using System;

namespace Xylem.Common.Utils.Crc16Ccitt
{
    /// <summary>
    /// Class for CRC16 CCITT  calculation
    /// </summary>
    public static class Crc16Ccitt
    {
        /// <summary>
        ///     Initial CRC value
        /// </summary>
        private const UInt16 CrcSeedFfff = 0xFFFF;
        /// <summary>
        ///     Initial CRC value
        /// </summary>
        private const UInt16 CrcSeed3791 = 0x3791;

        /// <summary>
        ///     Generator polynomial MagFlux - Modbus RTU
        /// </summary>
        private const UInt16 CrcGpA001 = 0xA001;
        /// <summary>
        ///     Generator polynomial GENESIS
        /// </summary>
        private const UInt16 CrcGp1021 = 0x1021;

        /// <summary>
        ///     Generator polynomial RFID
        /// </summary>
        private const UInt16 CrcGp0408 = 0x0408;

        /// <summary>
        ///     Generator polynomial for reversed IrDA
        /// </summary>
        private const UInt16 CrcGp8408 = 0x8408;

        /// <summary>
        ///     Calculates the CRC (LSB first, reversed, CRC CCITT 0x8408) from a data array of bytes.
        /// </summary>
        /// <param name="data">data to be processed</param>
        /// <returns>Calculated CRC</returns>
        public static UInt16 CalculateReversedLsb8408(Byte[] data)
        {
            var crc = CrcSeedFfff;
            foreach (var t in data)
            {
                //copy data byte to lower word because of LSB will be XORed
                var dataWord = (UInt16)(t & 0x00FF);
                for (var i = 0; i < 8; i++)
                {
                    //test if CRC lowest bit is XORed set to one
                    if (0x0001 == ((crc ^ dataWord) & 0x0001))
                    {
                        crc >>= 1;
                        //apply generator polynomial
                        crc ^= CrcGp8408;
                    }
                    else
                    {
                        crc >>= 1;
                    }

                    dataWord >>= 1;
                }
            }
            return ((UInt16)~crc);
        }
        /// <summary>
        ///     Calculates the bitwise inverted CRC (MSB first, CRCCCITT 0x1021) 
        ///     from a data array of bytes.
        /// </summary>
        /// <param name="data">data to be processed</param>
        /// <returns>Calculated CRC</returns>
        public static UInt16 CalculateInvertedMsb1021(Byte[] data)
        {
            return (UInt16)~CalculateMsb1021(data);
        }

        /// <summary>
        ///     Calculates the Modbus RTU CRC (LSB first, CRCCCITT 0xA001) 
        ///     from a data array of bytes.
        /// </summary>
        /// <param name="data">data to be processed</param>
        /// <returns>Calculated CRC</returns>
        public static UInt16 ModbusRtuLsbA001(Byte[] data)
        {
            var crc = CrcSeedFfff;
  
            for (var t = 0; t < data.Length; t++) 
            {
                crc ^= data[t];          // XOR byte into least sig. byte of crc
  
                for (var i = 8; i != 0; i--) 
                {    
                    if ((crc & 0x0001) != 0) 
                    {      
                        crc >>= 1;      
                        crc ^= CrcGpA001;
                    }
                    else
                    {
                        crc >>= 1;
                    }
                }
            }
            return crc;  
        }
        
        /// <summary>
        ///     Calculates the CRC (MSB first, CRCCCITT 0x1021) from a data array of bytes.
        /// </summary>
        /// <param name="data">data to be processed</param>
        /// <returns>Calculated CRC</returns>
        public static UInt16 CalculateMsb1021(Byte[] data)
        {
            var crc = CrcSeedFfff;
            foreach (var t in data)
            {
                //copy data byte to higher word because of MSB will be XORed
                var dataWord = (UInt16)((t << 8) & 0xFF00);
                for (var i = 0; i < 8; i++)
                {
                    //test if CRC highest bit or data input highest bit is XORed set to one
                    if (0x8000 == ((crc ^ dataWord) & 0x8000)) //shifted in MSB first
                    {
                        //shift CRC high bit out
                        crc <<= 1;
                        //apply generator polynomial
                        crc ^= CrcGp1021;
                    }
                    else //CRC highest bit and data input highest bit is equal
                    {
                        crc <<= 1;
                    }
                    dataWord <<= 1; //MSB first shifted out
                }
            }
            return crc;
        }

        /// <summary>
        ///     Calculates the CRC (LSB first, CRCCCITT 0x0408) from a data array of bytes.
        /// </summary>
        /// <param name="data">data to be processed</param>
        /// <returns>Calculated CRC</returns>
        public static UInt16 CalculateLsb0408(Byte[] data)
        {
            var crc = CrcSeed3791;
            foreach (var t in data)
            {
                var dataByte = t;
                for (var i = 0; i < 8; i++)
                {
                    //test if CRC lowest bit is set to one
                    if (1 == (crc & 1))
                    {
                        //shift CRC low bit out
                        crc >>= 1;
                        //test lowest bit of data word
                        if (1 == (dataByte & 1))
                            crc |= 0x8000;
                        //invert CRC MSB
                        crc ^= 0x8000;
                    }
                    else //CRC lowest bit not set
                    {
                        //shift CRC low bit out
                        crc >>= 1;
                        //test lowest bit of data word
                        if (1 == (dataByte & 1))
                            crc |= 0x8000;
                    }
                    if (0x8000 == (crc & 0x8000))
                        //apply generator polynomial
                        crc ^= CrcGp0408;

                    dataByte >>= 1; //LSB first shifted out
                }
            }
            return crc;
        }
    }
}