Produces 32-bit hash for hash table lookup. (Jenkins Hash Function) : Hash Code « Development Class « Java






Produces 32-bit hash for hash table lookup. (Jenkins Hash Function)

      
/*
 * 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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