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.hbase.classification.InterfaceAudience; 28 import org.apache.hadoop.hbase.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.Private 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 try { 257 for (int length = in.read(bytes); length > 0; length = in.read(bytes)) { 258 value = hash.hash(bytes, length, value); 259 } 260 } finally { 261 in.close(); 262 } 263 System.out.println(Math.abs(value)); 264 } 265 }