lucene/codecs/src/java/org/apache/lucene/codecs/bloom/MurmurHash2.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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.apache.lucene.codecs.bloom;

import org.apache.lucene.util.BytesRef;

/**
 * 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>
 * <p>
 *  The code from getopt.org was adapted by Mark Harwood in the form here as one of a pluggable choice of 
 *  hashing functions as the core function had to be adapted to work with BytesRefs with offsets and lengths
 *  rather than raw byte arrays.  
 * </p>
 * @lucene.experimental
 */
public final class MurmurHash2 extends HashFunction{

  public static final MurmurHash2 INSTANCE = new MurmurHash2();

  private MurmurHash2() {}

  public static int hash(byte[] data, int seed, int offset, int len) {
    int m = 0x5bd1e995;
    int r = 24;
    int h = seed ^ len;
    int len_4 = len >> 2;
    for (int i = 0; i < len_4; i++) {
      int i_4 = offset + (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;
    }
    int len_m = len_4 << 2;
    int left = len - len_m;
    if (left != 0) {
      if (left >= 3) {
        h ^= data[offset + len - 3] << 16;
      }
      if (left >= 2) {
        h ^= data[offset + len - 2] << 8;
      }
      if (left >= 1) {
        h ^= data[offset + len - 1];
      }
      h *= m;
    }
    h ^= h >>> 13;
    h *= m;
    h ^= h >>> 15;
    return h;
  }
  
  /**
   * Generates 32 bit hash from byte array with default seed value.
   * 
   * @param data 
   *          byte array to hash
   * @param offset
   *          the start position in the array to hash
   * @param len
   *          length of the array elements to hash
   * @return 32 bit hash of the given array
   */
  public static final int hash32(final byte[] data, int offset, int len) {
    return MurmurHash2.hash(data, 0x9747b28c, offset, len);
  }
  

  @Override
  public final int hash(BytesRef br) {
    return hash32(br.bytes, br.offset, br.length);
  }

  @Override
  public String toString() {
    return getClass().getSimpleName();
  }  
}