ICSharpCode.SharpZipLib.Checksums.Adler32 Class Reference

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. More...

Inheritance diagram for ICSharpCode.SharpZipLib.Checksums.Adler32:

Collaboration diagram for ICSharpCode.SharpZipLib.Checksums.Adler32:

Public Member Functions
	Adler32 ()
	Creates a new instance of the Adler32 class. The checksum starts off with a value of 1.
void	Reset ()
	Resets the Adler32 checksum to the initial value.
void	Update (int value)
	Updates the checksum with a byte value.
void	Update (byte[] buffer)
	Updates the checksum with an array of bytes.
void	Update (byte[] buffer, int offset, int count)
	Updates the checksum with the bytes taken from the array.

Properties
long	Value [get]
	Returns the Adler32 data checksum computed so far.

Private Attributes
uint	checksum

Static Private Attributes
const uint	BASE = 65521
	largest prime smaller than 65536

Detailed Description

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.)"

ICSharpCode.SharpZipLib.Zip.Compression.Streams.InflaterInputStream ICSharpCode.SharpZipLib.Zip.Compression.Streams.DeflaterOutputStream

Constructor & Destructor Documentation

Adler32()

ICSharpCode.SharpZipLib.Checksums.Adler32.Adler32

(

)

Creates a new instance of the Adler32 class. The checksum starts off with a value of 1.

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Member Function Documentation

Reset()

void ICSharpCode.SharpZipLib.Checksums.Adler32.Reset

(

)

Resets the Adler32 checksum to the initial value.

Implements ICSharpCode.SharpZipLib.Checksums.IChecksum.

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Update() [1/3]

void ICSharpCode.SharpZipLib.Checksums.Adler32.Update

(

byte[]

buffer

)

Updates the checksum with an array of bytes.

Parameters

buffer

The source of the data to update with.

Implements ICSharpCode.SharpZipLib.Checksums.IChecksum.

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Update() [2/3]

void ICSharpCode.SharpZipLib.Checksums.Adler32.Update	(	byte[]	buffer,
		int	offset,
		int	count )

Updates the checksum with the bytes taken from the array.

Parameters

buffer	an array of bytes
offset	the start of the data used for this update
count	the number of bytes to use for this update

Implements ICSharpCode.SharpZipLib.Checksums.IChecksum.

Update() [3/3]

void ICSharpCode.SharpZipLib.Checksums.Adler32.Update

(

int

value

)

Updates the checksum with a byte value.

Parameters

value

The data value to add. The high byte of the int is ignored.

Implements ICSharpCode.SharpZipLib.Checksums.IChecksum.

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Member Data Documentation

BASE

const uint ICSharpCode.SharpZipLib.Checksums.Adler32.BASE = 65521

staticprivate

largest prime smaller than 65536

checksum

uint ICSharpCode.SharpZipLib.Checksums.Adler32.checksum

private

Property Documentation

Value

long ICSharpCode.SharpZipLib.Checksums.Adler32.Value

get

Returns the Adler32 data checksum computed so far.

Implements ICSharpCode.SharpZipLib.Checksums.IChecksum.

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The documentation for this class was generated from the following file:

• ICSharpCode.SharpZLib/Checksums/Adler32.cs