix 3 32-bit values reversibly. + +This is reversible, so any information in (a,b,c) before mix() is +still in (a,b,c) after mix(). + +If four pairs of (a,b,c) inputs are run through mix(), or through +mix() in reverse, there are at least 32 bits of the output that +are sometimes the same for one pair and different for another pair. +This was tested for: +* pairs that differed by one bit, by two bits, in any combination + of top bits of (a,b,c), or in any combination of bottom bits of + (a,b,c). +* "differ" is defined as +, -, ^, or ~^. For + and -, I transformed + the output delta to a Gray code (a^(a>>1)) so a string of 1's (as + is commonly produced by subtraction) look like a single 1-bit + difference. +* the base values were pseudorandom, all zero but one bit set, or + all zero plus a counter that starts at zero. + +Some k values for my "a-=c; a^=rot(c,k); c+=b;" arrangement that +satisfy this are + 4 6 8 16 19 4 + 9 15 3 18 27 15 + 14 9 3 7 17 3 +Well, "9 15 3 18 27 15" didn't quite get 32 bits diffing +for "differ" defined as + with a one-bit base and a two-bit delta. I +used http://burtleburtle.net/bob/hash/avalanche.html to choose +the operations, constants, and arrangements of the variables. + +This does not achieve avalanche. There are input bits of (a,b,c) +that fail to affect some output bits of (a,b,c), especially of a. The +most thoroughly mixed value is c, but it doesn't really even achieve +avalanche in c. + +This allows some parallelism. Read-after-writes are good at doubling +the number of bits affected, so the goal of mixing pulls in the opposite +direction as the goal of parallelism. I did what I could. Rotates +seem to cost as much as shifts on every machine I could lay my hands +on, and rotates are much kinder to the top and bottom bits, so I used +rotates. +------------------------------------------------------------------------------- +*/ +#define mix(a,b,c) \ +{ \ + a -= c; a ^= rot(c, 4); c += b; \ + b -= a; b ^= rot(a, 6); a += c; \ + c -= b; c ^= rot(b, 8); b += a; \ + a -= c; a ^= rot(c,16); c += b; \ + b -= a; b ^= rot(a,19); a += c; \ + c -= b; c ^= rot(b, 4); b += a; \ +} + +/* +------------------------------------------------------------------------------- +final -- final mixing of 3 32-bit values (a,b,c) into c + +Pairs of (a,b,c) values differing in only a few bits will usually +produce values of c that look totally different. This was tested for +* pairs that differed by one bit, by two bits, in any combination + of top bits of (a,b,c), or in any combination of bottom bits of + (a,b,c). +* "differ" is defined as +, -, ^, or ~^. For + and -, I transformed + the output delta to a Gray code (a^(a>>1)) so a string of 1's (as + is commonly produced by subtraction) look like a single 1-bit + difference. +* the base values were pseudorandom, all zero but one bit set, or + all zero plus a counter that starts at zero. + +These constants passed: + 14 11 25 16 4 14 24 + 12 14 25 16 4 14 24 +and these came close: + 4 8 15 26 3 22 24 + 10 8 15 26 3 22 24 + 11 8 15 26 3 22 24 +------------------------------------------------------------------------------- +*/ +#define final(a,b,c) \ +{ \ + c ^= b; c -= rot(b,14); \ + a ^= c; a -= rot(c,11); \ + b ^= a; b -= rot(a,25); \ + c ^= b; c -= rot(b,16); \ + a ^= c; a -= rot(c,4); \ + b ^= a; b -= rot(a,14); \ + c ^= b; c -= rot(b,24); \ +} + +/* +------------------------------------------------------------------------------- +hashlittle() -- hash a variable-length key into a 32-bit value + k : the key (the unaligned variable-length array of bytes) + length : the length of the key, counting by bytes + val2 : IN: can be any 4-byte value OUT: second 32 bit hash. +Returns a 32-bit value. Every bit of the key affects every bit of +the return value. Two keys differing by one or two bits will have +totally different hash values. Note that the return value is better +mixed than val2, so use that first. + +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 (uint8_t **)k, do it like this: + for (i=0, h=0; i<n; ++i) h = hashlittle( k[i], len[i], h); + +By Bob Jenkins, 2006. bob_jenkins@burtleburtle.net. You may use this +code any way you wish, private, educational, or commercial. It's free. + +Use for hash table lookup, or anything where one collision in 2^^32 is +acceptable. Do NOT use for cryptographic purposes. +------------------------------------------------------------------------------- +*/ + +static uint32_t hashlittle(const void *key, size_t length) +{ + uint32_t a, b, c; /* internal state */ + union { + const void *ptr; + size_t i; + } u; /* needed for Mac Powerbook G4 */ + + /* Set up the internal state */ + a = b = c = 0xdeadbeef + ((uint32_t) length); + + u.ptr = key; + if (HASH_LITTLE_ENDIAN && ((u.i & 0x3) == 0)) { + const uint32_t *k = (const uint32_t *)key; /* read 32-bit chunks */ +#ifdef VALGRIND + const uint8_t *k8; +#endif + + /*------ all but last block: aligned reads and affect 32 bits of (a,b,c) */ + while (length > 12) { + a += k[0]; + b += k[1]; + c += k[2]; + mix(a, b, c); + length -= 12; + k += 3; + } + + /*----------------------------- handle the last (probably partial) block */ + /* + * "k[2]&0xffffff" actually reads beyond the end of the string, but + * then masks off the part it's not allowed to read. Because the + * string is aligned, the masked-off tail is in the same word as the + * rest of the string. Every machine with memory protection I've seen + * does it on word boundaries, so is OK with this. But VALGRIND will + * still catch it and complain. The masking trick does make the hash + * noticably faster for short strings (like English words). + */ +#ifndef VALGRIND + + switch (length) { + case 12: + c += k[2]; + b += k[1]; + a += k[0]; + break; + case 11: + c += k[2] & 0xffffff; + b += k[1]; + a += k[0]; + break; + case 10: + c += k[2] & 0xffff; + b += k[1]; + a += k[0]; + break; + case 9: + c += k[2] & 0xff; + b += k[1]; + a += k[0]; + break; + case 8: + b += k[1]; + a += k[0]; + break; + case 7: + b += k[1] & 0xffffff; + a += k[0]; + break; + case 6: + b += k[1] & 0xffff; + a += k[0]; + break; + case 5: + b += k[1] & 0xff; + a += k[0]; + break; + case 4: + a += k[0]; + break; + case 3: + a += k[0] & 0xffffff; + break; + case 2: + a += k[0] & 0xffff; + break; + case 1: + a += k[0] & 0xff; + break; + case 0: + return c; /* zero length strings require no mixing */ + } + +#else /* make valgrind happy */ + + k8 = (const uint8_t *)k; + switch (length) { + case 12: + c += k[2]; + b += k[1]; + a += k[0]; + break; + case 11: + c += ((uint32_t) k8[10]) << 16; /* fall through */ + case 10: + c += ((uint32_t) k8[9]) << 8; /* fall through */ + case 9: + c += k8[8]; /* fall through */ + case 8: + b += k[1]; + a += k[0]; + break; + case 7: + b += ((uint32_t) k8[6]) << 16; /* fall through */ + case 6: + b += ((uint32_t) k8[5]) << 8; /* fall through */ + case 5: + b += k8[4]; /* fall through */ + case 4: + a += k[0]; + break; + case 3: + a += ((uint32_t) k8[2]) << 16; /* fall through */ + case 2: + a += ((uint32_t) k8[1]) << 8; /* fall through */ + case 1: + a += k8[0]; + break; + case 0: + return c; + } + +#endif /* !valgrind */ + + } else if (HASH_LITTLE_ENDIAN && ((u.i & 0x1) == 0)) { + const uint16_t *k = (const uint16_t *)key; /* read 16-bit chunks */ + const uint8_t *k8; + + /*--------------- all but last block: aligned reads and different mixing */ + while (length > 12) { + a += k[0] + (((uint32_t) k[1]) << 16); + b += k[2] + (((uint32_t) k[3]) << 16); + c += k[4] + (((uint32_t) k[5]) << 16); + mix(a, b, c); + length -= 12; + k += 6; + } + + /*----------------------------- handle the last (probably partial) block */ + k8 = (const uint8_t *)k; + switch (length) { + case 12: + c += k[4] + (((uint32_t) k[5]) << 16); + b += k[2] + (((uint32_t) k[3]) << 16); + a += k[0] + (((uint32_t) k[1]) << 16); + break; + case 11: + c += ((uint32_t) k8[10]) << 16; /* fall through */ + case 10: + c += k[4]; + b += k[2] + (((uint32_t) k[3]) << 16); + a += k[0] + (((uint32_t) k[1]) << 16); + break; + case 9: + c += k8[8]; /* fall through */ + case 8: + b += k[2] + (((uint32_t) k[3]) << 16); + a += k[0] + (((uint32_t) k[1]) << 16); + break; + case 7: + b += ((uint32_t) k8[6]) << 16; /* fall through */ + case 6: + b += k[2]; + a += k[0] + (((uint32_t) k[1]) << 16); + break; + case 5: + b += k8[4]; /* fall through */ + case 4: + a += k[0] + (((uint32_t) k[1]) << 16); + break; + case 3: + a += ((uint32_t) k8[2]) << 16; /* fall through */ + case 2: + a += k[0]; + break; + case 1: + a += k8[0]; + break; + case 0: + return c; /* zero length requires no mixing */ + } + + } else { /* need to read the key one byte at a time */ + const uint8_t *k = (const uint8_t *)key; + + /*--------------- all but the last block: affect some 32 bits of (a,b,c) */ + while (length > 12) { + a += k[0]; + a += ((uint32_t) k[1]) << 8; + a += ((uint32_t) k[2]) << 16; + a += ((uint32_t) k[3]) << 24; + b += k[4]; + b += ((uint32_t) k[5]) << 8; + b += ((uint32_t) k[6]) << 16; + b += ((uint32_t) k[7]) << 24; + c += k[8]; + c += ((uint32_t) k[9]) << 8; + c += ((uint32_t) k[10]) << 16; + c += ((uint32_t) k[11]) << 24; + mix(a, b, c); + length -= 12; + k += 12; + } + + /*-------------------------------- last block: affect all 32 bits of (c) */ + switch (length) { /* all the case statements fall through */ + case 12: + c += ((uint32_t) k[11]) << 24; + case 11: + c += ((uint32_t) k[10]) << 16; + case 10: + c += ((uint32_t) k[9]) << 8; + case 9: + c += k[8]; + case 8: + b += ((uint32_t) k[7]) << 24; + case 7: + b += ((uint32_t) k[6]) << 16; + case 6: + b += ((uint32_t) k[5]) << 8; + case 5: + b += k[4]; + case 4: + a += ((uint32_t) k[3]) << 24; + case 3: + a += ((uint32_t) k[2]) << 16; + case 2: + a += ((uint32_t) k[1]) << 8; + case 1: + a += k[0]; + break; + case 0: + return c; + } + } + + final(a, b, c); + return c; +} + +unsigned int rbtdb_jenkins_lookup3(TDB_DATA * key) +{ + return hashlittle(key->dptr, key->dsize); +} +#endif /* !HAVE_TDB_JENKINS_HASH */ |