001/**
002 *
003 * Licensed to the Apache Software Foundation (ASF) under one
004 * or more contributor license agreements.  See the NOTICE file
005 * distributed with this work for additional information
006 * regarding copyright ownership.  The ASF licenses this file
007 * to you under the Apache License, Version 2.0 (the
008 * "License"); you may not use this file except in compliance
009 * with the License.  You may obtain a copy of the License at
010 *
011 *     http://www.apache.org/licenses/LICENSE-2.0
012 *
013 * Unless required by applicable law or agreed to in writing, software
014 * distributed under the License is distributed on an "AS IS" BASIS,
015 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
016 * See the License for the specific language governing permissions and
017 * limitations under the License.
018 */
019
020package org.apache.hadoop.hbase.util;
021
022import org.apache.yetus.audience.InterfaceAudience;
023import org.apache.yetus.audience.InterfaceStability;
024
025/**
026 * This is a very fast, non-cryptographic hash suitable for general hash-based
027 * lookup.  See http://murmurhash.googlepages.com/ for more details.
028 *
029 * <p>The C version of MurmurHash 2.0 found at that site was ported
030 * to Java by Andrzej Bialecki (ab at getopt org).</p>
031 */
032@InterfaceAudience.Private
033@InterfaceStability.Stable
034public class MurmurHash extends Hash {
035  private static MurmurHash _instance = new MurmurHash();
036
037  public static Hash getInstance() {
038    return _instance;
039  }
040
041  @Override
042  public <T> int hash(HashKey<T> hashKey, int seed) {
043    int m = 0x5bd1e995;
044    int r = 24;
045    int length = hashKey.length();
046    int h = seed ^ length;
047
048    int len_4 = length >> 2;
049
050    for (int i = 0; i < len_4; i++) {
051      int i_4 = (i << 2);
052      int k = hashKey.get(i_4 + 3);
053      k = k << 8;
054      k = k | (hashKey.get(i_4 + 2) & 0xff);
055      k = k << 8;
056      k = k | (hashKey.get(i_4 + 1) & 0xff);
057      k = k << 8;
058      // noinspection PointlessArithmeticExpression
059      k = k | (hashKey.get(i_4 + 0) & 0xff);
060      k *= m;
061      k ^= k >>> r;
062      k *= m;
063      h *= m;
064      h ^= k;
065    }
066
067    // avoid calculating modulo
068    int len_m = len_4 << 2;
069    int left = length - len_m;
070    int i_m = len_m;
071
072    if (left != 0) {
073      if (left >= 3) {
074        h ^= hashKey.get(i_m + 2) << 16;
075      }
076      if (left >= 2) {
077        h ^= hashKey.get(i_m + 1) << 8;
078      }
079      if (left >= 1) {
080        h ^= hashKey.get(i_m);
081      }
082
083      h *= m;
084    }
085
086    h ^= h >>> 13;
087    h *= m;
088    h ^= h >>> 15;
089
090    return h;
091  }
092}