1 /**
2 *
3 * Licensed to the Apache Software Foundation (ASF) under one
4 * or more contributor license agreements. See the NOTICE file
5 * distributed with this work for additional information
6 * regarding copyright ownership. The ASF licenses this file
7 * to you under the Apache License, Version 2.0 (the
8 * "License"); you may not use this file except in compliance
9 * with the License. You may obtain a copy of the License at
10 *
11 * http://www.apache.org/licenses/LICENSE-2.0
12 *
13 * Unless required by applicable law or agreed to in writing, software
14 * distributed under the License is distributed on an "AS IS" BASIS,
15 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
16 * See the License for the specific language governing permissions and
17 * limitations under the License.
18 */
19
20 package org.apache.hadoop.hbase.util;
21
22 import static java.lang.Integer.rotateLeft;
23
24 import java.io.FileInputStream;
25 import java.io.IOException;
26
27 import org.apache.hadoop.classification.InterfaceAudience;
28 import org.apache.hadoop.classification.InterfaceStability;
29
30 /**
31 * Produces 32-bit hash for hash table lookup.
32 *
33 * <pre>lookup3.c, by Bob Jenkins, May 2006, Public Domain.
34 *
35 * You can use this free for any purpose. It's in the public domain.
36 * It has no warranty.
37 * </pre>
38 *
39 * @see <a href="http://burtleburtle.net/bob/c/lookup3.c">lookup3.c</a>
40 * @see <a href="http://www.ddj.com/184410284">Hash Functions (and how this
41 * function compares to others such as CRC, MD?, etc</a>
42 * @see <a href="http://burtleburtle.net/bob/hash/doobs.html">Has update on the
43 * Dr. Dobbs Article</a>
44 */
45 @InterfaceAudience.Public
46 @InterfaceStability.Stable
47 public class JenkinsHash extends Hash {
48 private static final int BYTE_MASK = 0xff;
49
50 private static JenkinsHash _instance = new JenkinsHash();
51
52 public static Hash getInstance() {
53 return _instance;
54 }
55
56 /**
57 * taken from hashlittle() -- hash a variable-length key into a 32-bit value
58 *
59 * @param key the key (the unaligned variable-length array of bytes)
60 * @param nbytes number of bytes to include in hash
61 * @param initval can be any integer value
62 * @return a 32-bit value. Every bit of the key affects every bit of the
63 * return value. Two keys differing by one or two bits will have totally
64 * different hash values.
65 *
66 * <p>The best hash table sizes are powers of 2. There is no need to do mod
67 * a prime (mod is sooo slow!). If you need less than 32 bits, use a bitmask.
68 * For example, if you need only 10 bits, do
69 * <code>h = (h & hashmask(10));</code>
70 * In which case, the hash table should have hashsize(10) elements.
71 *
72 * <p>If you are hashing n strings byte[][] k, do it like this:
73 * for (int i = 0, h = 0; i < n; ++i) h = hash( k[i], h);
74 *
75 * <p>By Bob Jenkins, 2006. bob_jenkins@burtleburtle.net. You may use this
76 * code any way you wish, private, educational, or commercial. It's free.
77 *
78 * <p>Use for hash table lookup, or anything where one collision in 2^^32 is
79 * acceptable. Do NOT use for cryptographic purposes.
80 */
81 @Override
82 @SuppressWarnings("fallthrough")
83 public int hash(byte[] key, int off, int nbytes, int initval) {
84 int length = nbytes;
85 int a, b, c;
86 a = b = c = 0xdeadbeef + length + initval;
87 int offset = off;
88 for (; length > 12; offset += 12, length -= 12) {
89 a += (key[offset] & BYTE_MASK);
90 a += ((key[offset + 1] & BYTE_MASK) << 8);
91 a += ((key[offset + 2] & BYTE_MASK) << 16);
92 a += ((key[offset + 3] & BYTE_MASK) << 24);
93 b += (key[offset + 4] & BYTE_MASK);
94 b += ((key[offset + 5] & BYTE_MASK) << 8);
95 b += ((key[offset + 6] & BYTE_MASK) << 16);
96 b += ((key[offset + 7] & BYTE_MASK) << 24);
97 c += (key[offset + 8] & BYTE_MASK);
98 c += ((key[offset + 9] & BYTE_MASK) << 8);
99 c += ((key[offset + 10] & BYTE_MASK) << 16);
100 c += ((key[offset + 11] & BYTE_MASK) << 24);
101
102 /*
103 * mix -- mix 3 32-bit values reversibly.
104 * This is reversible, so any information in (a,b,c) before mix() is
105 * still in (a,b,c) after mix().
106 *
107 * If four pairs of (a,b,c) inputs are run through mix(), or through
108 * mix() in reverse, there are at least 32 bits of the output that
109 * are sometimes the same for one pair and different for another pair.
110 *
111 * This was tested for:
112 * - pairs that differed by one bit, by two bits, in any combination
113 * of top bits of (a,b,c), or in any combination of bottom bits of
114 * (a,b,c).
115 * - "differ" is defined as +, -, ^, or ~^. For + and -, I transformed
116 * the output delta to a Gray code (a^(a>>1)) so a string of 1's (as
117 * is commonly produced by subtraction) look like a single 1-bit
118 * difference.
119 * - the base values were pseudorandom, all zero but one bit set, or
120 * all zero plus a counter that starts at zero.
121 *
122 * Some k values for my "a-=c; a^=rot(c,k); c+=b;" arrangement that
123 * satisfy this are
124 * 4 6 8 16 19 4
125 * 9 15 3 18 27 15
126 * 14 9 3 7 17 3
127 * Well, "9 15 3 18 27 15" didn't quite get 32 bits diffing for
128 * "differ" defined as + with a one-bit base and a two-bit delta. I
129 * used http://burtleburtle.net/bob/hash/avalanche.html to choose
130 * the operations, constants, and arrangements of the variables.
131 *
132 * This does not achieve avalanche. There are input bits of (a,b,c)
133 * that fail to affect some output bits of (a,b,c), especially of a.
134 * The most thoroughly mixed value is c, but it doesn't really even
135 * achieve avalanche in c.
136 *
137 * This allows some parallelism. Read-after-writes are good at doubling
138 * the number of bits affected, so the goal of mixing pulls in the
139 * opposite direction as the goal of parallelism. I did what I could.
140 * Rotates seem to cost as much as shifts on every machine I could lay
141 * my hands on, and rotates are much kinder to the top and bottom bits,
142 * so I used rotates.
143 *
144 * #define mix(a,b,c) \
145 * { \
146 * a -= c; a ^= rot(c, 4); c += b; \
147 * b -= a; b ^= rot(a, 6); a += c; \
148 * c -= b; c ^= rot(b, 8); b += a; \
149 * a -= c; a ^= rot(c,16); c += b; \
150 * b -= a; b ^= rot(a,19); a += c; \
151 * c -= b; c ^= rot(b, 4); b += a; \
152 * }
153 *
154 * mix(a,b,c);
155 */
156 a -= c; a ^= rotateLeft(c, 4); c += b;
157 b -= a; b ^= rotateLeft(a, 6); a += c;
158 c -= b; c ^= rotateLeft(b, 8); b += a;
159 a -= c; a ^= rotateLeft(c, 16); c += b;
160 b -= a; b ^= rotateLeft(a, 19); a += c;
161 c -= b; c ^= rotateLeft(b, 4); b += a;
162 }
163
164 //-------------------------------- last block: affect all 32 bits of (c)
165 switch (length) { // all the case statements fall through
166 case 12:
167 c += ((key[offset + 11] & BYTE_MASK) << 24);
168 case 11:
169 c += ((key[offset + 10] & BYTE_MASK) << 16);
170 case 10:
171 c += ((key[offset + 9] & BYTE_MASK) << 8);
172 case 9:
173 c += (key[offset + 8] & BYTE_MASK);
174 case 8:
175 b += ((key[offset + 7] & BYTE_MASK) << 24);
176 case 7:
177 b += ((key[offset + 6] & BYTE_MASK) << 16);
178 case 6:
179 b += ((key[offset + 5] & BYTE_MASK) << 8);
180 case 5:
181 b += (key[offset + 4] & BYTE_MASK);
182 case 4:
183 a += ((key[offset + 3] & BYTE_MASK) << 24);
184 case 3:
185 a += ((key[offset + 2] & BYTE_MASK) << 16);
186 case 2:
187 a += ((key[offset + 1] & BYTE_MASK) << 8);
188 case 1:
189 //noinspection PointlessArithmeticExpression
190 a += (key[offset + 0] & BYTE_MASK);
191 break;
192 case 0:
193 return c;
194 }
195 /*
196 * final -- final mixing of 3 32-bit values (a,b,c) into c
197 *
198 * Pairs of (a,b,c) values differing in only a few bits will usually
199 * produce values of c that look totally different. This was tested for
200 * - pairs that differed by one bit, by two bits, in any combination
201 * of top bits of (a,b,c), or in any combination of bottom bits of
202 * (a,b,c).
203 *
204 * - "differ" is defined as +, -, ^, or ~^. For + and -, I transformed
205 * the output delta to a Gray code (a^(a>>1)) so a string of 1's (as
206 * is commonly produced by subtraction) look like a single 1-bit
207 * difference.
208 *
209 * - the base values were pseudorandom, all zero but one bit set, or
210 * all zero plus a counter that starts at zero.
211 *
212 * These constants passed:
213 * 14 11 25 16 4 14 24
214 * 12 14 25 16 4 14 24
215 * and these came close:
216 * 4 8 15 26 3 22 24
217 * 10 8 15 26 3 22 24
218 * 11 8 15 26 3 22 24
219 *
220 * #define final(a,b,c) \
221 * {
222 * c ^= b; c -= rot(b,14); \
223 * a ^= c; a -= rot(c,11); \
224 * b ^= a; b -= rot(a,25); \
225 * c ^= b; c -= rot(b,16); \
226 * a ^= c; a -= rot(c,4); \
227 * b ^= a; b -= rot(a,14); \
228 * c ^= b; c -= rot(b,24); \
229 * }
230 *
231 */
232 c ^= b; c -= rotateLeft(b, 14);
233 a ^= c; a -= rotateLeft(c, 11);
234 b ^= a; b -= rotateLeft(a, 25);
235 c ^= b; c -= rotateLeft(b, 16);
236 a ^= c; a -= rotateLeft(c, 4);
237 b ^= a; b -= rotateLeft(a, 14);
238 c ^= b; c -= rotateLeft(b, 24);
239 return c;
240 }
241
242 /**
243 * Compute the hash of the specified file
244 * @param args name of file to compute hash of.
245 * @throws IOException e
246 */
247 public static void main(String[] args) throws IOException {
248 if (args.length != 1) {
249 System.err.println("Usage: JenkinsHash filename");
250 System.exit(-1);
251 }
252 FileInputStream in = new FileInputStream(args[0]);
253 byte[] bytes = new byte[512];
254 int value = 0;
255 JenkinsHash hash = new JenkinsHash();
256 for (int length = in.read(bytes); length > 0; length = in.read(bytes)) {
257 value = hash.hash(bytes, length, value);
258 }
259 System.out.println(Math.abs(value));
260 }
261 }