001/*
002 * Licensed to the Apache Software Foundation (ASF) under one
003 * or more contributor license agreements.  See the NOTICE file
004 * distributed with this work for additional information
005 * regarding copyright ownership.  The ASF licenses this file
006 * to you under the Apache License, Version 2.0 (the
007 * "License"); you may not use this file except in compliance
008 * with the License.  You may obtain a copy of the License at
009 *
010 *     http://www.apache.org/licenses/LICENSE-2.0
011 *
012 * Unless required by applicable law or agreed to in writing, software
013 * distributed under the License is distributed on an "AS IS" BASIS,
014 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
015 * See the License for the specific language governing permissions and
016 * limitations under the License.
017 */
018package org.apache.hadoop.hbase.util;
019
020import org.apache.yetus.audience.InterfaceAudience;
021import org.apache.yetus.audience.InterfaceStability;
022
023/**
024 * This is a very fast, non-cryptographic hash suitable for general hash-based lookup. See
025 * http://murmurhash.googlepages.com/ for more details.
026 * <p>
027 * The C version of MurmurHash 2.0 found at that site was ported to Java by Andrzej Bialecki (ab at
028 * getopt org).
029 * </p>
030 */
031@InterfaceAudience.Private
032@InterfaceStability.Stable
033public class MurmurHash extends Hash {
034  private static MurmurHash _instance = new MurmurHash();
035
036  public static Hash getInstance() {
037    return _instance;
038  }
039
040  @Override
041  public <T> int hash(HashKey<T> hashKey, int seed) {
042    int m = 0x5bd1e995;
043    int r = 24;
044    int length = hashKey.length();
045    int h = seed ^ length;
046
047    int len_4 = length >> 2;
048
049    for (int i = 0; i < len_4; i++) {
050      int k = hashKey.getIntLE(i << 2);
051      k *= m;
052      k ^= k >>> r;
053      k *= m;
054      h *= m;
055      h ^= k;
056    }
057
058    // avoid calculating modulo
059    int len_m = len_4 << 2;
060    int left = length - len_m;
061    int i_m = len_m;
062
063    if (left != 0) {
064      if (left >= 3) {
065        h ^= hashKey.get(i_m + 2) << 16;
066      }
067      if (left >= 2) {
068        h ^= hashKey.get(i_m + 1) << 8;
069      }
070      if (left >= 1) {
071        h ^= hashKey.get(i_m);
072      }
073
074      h *= m;
075    }
076
077    h ^= h >>> 13;
078    h *= m;
079    h ^= h >>> 15;
080
081    return h;
082  }
083}