diff options
Diffstat (limited to 'util/hash.cc')
| -rw-r--r-- | util/hash.cc | 231 |
1 files changed, 0 insertions, 231 deletions
diff --git a/util/hash.cc b/util/hash.cc deleted file mode 100644 index dfef7b7..0000000 --- a/util/hash.cc +++ /dev/null @@ -1,231 +0,0 @@ -// Modified by Russ Cox to add "namespace re2". -// Also threw away all but hashword and hashword2. -// http://burtleburtle.net/bob/c/lookup3.c - -/* -------------------------------------------------------------------------------- -lookup3.c, by Bob Jenkins, May 2006, Public Domain. - -These are functions for producing 32-bit hashes for hash table lookup. -hashword(), 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 hashword(). 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. -------------------------------------------------------------------------------- -*/ - -#include "util/util.h" - -#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); \ -} - -namespace re2 { - -/* --------------------------------------------------------------------- - This works on all machines. To be useful, it requires - -- that the key be an array of uint32_t's, and - -- that the length be the number of uint32_t's in the key - - The function hashword() is identical to hashlittle() on little-endian - machines, and identical to hashbig() on big-endian machines, - except that the length has to be measured in uint32_ts rather than in - bytes. hashlittle() is more complicated than hashword() only because - hashlittle() has to dance around fitting the key bytes into registers. --------------------------------------------------------------------- -*/ -uint32 hashword( -const uint32 *k, /* the key, an array of uint32_t values */ -size_t length, /* the length of the key, in uint32_ts */ -uint32 initval) /* the previous hash, or an arbitrary value */ -{ - uint32_t a,b,c; - - /* Set up the internal state */ - a = b = c = 0xdeadbeef + (((uint32_t)length)<<2) + initval; - - /*------------------------------------------------- handle most of the key */ - while (length > 3) - { - a += k[0]; - b += k[1]; - c += k[2]; - mix(a,b,c); - length -= 3; - k += 3; - } - - /*------------------------------------------- handle the last 3 uint32_t's */ - switch(length) /* all the case statements fall through */ - { - case 3 : c+=k[2]; - case 2 : b+=k[1]; - case 1 : a+=k[0]; - final(a,b,c); - case 0: /* case 0: nothing left to add */ - break; - } - /*------------------------------------------------------ report the result */ - return c; -} - - -/* --------------------------------------------------------------------- -hashword2() -- same as hashword(), but take two seeds and return two -32-bit values. pc and pb must both be nonnull, and *pc and *pb must -both be initialized with seeds. If you pass in (*pb)==0, the output -(*pc) will be the same as the return value from hashword(). --------------------------------------------------------------------- -*/ -void hashword2 ( -const uint32 *k, /* the key, an array of uint32_t values */ -size_t length, /* the length of the key, in uint32_ts */ -uint32 *pc, /* IN: seed OUT: primary hash value */ -uint32 *pb) /* IN: more seed OUT: secondary hash value */ -{ - uint32_t a,b,c; - - /* Set up the internal state */ - a = b = c = 0xdeadbeef + ((uint32_t)(length<<2)) + *pc; - c += *pb; - - /*------------------------------------------------- handle most of the key */ - while (length > 3) - { - a += k[0]; - b += k[1]; - c += k[2]; - mix(a,b,c); - length -= 3; - k += 3; - } - - /*------------------------------------------- handle the last 3 uint32_t's */ - switch(length) /* all the case statements fall through */ - { - case 3 : c+=k[2]; - case 2 : b+=k[1]; - case 1 : a+=k[0]; - final(a,b,c); - case 0: /* case 0: nothing left to add */ - break; - } - /*------------------------------------------------------ report the result */ - *pc=c; *pb=b; -} - -} // namespace re2 |