namespace BLEProtocol
{
    /// <summary>
    /// CRC16校验计算 (CRC16-CCITT协议)
    /// 多项式: 0x1021, 初始值: 0xFFFF
    /// </summary>
    public static class CRC16
    {
        /// <summary>
        /// 计算CRC16校验值 (CRC16-CCITT)
        /// 使用直接计算法，不使用查找表
        /// </summary>
        /// <param name="data">数据字节数组</param>
        /// <param name="offset">起始偏移</param>
        /// <param name="length">数据长度</param>
        /// <returns>CRC16校验值</returns>
        public static ushort Calculate(byte[] data, int offset, int length)
        {
            ushort crc = 0xFFFF;

            for (int i = 0; i < length; i++)
            {
                crc ^= (ushort)(data[offset + i] << 8);
                for (int j = 0; j < 8; j++)
                {
                    if ((crc & 0x8000) != 0)
                    {
                        crc = (ushort)((crc << 1) ^ 0x1021);
                    }
                    else
                    {
                        crc <<= 1;
                    }
                }
            }

            return crc;
        }

        /// <summary>
        /// 计算CRC16校验值 (整个数组)
        /// </summary>
        public static ushort Calculate(byte[] data)
        {
            if (data == null || data.Length == 0)
                return 0;
            return Calculate(data, 0, data.Length);
        }

        /// <summary>
        /// 验证CRC16校验
        /// </summary>
        /// <param name="data">数据字节数组(包含CRC)</param>
        /// <param name="length">数据长度(包含CRC的2字节)</param>
        /// <returns>校验是否通过</returns>
        public static bool Validate(byte[] data, int length)
        {
            if (data == null || length < 3)
                return false;

            ushort calculatedCrc = Calculate(data, 0, length - 2);
            ushort receivedCrc = (ushort)(data[length - 2] | (data[length - 1] << 8));

            return calculatedCrc == receivedCrc;
        }
    }
}