using System;
using System.Collections.Generic;
using System.Text;

namespace myCRC
{
	public enum CRCMethod { CRC16, CRC16Kermit, CRC16DNP, CRC16CCITT, CRC32 };
	public enum DataType { TEXT, HEX };

	class MyCRC
	{
		private uint[] crctab = new uint[256];

		public Encoding Encoding = Encoding.Default;

		public string CalcCRC(string data, DataType datatype, CRCMethod methodCRC, string Polynomial, string crcinit, bool revert)
		{
			ushort CRC16 = 0;
			uint CRC32 = 0;
			byte Q;
			ushort HI_, LO_;

			CrTabl(methodCRC, Convert.ToUInt32(Polynomial, 16));

			if (methodCRC == CRCMethod.CRC32)
				CRC32 = Convert.ToUInt32(crcinit, 16);
			else
				CRC16 = Convert.ToUInt16(crcinit, 16);

			if (data.Length > 0)
			{
				if (datatype == DataType.TEXT)
				{
					foreach (byte b in this.Encoding.GetBytes(data))
					{
						switch (methodCRC)
						{
							case CRCMethod.CRC16:
							case CRCMethod.CRC16Kermit:
							case CRCMethod.CRC16DNP:
								CRC16 = (ushort)(HI(CRC16) ^ Lo(crctab[b ^ LO(CRC16)]));
								break;
							case CRCMethod.CRC16CCITT:
								CRC16 = (ushort)((CRC16 << 8) ^ Lo(crctab[b ^ (CRC16 >> 8)]));
								break;
							case CRCMethod.CRC32:
								CRC32 = (uint)((CRC32 >> 8) ^ crctab[b ^ (CRC32 & 0x000000FF)]);
								break;
						}
					}
				}
				else
				{
					for (int i = 0; i < data.Length; i += 2)
					{
						Q = Hex2Byte(data.Substring(i, 2));
						switch (methodCRC)
						{
							case CRCMethod.CRC16:
							case CRCMethod.CRC16Kermit:
							case CRCMethod.CRC16DNP:
								CRC16 = (ushort)(HI(CRC16) ^ Lo(crctab[Q ^ LO(CRC16)]));
								break;
							case CRCMethod.CRC16CCITT:
								CRC16 = (ushort)((CRC16 << 8) ^ Lo(crctab[Q ^ (CRC16 >> 8)]));
								break;
							case CRCMethod.CRC32:
								CRC32 = (uint)((CRC32 >> 8) ^ crctab[Q ^ (CRC32 & 0x000000FF)]);
								break;
						}
					}
				}
			}
			switch (methodCRC)
			{
				case CRCMethod.CRC16:
				case CRCMethod.CRC16CCITT:
					break;
				case CRCMethod.CRC16Kermit:
					LO_ = (ushort)((CRC16 & 0xff00) >> 8);
					HI_ = (ushort)((CRC16 & 0x00ff) << 8);
					CRC16 = (ushort)(LO_ | HI_);
					break;
				case CRCMethod.CRC16DNP:
					CRC16 = (ushort)(CRC16 ^ 0xFFFF);
					LO_ = (ushort)((CRC16 & 0xff00) >> 8);
					HI_ = (ushort)((CRC16 & 0x00ff) << 8);
					CRC16 = (ushort)(LO_ | HI_);
					break;
				case CRCMethod.CRC32:
					CRC32 = (uint)(CRC32 ^ 0xFFFFFFFF);
					break;
			}
			if (methodCRC == CRCMethod.CRC32)
				return CRC32.ToString("X8");
			else
			{
				if (revert)
				{
					LO_ = (ushort)((CRC16 & 0xff00) >> 8);
					HI_ = (ushort)((CRC16 & 0x00ff) << 8);
					CRC16 = (ushort)(LO_ | HI_);
				}
				return CRC16.ToString("X4");
			}
		}

		private byte Hex2Byte(string p)
		{
			int hi = Uri.FromHex(p[0]);
			int lo = Uri.FromHex(p[1]);
			return (byte)(lo | (hi << 4));
		}

		private ushort Lo(uint p)
		{
			return BitConverter.ToUInt16(BitConverter.GetBytes(p & 0xFFFF), 0);
		}

		private byte HI(ushort p)
		{
			return (byte)(p >> 8);
		}

		private byte LO(ushort p)
		{
			return (byte)(p & 0xFF);
		}

		private void CrTabl(CRCMethod methodCRC, uint POLY)
		{
			uint crc;
			uint L = 0;
			for (uint i = 0; i <= 255; i++)
			{
				crc = 0;
				switch (methodCRC)
				{
					case CRCMethod.CRC16:
					case CRCMethod.CRC16Kermit:
					case CRCMethod.CRC16DNP:
						L = i;
						break;
					case CRCMethod.CRC16CCITT:
						L = i << 8;
						break;
					case CRCMethod.CRC32:
						crc = i;
						break;
				}
				for (int j = 0; j <= 7; j++)
				{
					switch (methodCRC)
					{
						case CRCMethod.CRC16:
						case CRCMethod.CRC16Kermit:
						case CRCMethod.CRC16DNP:
							if (((crc ^ L) & 0x00000001) != 0)
								crc = (crc >> 1) ^ POLY;
							else
								crc = crc >> 1;
							break;
						case CRCMethod.CRC16CCITT:
							if (((crc ^ L) & 0x00008000) != 0)
								crc = (crc << 1) ^ POLY;
							else
								crc = crc << 1;
							break;
						case CRCMethod.CRC32:
							if ((crc & 0x00000001) != 0)
								crc = (crc >> 1) ^ POLY;
							else
								crc = crc >> 1;
							break;
					}
					switch (methodCRC)
					{
						case CRCMethod.CRC16:
						case CRCMethod.CRC16Kermit:
						case CRCMethod.CRC16DNP:
							L = L >> 1;
							break;
						case CRCMethod.CRC16CCITT:
							L = L << 1;
							break;
						case CRCMethod.CRC32:
							crctab[i] = crc;
							break;
					}
				}
				if (methodCRC != CRCMethod.CRC32)
					crctab[i] = crc;
			}
		}
	}
}