This is a very fast, non-cryptographic hash suitable for general hash-based lookup. : Hash Code « Development Class « Java






This is a very fast, non-cryptographic hash suitable for general hash-based lookup.

      
/*
 * 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;

/**
 * This is a very fast, non-cryptographic hash suitable for general hash-based
 * lookup. See http://murmurhash.googlepages.com/ for more details.
 * 
 * <p>
 * The C version of MurmurHash 2.0 found at that site was ported to Java by
 * Andrzej Bialecki (ab at getopt org).
 * </p>
 * 
 * This is suitable for 32bit application, if 64bit we need to improve it.
 * 
 * @see http://murmurhash.googlepages.com/
 */
public class MurmurHashFunction extends AbstractHashFunction {

  public int hash(byte[] data, int length, int seed) {
    int m = 0x5bd1e995;
    int r = 24;
    int h = seed ^ length;
    int len_4 = length >> 2;

    for (int i = 0; i < len_4; i++) {
      int i_4 = i << 2;
      int k = data[i_4 + 3];
      k = k << 8;
      k = k | (data[i_4 + 2] & 0xff);
      k = k << 8;
      k = k | (data[i_4 + 1] & 0xff);
      k = k << 8;
      k = k | (data[i_4 + 0] & 0xff);
      k *= m;
      k ^= k >>> r;
      k *= m;
      h *= m;
      h ^= k;
    }

    // avoid calculating modulo
    int len_m = len_4 << 2;
    int left = length - len_m;

    if (left != 0) {
      if (left >= 3) {
        h ^= (int) data[length - 3] << 16;
      }
      if (left >= 2) {
        h ^= (int) data[length - 2] << 8;
      }
      if (left >= 1) {
        h ^= (int) data[length - 1];
      }

      h *= m;
    }

    h ^= h >>> 13;
    h *= m;
    h ^= h >>> 15;

    return h;
  }
}
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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