forked from Qortal/Brooklyn
162 lines
6.8 KiB
C
162 lines
6.8 KiB
C
/*
|
|
Copyright (c) 2012, Broadcom Europe Ltd
|
|
All rights reserved.
|
|
|
|
Redistribution and use in source and binary forms, with or without
|
|
modification, are permitted provided that the following conditions are met:
|
|
* Redistributions of source code must retain the above copyright
|
|
notice, this list of conditions and the following disclaimer.
|
|
* Redistributions in binary form must reproduce the above copyright
|
|
notice, this list of conditions and the following disclaimer in the
|
|
documentation and/or other materials provided with the distribution.
|
|
* Neither the name of the copyright holder nor the
|
|
names of its contributors may be used to endorse or promote products
|
|
derived from this software without specific prior written permission.
|
|
|
|
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
|
|
ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
|
|
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
|
|
DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY
|
|
DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
|
|
(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
|
|
LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
|
|
ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
|
|
(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
|
|
SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
|
|
*/
|
|
|
|
#ifndef KHRN_INT_HASH_H
|
|
#define KHRN_INT_HASH_H
|
|
|
|
/*
|
|
-------------------------------------------------------------------------------
|
|
These are functions for producing 32-bit hashes for hash table lookup.
|
|
khrn_hashword(), khrn_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 khrn_hashlittle(). khrn_hashlittle() and hashbig()
|
|
hash byte arrays. khrn_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
|
|
khrn_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 khrn_hashword(). If you have a byte array (like
|
|
a character string), use khrn_hashlittle(). If you have several byte arrays, or
|
|
a mix of things, see the comments above khrn_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 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) \
|
|
do { \
|
|
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; \
|
|
} while (0)
|
|
|
|
/*
|
|
-------------------------------------------------------------------------------
|
|
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) \
|
|
do { \
|
|
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); \
|
|
} while (0)
|
|
|
|
uint32_t khrn_hashword(const uint32_t *key, int length, uint32_t initval);
|
|
uint32_t khrn_hashlittle(const void *key, int length, uint32_t initval);
|
|
|
|
#endif
|
|
|