using System;

namespace ICSharpCode.SharpZipLib.Checksum
{
	/// <summary>
	/// CRC-32 with unreversed data and reversed output
	/// </summary>
	/// <remarks>
	/// Generate a table for a byte-wise 32-bit CRC calculation on the polynomial:
	/// x^32+x^26+x^23+x^22+x^16+x^12+x^11+x^10+x^8+x^7+x^5+x^4+x^2+x^1+x^0.
	///
	/// Polynomials over GF(2) are represented in binary, one bit per coefficient,
	/// with the lowest powers in the most significant bit.  Then adding polynomials
	/// is just exclusive-or, and multiplying a polynomial by x is a right shift by
	/// one.  If we call the above polynomial p, and represent a byte as the
	/// polynomial q, also with the lowest power in the most significant bit (so the
	/// byte 0xb1 is the polynomial x^7+x^3+x+1), then the CRC is (q*x^32) mod p,
	/// where a mod b means the remainder after dividing a by b.
	///
	/// This calculation is done using the shift-register method of multiplying and
	/// taking the remainder.  The register is initialized to zero, and for each
	/// incoming bit, x^32 is added mod p to the register if the bit is a one (where
	/// x^32 mod p is p+x^32 = x^26+...+1), and the register is multiplied mod p by
	/// x (which is shifting right by one and adding x^32 mod p if the bit shifted
	/// out is a one).  We start with the highest power (least significant bit) of
	/// q and repeat for all eight bits of q.
	///
	/// The table is simply the CRC of all possible eight bit values.  This is all
	/// the information needed to generate CRC's on data a byte at a time for all
	/// combinations of CRC register values and incoming bytes.
	/// </remarks>
	public sealed class BZip2Crc : IChecksum
	{
		#region Instance Fields

		private const uint crcInit = 0xFFFFFFFF;
		//const uint crcXor = 0x00000000;

		private static readonly uint[] crcTable = {
			0X00000000, 0X04C11DB7, 0X09823B6E, 0X0D4326D9,
			0X130476DC, 0X17C56B6B, 0X1A864DB2, 0X1E475005,
			0X2608EDB8, 0X22C9F00F, 0X2F8AD6D6, 0X2B4BCB61,
			0X350C9B64, 0X31CD86D3, 0X3C8EA00A, 0X384FBDBD,
			0X4C11DB70, 0X48D0C6C7, 0X4593E01E, 0X4152FDA9,
			0X5F15ADAC, 0X5BD4B01B, 0X569796C2, 0X52568B75,
			0X6A1936C8, 0X6ED82B7F, 0X639B0DA6, 0X675A1011,
			0X791D4014, 0X7DDC5DA3, 0X709F7B7A, 0X745E66CD,
			0X9823B6E0, 0X9CE2AB57, 0X91A18D8E, 0X95609039,
			0X8B27C03C, 0X8FE6DD8B, 0X82A5FB52, 0X8664E6E5,
			0XBE2B5B58, 0XBAEA46EF, 0XB7A96036, 0XB3687D81,
			0XAD2F2D84, 0XA9EE3033, 0XA4AD16EA, 0XA06C0B5D,
			0XD4326D90, 0XD0F37027, 0XDDB056FE, 0XD9714B49,
			0XC7361B4C, 0XC3F706FB, 0XCEB42022, 0XCA753D95,
			0XF23A8028, 0XF6FB9D9F, 0XFBB8BB46, 0XFF79A6F1,
			0XE13EF6F4, 0XE5FFEB43, 0XE8BCCD9A, 0XEC7DD02D,
			0X34867077, 0X30476DC0, 0X3D044B19, 0X39C556AE,
			0X278206AB, 0X23431B1C, 0X2E003DC5, 0X2AC12072,
			0X128E9DCF, 0X164F8078, 0X1B0CA6A1, 0X1FCDBB16,
			0X018AEB13, 0X054BF6A4, 0X0808D07D, 0X0CC9CDCA,
			0X7897AB07, 0X7C56B6B0, 0X71159069, 0X75D48DDE,
			0X6B93DDDB, 0X6F52C06C, 0X6211E6B5, 0X66D0FB02,
			0X5E9F46BF, 0X5A5E5B08, 0X571D7DD1, 0X53DC6066,
			0X4D9B3063, 0X495A2DD4, 0X44190B0D, 0X40D816BA,
			0XACA5C697, 0XA864DB20, 0XA527FDF9, 0XA1E6E04E,
			0XBFA1B04B, 0XBB60ADFC, 0XB6238B25, 0XB2E29692,
			0X8AAD2B2F, 0X8E6C3698, 0X832F1041, 0X87EE0DF6,
			0X99A95DF3, 0X9D684044, 0X902B669D, 0X94EA7B2A,
			0XE0B41DE7, 0XE4750050, 0XE9362689, 0XEDF73B3E,
			0XF3B06B3B, 0XF771768C, 0XFA325055, 0XFEF34DE2,
			0XC6BCF05F, 0XC27DEDE8, 0XCF3ECB31, 0XCBFFD686,
			0XD5B88683, 0XD1799B34, 0XDC3ABDED, 0XD8FBA05A,
			0X690CE0EE, 0X6DCDFD59, 0X608EDB80, 0X644FC637,
			0X7A089632, 0X7EC98B85, 0X738AAD5C, 0X774BB0EB,
			0X4F040D56, 0X4BC510E1, 0X46863638, 0X42472B8F,
			0X5C007B8A, 0X58C1663D, 0X558240E4, 0X51435D53,
			0X251D3B9E, 0X21DC2629, 0X2C9F00F0, 0X285E1D47,
			0X36194D42, 0X32D850F5, 0X3F9B762C, 0X3B5A6B9B,
			0X0315D626, 0X07D4CB91, 0X0A97ED48, 0X0E56F0FF,
			0X1011A0FA, 0X14D0BD4D, 0X19939B94, 0X1D528623,
			0XF12F560E, 0XF5EE4BB9, 0XF8AD6D60, 0XFC6C70D7,
			0XE22B20D2, 0XE6EA3D65, 0XEBA91BBC, 0XEF68060B,
			0XD727BBB6, 0XD3E6A601, 0XDEA580D8, 0XDA649D6F,
			0XC423CD6A, 0XC0E2D0DD, 0XCDA1F604, 0XC960EBB3,
			0XBD3E8D7E, 0XB9FF90C9, 0XB4BCB610, 0XB07DABA7,
			0XAE3AFBA2, 0XAAFBE615, 0XA7B8C0CC, 0XA379DD7B,
			0X9B3660C6, 0X9FF77D71, 0X92B45BA8, 0X9675461F,
			0X8832161A, 0X8CF30BAD, 0X81B02D74, 0X857130C3,
			0X5D8A9099, 0X594B8D2E, 0X5408ABF7, 0X50C9B640,
			0X4E8EE645, 0X4A4FFBF2, 0X470CDD2B, 0X43CDC09C,
			0X7B827D21, 0X7F436096, 0X7200464F, 0X76C15BF8,
			0X68860BFD, 0X6C47164A, 0X61043093, 0X65C52D24,
			0X119B4BE9, 0X155A565E, 0X18197087, 0X1CD86D30,
			0X029F3D35, 0X065E2082, 0X0B1D065B, 0X0FDC1BEC,
			0X3793A651, 0X3352BBE6, 0X3E119D3F, 0X3AD08088,
			0X2497D08D, 0X2056CD3A, 0X2D15EBE3, 0X29D4F654,
			0XC5A92679, 0XC1683BCE, 0XCC2B1D17, 0XC8EA00A0,
			0XD6AD50A5, 0XD26C4D12, 0XDF2F6BCB, 0XDBEE767C,
			0XE3A1CBC1, 0XE760D676, 0XEA23F0AF, 0XEEE2ED18,
			0XF0A5BD1D, 0XF464A0AA, 0XF9278673, 0XFDE69BC4,
			0X89B8FD09, 0X8D79E0BE, 0X803AC667, 0X84FBDBD0,
			0X9ABC8BD5, 0X9E7D9662, 0X933EB0BB, 0X97FFAD0C,
			0XAFB010B1, 0XAB710D06, 0XA6322BDF, 0XA2F33668,
			0XBCB4666D, 0XB8757BDA, 0XB5365D03, 0XB1F740B4
		};

		/// <summary>
		/// The CRC data checksum so far.
		/// </summary>
		private uint checkValue;

		#endregion Instance Fields

		/// <summary>
		/// Initialise a default instance of <see cref="BZip2Crc"></see>
		/// </summary>
		public BZip2Crc()
		{
			Reset();
		}

		/// <summary>
		/// Resets the CRC data checksum as if no update was ever called.
		/// </summary>
		public void Reset()
		{
			checkValue = crcInit;
		}

		/// <summary>
		/// Returns the CRC data checksum computed so far.
		/// </summary>
		/// <remarks>Reversed Out = true</remarks>
		public long Value
		{
			get
			{
				// Tehcnically, the output should be:
				//return (long)(~checkValue ^ crcXor);
				// but x ^ 0 = x, so there is no point in adding
				// the XOR operation
				return (long)(~checkValue);
			}
		}

		/// <summary>
		/// Updates the checksum with the int bval.
		/// </summary>
		/// <param name = "bval">
		/// the byte is taken as the lower 8 bits of bval
		/// </param>
		/// <remarks>Reversed Data = false</remarks>
		public void Update(int bval)
		{
			checkValue = unchecked(crcTable[(byte)(((checkValue >> 24) & 0xFF) ^ bval)] ^ (checkValue << 8));
		}

		/// <summary>
		/// Updates the CRC data checksum with the bytes taken from
		/// a block of data.
		/// </summary>
		/// <param name="buffer">Contains the data to update the CRC with.</param>
		public void Update(byte[] buffer)
		{
			if (buffer == null)
			{
				throw new ArgumentNullException(nameof(buffer));
			}

			Update(new ArraySegment<byte>(buffer, 0, buffer.Length));
		}

		/// <summary>
		/// Update CRC data checksum based on a portion of a block of data
		/// </summary>
		/// <param name = "segment">
		/// The chunk of data to add
		/// </param>
		public void Update(ArraySegment<byte> segment)
		{
			var count = segment.Count;
			var offset = segment.Offset;

			while (--count >= 0)
				Update(segment.Array[offset++]);
		}
	}
}
