1 /**
2 * Copyright 2010 The Apache Software Foundation
3 *
4 * Licensed to the Apache Software Foundation (ASF) under one
5 * or more contributor license agreements. See the NOTICE file
6 * distributed with this work for additional information
7 * regarding copyright ownership. The ASF licenses this file
8 * to you under the Apache License, Version 2.0 (the
9 * "License"); you may not use this file except in compliance
10 * with the License. You may obtain a copy of the License at
11 *
12 * http://www.apache.org/licenses/LICENSE-2.0
13 *
14 * Unless required by applicable law or agreed to in writing, software
15 * distributed under the License is distributed on an "AS IS" BASIS,
16 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
17 * See the License for the specific language governing permissions and
18 * limitations under the License.
19 */
20
21 package org.apache.hadoop.hbase.util;
22
23 import static java.lang.Integer.rotateLeft;
24
25 import java.io.FileInputStream;
26 import java.io.IOException;
27
28 /**
29 * Produces 32-bit hash for hash table lookup.
30 *
31 * <pre>lookup3.c, by Bob Jenkins, May 2006, Public Domain.
32 *
33 * You can use this free for any purpose. It's in the public domain.
34 * It has no warranty.
35 * </pre>
36 *
37 * @see <a href="http://burtleburtle.net/bob/c/lookup3.c">lookup3.c</a>
38 * @see <a href="http://www.ddj.com/184410284">Hash Functions (and how this
39 * function compares to others such as CRC, MD?, etc</a>
40 * @see <a href="http://burtleburtle.net/bob/hash/doobs.html">Has update on the
41 * Dr. Dobbs Article</a>
42 */
43 public class JenkinsHash extends Hash {
44 private static final int BYTE_MASK = 0xff;
45
46 private static JenkinsHash _instance = new JenkinsHash();
47
48 public static Hash getInstance() {
49 return _instance;
50 }
51
52 /**
53 * taken from hashlittle() -- hash a variable-length key into a 32-bit value
54 *
55 * @param key the key (the unaligned variable-length array of bytes)
56 * @param nbytes number of bytes to include in hash
57 * @param initval can be any integer value
58 * @return a 32-bit value. Every bit of the key affects every bit of the
59 * return value. Two keys differing by one or two bits will have totally
60 * different hash values.
61 *
62 * <p>The best hash table sizes are powers of 2. There is no need to do mod
63 * a prime (mod is sooo slow!). If you need less than 32 bits, use a bitmask.
64 * For example, if you need only 10 bits, do
65 * <code>h = (h & hashmask(10));</code>
66 * In which case, the hash table should have hashsize(10) elements.
67 *
68 * <p>If you are hashing n strings byte[][] k, do it like this:
69 * for (int i = 0, h = 0; i < n; ++i) h = hash( k[i], h);
70 *
71 * <p>By Bob Jenkins, 2006. bob_jenkins@burtleburtle.net. You may use this
72 * code any way you wish, private, educational, or commercial. It's free.
73 *
74 * <p>Use for hash table lookup, or anything where one collision in 2^^32 is
75 * acceptable. Do NOT use for cryptographic purposes.
76 */
77 @Override
78 @SuppressWarnings("fallthrough")
79 public int hash(byte[] key, int off, int nbytes, int initval) {
80 int length = nbytes;
81 int a, b, c;
82 a = b = c = 0xdeadbeef + length + initval;
83 int offset = off;
84 for (; length > 12; offset += 12, length -= 12) {
85 a += (key[offset] & BYTE_MASK);
86 a += ((key[offset + 1] & BYTE_MASK) << 8);
87 a += ((key[offset + 2] & BYTE_MASK) << 16);
88 a += ((key[offset + 3] & BYTE_MASK) << 24);
89 b += (key[offset + 4] & BYTE_MASK);
90 b += ((key[offset + 5] & BYTE_MASK) << 8);
91 b += ((key[offset + 6] & BYTE_MASK) << 16);
92 b += ((key[offset + 7] & BYTE_MASK) << 24);
93 c += (key[offset + 8] & BYTE_MASK);
94 c += ((key[offset + 9] & BYTE_MASK) << 8);
95 c += ((key[offset + 10] & BYTE_MASK) << 16);
96 c += ((key[offset + 11] & BYTE_MASK) << 24);
97
98 /*
99 * mix -- mix 3 32-bit values reversibly.
100 * This is reversible, so any information in (a,b,c) before mix() is
101 * still in (a,b,c) after mix().
102 *
103 * If four pairs of (a,b,c) inputs are run through mix(), or through
104 * mix() in reverse, there are at least 32 bits of the output that
105 * are sometimes the same for one pair and different for another pair.
106 *
107 * This was tested for:
108 * - pairs that differed by one bit, by two bits, in any combination
109 * of top bits of (a,b,c), or in any combination of bottom bits of
110 * (a,b,c).
111 * - "differ" is defined as +, -, ^, or ~^. For + and -, I transformed
112 * the output delta to a Gray code (a^(a>>1)) so a string of 1's (as
113 * is commonly produced by subtraction) look like a single 1-bit
114 * difference.
115 * - the base values were pseudorandom, all zero but one bit set, or
116 * all zero plus a counter that starts at zero.
117 *
118 * Some k values for my "a-=c; a^=rot(c,k); c+=b;" arrangement that
119 * satisfy this are
120 * 4 6 8 16 19 4
121 * 9 15 3 18 27 15
122 * 14 9 3 7 17 3
123 * Well, "9 15 3 18 27 15" didn't quite get 32 bits diffing for
124 * "differ" defined as + with a one-bit base and a two-bit delta. I
125 * used http://burtleburtle.net/bob/hash/avalanche.html to choose
126 * the operations, constants, and arrangements of the variables.
127 *
128 * This does not achieve avalanche. There are input bits of (a,b,c)
129 * that fail to affect some output bits of (a,b,c), especially of a.
130 * The most thoroughly mixed value is c, but it doesn't really even
131 * achieve avalanche in c.
132 *
133 * This allows some parallelism. Read-after-writes are good at doubling
134 * the number of bits affected, so the goal of mixing pulls in the
135 * opposite direction as the goal of parallelism. I did what I could.
136 * Rotates seem to cost as much as shifts on every machine I could lay
137 * my hands on, and rotates are much kinder to the top and bottom bits,
138 * so I used rotates.
139 *
140 * #define mix(a,b,c) \
141 * { \
142 * a -= c; a ^= rot(c, 4); c += b; \
143 * b -= a; b ^= rot(a, 6); a += c; \
144 * c -= b; c ^= rot(b, 8); b += a; \
145 * a -= c; a ^= rot(c,16); c += b; \
146 * b -= a; b ^= rot(a,19); a += c; \
147 * c -= b; c ^= rot(b, 4); b += a; \
148 * }
149 *
150 * mix(a,b,c);
151 */
152 a -= c; a ^= rotateLeft(c, 4); c += b;
153 b -= a; b ^= rotateLeft(a, 6); a += c;
154 c -= b; c ^= rotateLeft(b, 8); b += a;
155 a -= c; a ^= rotateLeft(c, 16); c += b;
156 b -= a; b ^= rotateLeft(a, 19); a += c;
157 c -= b; c ^= rotateLeft(b, 4); b += a;
158 }
159
160 //-------------------------------- last block: affect all 32 bits of (c)
161 switch (length) { // all the case statements fall through
162 case 12:
163 c += ((key[offset + 11] & BYTE_MASK) << 24);
164 case 11:
165 c += ((key[offset + 10] & BYTE_MASK) << 16);
166 case 10:
167 c += ((key[offset + 9] & BYTE_MASK) << 8);
168 case 9:
169 c += (key[offset + 8] & BYTE_MASK);
170 case 8:
171 b += ((key[offset + 7] & BYTE_MASK) << 24);
172 case 7:
173 b += ((key[offset + 6] & BYTE_MASK) << 16);
174 case 6:
175 b += ((key[offset + 5] & BYTE_MASK) << 8);
176 case 5:
177 b += (key[offset + 4] & BYTE_MASK);
178 case 4:
179 a += ((key[offset + 3] & BYTE_MASK) << 24);
180 case 3:
181 a += ((key[offset + 2] & BYTE_MASK) << 16);
182 case 2:
183 a += ((key[offset + 1] & BYTE_MASK) << 8);
184 case 1:
185 //noinspection PointlessArithmeticExpression
186 a += (key[offset + 0] & BYTE_MASK);
187 break;
188 case 0:
189 return c;
190 }
191 /*
192 * final -- final mixing of 3 32-bit values (a,b,c) into c
193 *
194 * Pairs of (a,b,c) values differing in only a few bits will usually
195 * produce values of c that look totally different. This was tested for
196 * - pairs that differed by one bit, by two bits, in any combination
197 * of top bits of (a,b,c), or in any combination of bottom bits of
198 * (a,b,c).
199 *
200 * - "differ" is defined as +, -, ^, or ~^. For + and -, I transformed
201 * the output delta to a Gray code (a^(a>>1)) so a string of 1's (as
202 * is commonly produced by subtraction) look like a single 1-bit
203 * difference.
204 *
205 * - the base values were pseudorandom, all zero but one bit set, or
206 * all zero plus a counter that starts at zero.
207 *
208 * These constants passed:
209 * 14 11 25 16 4 14 24
210 * 12 14 25 16 4 14 24
211 * and these came close:
212 * 4 8 15 26 3 22 24
213 * 10 8 15 26 3 22 24
214 * 11 8 15 26 3 22 24
215 *
216 * #define final(a,b,c) \
217 * {
218 * c ^= b; c -= rot(b,14); \
219 * a ^= c; a -= rot(c,11); \
220 * b ^= a; b -= rot(a,25); \
221 * c ^= b; c -= rot(b,16); \
222 * a ^= c; a -= rot(c,4); \
223 * b ^= a; b -= rot(a,14); \
224 * c ^= b; c -= rot(b,24); \
225 * }
226 *
227 */
228 c ^= b; c -= rotateLeft(b, 14);
229 a ^= c; a -= rotateLeft(c, 11);
230 b ^= a; b -= rotateLeft(a, 25);
231 c ^= b; c -= rotateLeft(b, 16);
232 a ^= c; a -= rotateLeft(c, 4);
233 b ^= a; b -= rotateLeft(a, 14);
234 c ^= b; c -= rotateLeft(b, 24);
235 return c;
236 }
237
238 /**
239 * Compute the hash of the specified file
240 * @param args name of file to compute hash of.
241 * @throws IOException e
242 */
243 public static void main(String[] args) throws IOException {
244 if (args.length != 1) {
245 System.err.println("Usage: JenkinsHash filename");
246 System.exit(-1);
247 }
248 FileInputStream in = new FileInputStream(args[0]);
249 byte[] bytes = new byte[512];
250 int value = 0;
251 JenkinsHash hash = new JenkinsHash();
252 for (int length = in.read(bytes); length > 0; length = in.read(bytes)) {
253 value = hash.hash(bytes, length, value);
254 }
255 System.out.println(Math.abs(value));
256 }
257 }