C#CRC16CCITTXModem

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核心方法，如果要返回两个字节，则把最后改为 return BitConverter.GetBytes(crc)

        private ushort Crc16Ccitt(byte[] bytes)
         {
            ushort poly = 0x1021;
            ushort[] table = new ushort[256];
            ushort initialValue = 0x0;
            ushort temp, a;
            ushort crc = initialValue;
            for (int i = 0; i < table.Length; ++i)
            {
                temp = 0;
                a = (ushort)(i << 8);
                for (int j = 0; j < 8; ++j)
                {
                    if (((temp ^ a) & 0x8000) != 0)
                        temp = (ushort)((temp << 1) ^ poly);
                    else
                        temp <<= 1;
                    a <<= 1;
                }
                table[i] = temp;
            }
            for (int i = 0; i < bytes.Length; ++i)
            {
                crc = (ushort)((crc << 8) ^ table[((crc >> 8) ^ (0xff & bytes[i]))]);
            }
            return crc;
        }

提取为类，把查询表提前初始化，避免每次调用都进行计算。

    /// <summary>
    /// CRC16 CCITT XModem计算校验值
    /// </summary>
    public static class CRC16
    {
        /// <summary>
        /// 多项式
        /// </summary>
        const ushort Polynominal = 0x1021;

        /// <summary>
        /// 查询表
        /// </summary>
        static ushort[] table = new ushort[256];

        /// <summary>
        /// 静态构造函数，初始化查询表
        /// </summary>
        static CRC16()
        {
            ushort temp, a;
            for (int i = 0; i < table.Length; ++i)
            {
                temp = 0;
                a = (ushort)(i << 8);
                for (int j = 0; j < 8; ++j)
                {
                    if (((temp ^ a) & 0x8000) != 0)
                        temp = (ushort)((temp << 1) ^ Polynominal);
                    else
                        temp <<= 1;
                    a <<= 1;
                }
                table[i] = temp;
            }
        }

        /// <summary>
        /// 计算校验值
        /// </summary>
        /// <param name="data"></param>
        /// <returns></returns>
        public static byte[] CheckSum(List<byte> data)
        {
            ushort crc = 0x0;

            for (int i = 0; i < data.Count; ++i)
            {
                crc = (ushort)((crc << 8) ^ table[((crc >> 8) ^ (0xff & data[i]))]);
            }
            return BitConverter.GetBytes(crc);
        }

        /// <summary>
        /// 计算部分区域的校验值
        /// </summary>
        /// <param name="data"></param>
        /// <param name="startIndex"></param>
        /// <param name="count"></param>
        /// <returns></returns>
        public static byte[] CheckSumRange(List<byte> data, int startIndex, int count)
        {
            return CheckSum(data.GetRange(startIndex, count));
        }
    }