Mega Code Archive
Produces 32-bit hash for hash table lookup (Jenkins Hash Function)
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/*
* Copyright 2008-2010 the T2 Project ant the Others.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
//package org.t2framework.commons.util;
/**
* Produces 32-bit hash for hash table lookup.
*
* <pre>
* lookup3.c, by Bob Jenkins, May 2006, Public Domain.
* You can use this free for any purpose. It's in the public domain.
* It has no warranty.
* </pre>
*
* @see <a href="http://burtleburtle.net/bob/c/lookup3.c">lookup3.c</a>
* @see <a href="http://www.ddj.com/184410284">Hash Functions (and how this
* function compares to others such as CRC, MD?, etc</a>
* @see <a href="http://burtleburtle.net/bob/hash/doobs.html">Has update on the
* Dr. Dobbs Article</a>
*/
public class JenkinsHashFunction extends AbstractHashFunction {
private static long INT_MASK = 0x00000000ffffffffL;
private static long BYTE_MASK = 0x00000000000000ffL;
private static long rot(long val, int pos) {
return ((Integer.rotateLeft((int) (val & INT_MASK), pos)) & INT_MASK);
}
/**
* taken from hashlittle() -- hash a variable-length key into a 32-bit value
*
* @param key
* the key (the unaligned variable-length array of bytes)
* @param nbytes
* number of bytes to include in hash
* @param initval
* can be any integer value
* @return 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.
*
* <p>
* 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
* <code>h = (h & hashmask(10));</code> In which case, the hash
* table should have hashsize(10) elements.
*
* <p>
* If you are hashing n strings byte[][] k, do it like this: for
* (int i = 0, h = 0; i < n; ++i) h = hash( k[i], h);
*
* <p>
* By Bob Jenkins, 2006. bob_jenkins@burtleburtle.net. You may use
* this code any way you wish, private, educational, or commercial.
* It's free.
*
* <p>
* Use for hash table lookup, or anything where one collision in
* 2^^32 is acceptable. Do NOT use for cryptographic purposes.
*/
@SuppressWarnings("fallthrough")
public int hash(byte[] key, int nbytes, int initval) {
int length = nbytes;
long a, b, c; // We use longs because we don't have unsigned ints
a = b = c = (0x00000000deadbeefL + length + initval) & INT_MASK;
int offset = 0;
for (; length > 12; offset += 12, length -= 12) {
a = (a + (key[offset + 0] & BYTE_MASK)) & INT_MASK;
a = (a + (((key[offset + 1] & BYTE_MASK) << 8) & INT_MASK))
& INT_MASK;
a = (a + (((key[offset + 2] & BYTE_MASK) << 16) & INT_MASK))
& INT_MASK;
a = (a + (((key[offset + 3] & BYTE_MASK) << 24) & INT_MASK))
& INT_MASK;
b = (b + (key[offset + 4] & BYTE_MASK)) & INT_MASK;
b = (b + (((key[offset + 5] & BYTE_MASK) << 8) & INT_MASK))
& INT_MASK;
b = (b + (((key[offset + 6] & BYTE_MASK) << 16) & INT_MASK))
& INT_MASK;
b = (b + (((key[offset + 7] & BYTE_MASK) << 24) & INT_MASK))
& INT_MASK;
c = (c + (key[offset + 8] & BYTE_MASK)) & INT_MASK;
c = (c + (((key[offset + 9] & BYTE_MASK) << 8) & INT_MASK))
& INT_MASK;
c = (c + (((key[offset + 10] & BYTE_MASK) << 16) & INT_MASK))
& INT_MASK;
c = (c + (((key[offset + 11] & BYTE_MASK) << 24) & INT_MASK))
& INT_MASK;
/*
* 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; \ }
*
* mix(a,b,c);
*/
a = (a - c) & INT_MASK;
a ^= rot(c, 4);
c = (c + b) & INT_MASK;
b = (b - a) & INT_MASK;
b ^= rot(a, 6);
a = (a + c) & INT_MASK;
c = (c - b) & INT_MASK;
c ^= rot(b, 8);
b = (b + a) & INT_MASK;
a = (a - c) & INT_MASK;
a ^= rot(c, 16);
c = (c + b) & INT_MASK;
b = (b - a) & INT_MASK;
b ^= rot(a, 19);
a = (a + c) & INT_MASK;
c = (c - b) & INT_MASK;
c ^= rot(b, 4);
b = (b + a) & INT_MASK;
}
// -------------------------------- last block: affect all 32 bits of
// (c)
switch (length) { // all the case statements fall through
case 12:
c = (c + (((key[offset + 11] & BYTE_MASK) << 24) & INT_MASK))
& INT_MASK;
case 11:
c = (c + (((key[offset + 10] & BYTE_MASK) << 16) & INT_MASK))
& INT_MASK;
case 10:
c = (c + (((key[offset + 9] & BYTE_MASK) << 8) & INT_MASK))
& INT_MASK;
case 9:
c = (c + (key[offset + 8] & BYTE_MASK)) & INT_MASK;
case 8:
b = (b + (((key[offset + 7] & BYTE_MASK) << 24) & INT_MASK))
& INT_MASK;
case 7:
b = (b + (((key[offset + 6] & BYTE_MASK) << 16) & INT_MASK))
& INT_MASK;
case 6:
b = (b + (((key[offset + 5] & BYTE_MASK) << 8) & INT_MASK))
& INT_MASK;
case 5:
b = (b + (key[offset + 4] & BYTE_MASK)) & INT_MASK;
case 4:
a = (a + (((key[offset + 3] & BYTE_MASK) << 24) & INT_MASK))
& INT_MASK;
case 3:
a = (a + (((key[offset + 2] & BYTE_MASK) << 16) & INT_MASK))
& INT_MASK;
case 2:
a = (a + (((key[offset + 1] & BYTE_MASK) << 8) & INT_MASK))
& INT_MASK;
case 1:
a = (a + (key[offset + 0] & BYTE_MASK)) & INT_MASK;
break;
case 0:
return (int) (c & INT_MASK);
}
/*
* 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);
* \ }
*/
c ^= b;
c = (c - rot(b, 14)) & INT_MASK;
a ^= c;
a = (a - rot(c, 11)) & INT_MASK;
b ^= a;
b = (b - rot(a, 25)) & INT_MASK;
c ^= b;
c = (c - rot(b, 16)) & INT_MASK;
a ^= c;
a = (a - rot(c, 4)) & INT_MASK;
b ^= a;
b = (b - rot(a, 14)) & INT_MASK;
c ^= b;
c = (c - rot(b, 24)) & INT_MASK;
return (int) (c & INT_MASK);
}
}
abstract class AbstractHashFunction implements HashFunction {
public int hash(byte[] bytes, int initval) {
return hash(bytes, bytes.length, initval);
}
public int hash(byte[] bytes) {
return hash(bytes, bytes.length, -1);
}
}
interface HashFunction {
int hash(byte[] bytes);
int hash(byte[] bytes, int initval);
int hash(byte[] bytes, int length, int initval);
}
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