dmdb/ICSharpCode.SharpZipLib/Checksum/Crc32.cs

using System;

namespace ICSharpCode.SharpZipLib.Checksum
{
	/// <summary>
	/// CRC-32 with reversed data and unreversed 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 Crc32 : IChecksum
	{
		#region Instance Fields

		private static readonly uint crcInit = 0xFFFFFFFF;
		private static readonly uint crcXor = 0xFFFFFFFF;

		private static readonly uint[] crcTable = {
			0x00000000, 0x77073096, 0xEE0E612C, 0x990951BA, 0x076DC419,
			0x706AF48F, 0xE963A535, 0x9E6495A3, 0x0EDB8832, 0x79DCB8A4,
			0xE0D5E91E, 0x97D2D988, 0x09B64C2B, 0x7EB17CBD, 0xE7B82D07,
			0x90BF1D91, 0x1DB71064, 0x6AB020F2, 0xF3B97148, 0x84BE41DE,
			0x1ADAD47D, 0x6DDDE4EB, 0xF4D4B551, 0x83D385C7, 0x136C9856,
			0x646BA8C0, 0xFD62F97A, 0x8A65C9EC, 0x14015C4F, 0x63066CD9,
			0xFA0F3D63, 0x8D080DF5, 0x3B6E20C8, 0x4C69105E, 0xD56041E4,
			0xA2677172, 0x3C03E4D1, 0x4B04D447, 0xD20D85FD, 0xA50AB56B,
			0x35B5A8FA, 0x42B2986C, 0xDBBBC9D6, 0xACBCF940, 0x32D86CE3,
			0x45DF5C75, 0xDCD60DCF, 0xABD13D59, 0x26D930AC, 0x51DE003A,
			0xC8D75180, 0xBFD06116, 0x21B4F4B5, 0x56B3C423, 0xCFBA9599,
			0xB8BDA50F, 0x2802B89E, 0x5F058808, 0xC60CD9B2, 0xB10BE924,
			0x2F6F7C87, 0x58684C11, 0xC1611DAB, 0xB6662D3D, 0x76DC4190,
			0x01DB7106, 0x98D220BC, 0xEFD5102A, 0x71B18589, 0x06B6B51F,
			0x9FBFE4A5, 0xE8B8D433, 0x7807C9A2, 0x0F00F934, 0x9609A88E,
			0xE10E9818, 0x7F6A0DBB, 0x086D3D2D, 0x91646C97, 0xE6635C01,
			0x6B6B51F4, 0x1C6C6162, 0x856530D8, 0xF262004E, 0x6C0695ED,
			0x1B01A57B, 0x8208F4C1, 0xF50FC457, 0x65B0D9C6, 0x12B7E950,
			0x8BBEB8EA, 0xFCB9887C, 0x62DD1DDF, 0x15DA2D49, 0x8CD37CF3,
			0xFBD44C65, 0x4DB26158, 0x3AB551CE, 0xA3BC0074, 0xD4BB30E2,
			0x4ADFA541, 0x3DD895D7, 0xA4D1C46D, 0xD3D6F4FB, 0x4369E96A,
			0x346ED9FC, 0xAD678846, 0xDA60B8D0, 0x44042D73, 0x33031DE5,
			0xAA0A4C5F, 0xDD0D7CC9, 0x5005713C, 0x270241AA, 0xBE0B1010,
			0xC90C2086, 0x5768B525, 0x206F85B3, 0xB966D409, 0xCE61E49F,
			0x5EDEF90E, 0x29D9C998, 0xB0D09822, 0xC7D7A8B4, 0x59B33D17,
			0x2EB40D81, 0xB7BD5C3B, 0xC0BA6CAD, 0xEDB88320, 0x9ABFB3B6,
			0x03B6E20C, 0x74B1D29A, 0xEAD54739, 0x9DD277AF, 0x04DB2615,
			0x73DC1683, 0xE3630B12, 0x94643B84, 0x0D6D6A3E, 0x7A6A5AA8,
			0xE40ECF0B, 0x9309FF9D, 0x0A00AE27, 0x7D079EB1, 0xF00F9344,
			0x8708A3D2, 0x1E01F268, 0x6906C2FE, 0xF762575D, 0x806567CB,
			0x196C3671, 0x6E6B06E7, 0xFED41B76, 0x89D32BE0, 0x10DA7A5A,
			0x67DD4ACC, 0xF9B9DF6F, 0x8EBEEFF9, 0x17B7BE43, 0x60B08ED5,
			0xD6D6A3E8, 0xA1D1937E, 0x38D8C2C4, 0x4FDFF252, 0xD1BB67F1,
			0xA6BC5767, 0x3FB506DD, 0x48B2364B, 0xD80D2BDA, 0xAF0A1B4C,
			0x36034AF6, 0x41047A60, 0xDF60EFC3, 0xA867DF55, 0x316E8EEF,
			0x4669BE79, 0xCB61B38C, 0xBC66831A, 0x256FD2A0, 0x5268E236,
			0xCC0C7795, 0xBB0B4703, 0x220216B9, 0x5505262F, 0xC5BA3BBE,
			0xB2BD0B28, 0x2BB45A92, 0x5CB36A04, 0xC2D7FFA7, 0xB5D0CF31,
			0x2CD99E8B, 0x5BDEAE1D, 0x9B64C2B0, 0xEC63F226, 0x756AA39C,
			0x026D930A, 0x9C0906A9, 0xEB0E363F, 0x72076785, 0x05005713,
			0x95BF4A82, 0xE2B87A14, 0x7BB12BAE, 0x0CB61B38, 0x92D28E9B,
			0xE5D5BE0D, 0x7CDCEFB7, 0x0BDBDF21, 0x86D3D2D4, 0xF1D4E242,
			0x68DDB3F8, 0x1FDA836E, 0x81BE16CD, 0xF6B9265B, 0x6FB077E1,
			0x18B74777, 0x88085AE6, 0xFF0F6A70, 0x66063BCA, 0x11010B5C,
			0x8F659EFF, 0xF862AE69, 0x616BFFD3, 0x166CCF45, 0xA00AE278,
			0xD70DD2EE, 0x4E048354, 0x3903B3C2, 0xA7672661, 0xD06016F7,
			0x4969474D, 0x3E6E77DB, 0xAED16A4A, 0xD9D65ADC, 0x40DF0B66,
			0x37D83BF0, 0xA9BCAE53, 0xDEBB9EC5, 0x47B2CF7F, 0x30B5FFE9,
			0xBDBDF21C, 0xCABAC28A, 0x53B39330, 0x24B4A3A6, 0xBAD03605,
			0xCDD70693, 0x54DE5729, 0x23D967BF, 0xB3667A2E, 0xC4614AB8,
			0x5D681B02, 0x2A6F2B94, 0xB40BBE37, 0xC30C8EA1, 0x5A05DF1B,
			0x2D02EF8D
		};

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

		#endregion Instance Fields

		internal static uint ComputeCrc32(uint oldCrc, byte bval)
		{
			return (uint)(Crc32.crcTable[(oldCrc ^ bval) & 0xFF] ^ (oldCrc >> 8));
		}

		/// <summary>
		/// Initialise a default instance of <see cref="Crc32"></see>
		/// </summary>
		public Crc32()
		{
			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 = false</remarks>
		public long Value
		{
			get
			{
				return (long)(checkValue ^ crcXor);
			}
		}

		/// <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 = true</remarks>
		public void Update(int bval)
		{
			checkValue = unchecked(crcTable[(checkValue ^ bval) & 0xFF] ^ (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++]);
		}
	}
}