Class BZip2Crc

CRC-32 with unreversed data and reversed output

Inheritance

System.Object

BZip2Crc

Implements

IChecksum

Inherited Members

System.Object.Equals(System.Object)

System.Object.Equals(System.Object, System.Object)

System.Object.GetHashCode()

System.Object.GetType()

System.Object.MemberwiseClone()

System.Object.ReferenceEquals(System.Object, System.Object)

System.Object.ToString()

Namespace: ICSharpCode.SharpZipLib.Checksum

Assembly: ICSharpCode.SharpZipLib.dll

Syntax

public sealed class BZip2Crc : IChecksum

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.

This implementation uses sixteen lookup tables stored in one linear array to implement the slicing-by-16 algorithm, a variant of the slicing-by-8 algorithm described in this Intel white paper:

https://web.archive.org/web/20120722193753/http://download.intel.com/technology/comms/perfnet/download/slicing-by-8.pdf

The first lookup table is simply the CRC of all possible eight bit values. Each successive lookup table is derived from the original table generated by Sarwate's algorithm. Slicing a 16-bit input and XORing the outputs together will produce the same output as a byte-by-byte CRC loop with fewer arithmetic and bit manipulation operations, at the cost of increased memory consumed by the lookup tables. (Slicing-by-16 requires a 16KB table, which is still small enough to fit in most processors' L1 cache.)

Constructors

BZip2Crc()

Initialise a default instance of BZip2Crc

Declaration

public BZip2Crc()

Properties

Value

Returns the CRC data checksum computed so far.

Declaration

public long Value { get; }

Property Value

Type	Description
System.Int64

Remarks

Reversed Out = true

Methods

Reset()

Resets the CRC data checksum as if no update was ever called.

Declaration

public void Reset()

Update(ArraySegment<Byte>)

Update CRC data checksum based on a portion of a block of data

Declaration

public void Update(ArraySegment<byte> segment)

Parameters

Type	Name	Description
System.ArraySegment<System.Byte>	segment	The chunk of data to add

Update(Byte[])

Updates the CRC data checksum with the bytes taken from a block of data.

Declaration

public void Update(byte[] buffer)

Parameters

Type	Name	Description
System.Byte[]	buffer	Contains the data to update the CRC with.

Update(Int32)

Updates the checksum with the int bval.

Declaration

public void Update(int bval)

Parameters

Type	Name	Description
System.Int32	bval	the byte is taken as the lower 8 bits of bval

Remarks

Reversed Data = false

Implements

IChecksum