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Common functionality for HBase
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/**
 *
 * 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.hadoop.hbase.util;

import org.apache.hadoop.hbase.classification.InterfaceAudience;
import org.apache.hadoop.hbase.classification.InterfaceStability;

/**
 * This is a very fast, non-cryptographic hash suitable for general hash-based
 * lookup.  See http://code.google.com/p/smhasher/wiki/MurmurHash3 for details.
 *
 * MurmurHash3 is the successor to MurmurHash2. It comes in 3 variants, and
 * the 32-bit version targets low latency for hash table use.
 */
@InterfaceAudience.Private
@InterfaceStability.Stable
public class MurmurHash3 extends Hash {
  private static MurmurHash3 _instance = new MurmurHash3();

  public static Hash getInstance() {
    return _instance;
  }

  /** Returns the MurmurHash3_x86_32 hash. */
  @edu.umd.cs.findbugs.annotations.SuppressWarnings("SF")
  @Override
  public int hash(byte[] bytes, int offset, int length, int initval) {
    final int c1 = 0xcc9e2d51;
    final int c2 = 0x1b873593;

    int h1 = initval;
    int roundedEnd = offset + (length & 0xfffffffc); // round down to 4 byte block

    for (int i = offset; i < roundedEnd; i += 4) {
      // little endian load order
      int k1 = (bytes[i] & 0xff) | ((bytes[i + 1] & 0xff) << 8) | ((bytes[i + 2] & 0xff) << 16)
          | (bytes[i + 3] << 24);
      k1 *= c1;
      k1 = (k1 << 15) | (k1 >>> 17); // ROTL32(k1,15);
      k1 *= c2;

      h1 ^= k1;
      h1 = (h1 << 13) | (h1 >>> 19); // ROTL32(h1,13);
      h1 = h1 * 5 + 0xe6546b64;
    }

    // tail
    int k1 = 0;

    switch (length & 0x03) {
    case 3:
      k1 = (bytes[roundedEnd + 2] & 0xff) << 16;
      // FindBugs SF_SWITCH_FALLTHROUGH
    case 2:
      k1 |= (bytes[roundedEnd + 1] & 0xff) << 8;
      // FindBugs SF_SWITCH_FALLTHROUGH
    case 1:
      k1 |= (bytes[roundedEnd] & 0xff);
      k1 *= c1;
      k1 = (k1 << 15) | (k1 >>> 17); // ROTL32(k1,15);
      k1 *= c2;
      h1 ^= k1;
    }

    // finalization
    h1 ^= length;

    // fmix(h1);
    h1 ^= h1 >>> 16;
    h1 *= 0x85ebca6b;
    h1 ^= h1 >>> 13;
    h1 *= 0xc2b2ae35;
    h1 ^= h1 >>> 16;

    return h1;
  }
}