// 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 |