src/Lucene.Net.Codecs/Bloom/MurmurHash2.cs

using Lucene.Net.Util;

namespace Lucene.Net.Codecs.Bloom
{
    /*
     * 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.
     */

    /// <summary>
    /// This is a very fast, non-cryptographic hash suitable for general hash-based
    /// lookup. See http://murmurhash.googlepages.com/ for more details.
    /// <para/>
    /// The C version of MurmurHash 2.0 found at that site was ported to Java by
    /// Andrzej Bialecki (ab at getopt org).
    /// <para/>
    /// 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 <see cref="BytesRef"/>s with offsets and lengths
    /// rather than raw byte arrays.  
    /// <para/>
    /// @lucene.experimental
    /// </summary>
    public sealed class MurmurHash2 : HashFunction
    {
        public static readonly 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 = (int)(((uint)seed) ^ (long)len); // LUCENENET NOTE: Need to convert seed to uint (since the constant was a uint)
            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;
            }
            var len_m = len_4 << 2;
            var 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;
        }

        /// <summary>
        /// Generates 32 bit hash from byte array with default seed value.
        /// </summary>
        /// <param name="data">Byte array to hash.</param>
        /// <param name="offset">The start position in the array to hash.</param>
        /// <param name="len">Length of the array elements to hash.</param>
        /// <returns>32 bit hash of the given array.</returns>
        public static int Hash32(byte[] data, int offset, int len)
        {
            return Hash(data, unchecked((int)0x9747b28c), offset, len);
        }

        public override int Hash(BytesRef br)
        {
            return Hash32(br.Bytes, br.Offset, br.Length);
        }
    }
}