Class Adler32

Computes Adler32 checksum for a stream of data. An Adler32 checksum is not as reliable as a CRC32 checksum, but a lot faster to compute.

The specification for Adler32 may be found in RFC 1950. ZLIB Compressed Data Format Specification version 3.3)

From that document:

"ADLER32 (Adler-32 checksum) This contains a checksum value of the uncompressed data (excluding any dictionary data) computed according to Adler-32 algorithm. This algorithm is a 32-bit extension and improvement of the Fletcher algorithm, used in the ITU-T X.224 / ISO 8073 standard.

Adler-32 is composed of two sums accumulated per byte: s1 is the sum of all bytes, s2 is the sum of all s1 values. Both sums are done modulo 65521. s1 is initialized to 1, s2 to zero. The Adler-32 checksum is stored as s2*65536 + s1 in most- significant-byte first (network) order."

"8.2. The Adler-32 algorithm

The Adler-32 algorithm is much faster than the CRC32 algorithm yet still provides an extremely low probability of undetected errors.

The modulo on unsigned long accumulators can be delayed for 5552 bytes, so the modulo operation time is negligible. If the bytes are a, b, c, the second sum is 3a + 2b + c + 3, and so is position and order sensitive, unlike the first sum, which is just a checksum. That 65521 is prime is important to avoid a possible large class of two-byte errors that leave the check unchanged. (The Fletcher checksum uses 255, which is not prime and which also makes the Fletcher check insensitive to single byte changes 0 - 255.)

The sum s1 is initialized to 1 instead of zero to make the length of the sequence part of s2, so that the length does not have to be checked separately. (Any sequence of zeroes has a Fletcher checksum of zero.)"

Inheritance

System.Object

Adler32

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 Adler32 : IChecksum

Constructors

Adler32()

Initialise a default instance of Adler32

Declaration

public Adler32()

Properties

Value

Returns the Adler32 data checksum computed so far.

Declaration

public long Value { get; }

Property Value

Type	Description
System.Int64

Methods

Reset()

Resets the Adler32 data checksum as if no update was ever called.

Declaration

public void Reset()

Update(ArraySegment<Byte>)

Update Adler32 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 Adler32 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 checksum with.

Update(Int32)

Updates the checksum with the byte b.

Declaration

public void Update(int bval)

Parameters

Type	Name	Description
System.Int32	bval	The data value to add. The high byte of the int is ignored.

Implements

IChecksum