Java tutorial
/* * 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); }