C# CRC16 和汉明重量

最近在看redis之类的pdf，发现redis在做集群的时候，不同的key分到不同的主服务器，其中划分key的算法采用CRC16算法，所以特此整理一下其C#code如下：

  #region CRC16
        /// <summary>
        /// Computes the hash-slot that would be used by the given key
        /// </summary>
        public static unsafe int HashSlot(string key)
        {
            //HASH_SLOT = CRC16(key) mod 16384
            if (string.IsNullOrEmpty(key)) return -1;
            unchecked
            {
                //var blob = (byte[])key;
                var blob = Encoding.UTF8.GetBytes(key);
                fixed (byte* ptr = blob)
                {
                    int offset = 0, count = blob.Length, start, end;
                    if ((start = IndexOf(ptr, (byte)'{', 0, count - 1)) >= 0
                        && (end = IndexOf(ptr, (byte)'}', start + 1, count)) >= 0
                        && --end != start)
                    {
                        offset = start + 1;
                        count = end - start; // note we already subtracted one via --end
                    }

                    uint crc = 0;
                    for (int i = 0; i < count; i++)
                        crc = ((crc << 8) ^ crc16tab[((crc >> 8) ^ ptr[offset++]) & 0x00FF]) & 0x0000FFFF;
                    return (int)(crc % 16384);
                }
            }
        }
        static unsafe int IndexOf(byte* ptr, byte value, int start, int end)
        {
            for (int offset = start; offset < end; offset++)
                if (ptr[offset] == value) return offset;
            return -1;
        }
        static readonly ushort[] crc16tab =
          {
                0x0000,0x1021,0x2042,0x3063,0x4084,0x50a5,0x60c6,0x70e7,
                0x8108,0x9129,0xa14a,0xb16b,0xc18c,0xd1ad,0xe1ce,0xf1ef,
                0x1231,0x0210,0x3273,0x2252,0x52b5,0x4294,0x72f7,0x62d6,
                0x9339,0x8318,0xb37b,0xa35a,0xd3bd,0xc39c,0xf3ff,0xe3de,
                0x2462,0x3443,0x0420,0x1401,0x64e6,0x74c7,0x44a4,0x5485,
                0xa56a,0xb54b,0x8528,0x9509,0xe5ee,0xf5cf,0xc5ac,0xd58d,
                0x3653,0x2672,0x1611,0x0630,0x76d7,0x66f6,0x5695,0x46b4,
                0xb75b,0xa77a,0x9719,0x8738,0xf7df,0xe7fe,0xd79d,0xc7bc,
                0x48c4,0x58e5,0x6886,0x78a7,0x0840,0x1861,0x2802,0x3823,
                0xc9cc,0xd9ed,0xe98e,0xf9af,0x8948,0x9969,0xa90a,0xb92b,
                0x5af5,0x4ad4,0x7ab7,0x6a96,0x1a71,0x0a50,0x3a33,0x2a12,
                0xdbfd,0xcbdc,0xfbbf,0xeb9e,0x9b79,0x8b58,0xbb3b,0xab1a,
                0x6ca6,0x7c87,0x4ce4,0x5cc5,0x2c22,0x3c03,0x0c60,0x1c41,
                0xedae,0xfd8f,0xcdec,0xddcd,0xad2a,0xbd0b,0x8d68,0x9d49,
                0x7e97,0x6eb6,0x5ed5,0x4ef4,0x3e13,0x2e32,0x1e51,0x0e70,
                0xff9f,0xefbe,0xdfdd,0xcffc,0xbf1b,0xaf3a,0x9f59,0x8f78,
                0x9188,0x81a9,0xb1ca,0xa1eb,0xd10c,0xc12d,0xf14e,0xe16f,
                0x1080,0x00a1,0x30c2,0x20e3,0x5004,0x4025,0x7046,0x6067,
                0x83b9,0x9398,0xa3fb,0xb3da,0xc33d,0xd31c,0xe37f,0xf35e,
                0x02b1,0x1290,0x22f3,0x32d2,0x4235,0x5214,0x6277,0x7256,
                0xb5ea,0xa5cb,0x95a8,0x8589,0xf56e,0xe54f,0xd52c,0xc50d,
                0x34e2,0x24c3,0x14a0,0x0481,0x7466,0x6447,0x5424,0x4405,
                0xa7db,0xb7fa,0x8799,0x97b8,0xe75f,0xf77e,0xc71d,0xd73c,
                0x26d3,0x36f2,0x0691,0x16b0,0x6657,0x7676,0x4615,0x5634,
                0xd94c,0xc96d,0xf90e,0xe92f,0x99c8,0x89e9,0xb98a,0xa9ab,
                0x5844,0x4865,0x7806,0x6827,0x18c0,0x08e1,0x3882,0x28a3,
                0xcb7d,0xdb5c,0xeb3f,0xfb1e,0x8bf9,0x9bd8,0xabbb,0xbb9a,
                0x4a75,0x5a54,0x6a37,0x7a16,0x0af1,0x1ad0,0x2ab3,0x3a92,
                0xfd2e,0xed0f,0xdd6c,0xcd4d,0xbdaa,0xad8b,0x9de8,0x8dc9,
                0x7c26,0x6c07,0x5c64,0x4c45,0x3ca2,0x2c83,0x1ce0,0x0cc1,
                0xef1f,0xff3e,0xcf5d,0xdf7c,0xaf9b,0xbfba,0x8fd9,0x9ff8,
                0x6e17,0x7e36,0x4e55,0x5e74,0x2e93,0x3eb2,0x0ed1,0x1ef0
            };
        #endregion

在redis的bitcount函数里面有一个汉明重量的算法（其实就是看二进制数组中1的个数），其pdf中的code如下，redis里面提到的汉明重量code 只是考虑到了长度为32的字节数组。

   public static uint bitCount(uint i)
        {
            // HD, Figure 5-2
            i = (i & 0x55555555) + ((i >> 1) & 0x55555555);
            i = (i & 0x33333333) + ((i >> 2) & 0x33333333);
            i = (i & 0x0f0f0f0f) + (i >> 4) & 0x0f0f0f0f;
            i = (i * (0x01010101) >> 24);
            return i;
        }

网上也有该算法类似实现：

  public static int bitCount(int i)
        {        
            // HD, Figure 5-2
            i = i - ((i >> 1) & 0x55555555);
            i = (i & 0x33333333) + ((i >> 2) & 0x33333333);
            i = (i + (i >> 4)) & 0x0f0f0f0f;
            i = i + (i >> 8);
            i = i + (i >> 16);
            return i & 0x3f;
        }

网上也有一些简单的算法如：

       public static int bitCount(int i)
        {
            int num = 0;
            while (i != 0)
            {
                i &= (i - 1);
                num++;
            }
            return num;
        }

我们相爱撸代码的时候还是需要考虑code的时间复杂度和空间复杂度。