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:

ICSharpCode::SharpZipLib::Checksums::IChecksum List of all members.

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 bval)
 Updates the checksum with the byte b.
void Update (byte[] buffer)
 Updates the checksum with an array of bytes.
void Update (byte[] buf, int off, int len)
 Updates the checksum with the bytes taken from the array.

Properties

long Value
 Returns the Adler32 data checksum computed so far.

Private Attributes

uint checksum

Static Private Attributes

static readonly 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

Definition at line 89 of file Adler32.cs.


Constructor & Destructor Documentation

ICSharpCode::SharpZipLib::Checksums::Adler32::Adler32  )  [inline]
 

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

Definition at line 111 of file Adler32.cs.


Member Function Documentation

void ICSharpCode::SharpZipLib::Checksums::Adler32::Reset  )  [inline]
 

Resets the Adler32 checksum to the initial value.

Implements ICSharpCode::SharpZipLib::Checksums::IChecksum.

Definition at line 119 of file Adler32.cs.

void ICSharpCode::SharpZipLib::Checksums::Adler32::Update byte[]  buf,
int  off,
int  len
[inline]
 

Updates the checksum with the bytes taken from the array.

Parameters:
buf an array of bytes
off the start of the data used for this update
len the number of bytes to use for this update

Implements ICSharpCode::SharpZipLib::Checksums::IChecksum.

Definition at line 166 of file Adler32.cs.

void ICSharpCode::SharpZipLib::Checksums::Adler32::Update byte[]  buffer  )  [inline]
 

Updates the checksum with an array of bytes.

Parameters:
buffer The source of the data to update with.

Implements ICSharpCode::SharpZipLib::Checksums::IChecksum.

Definition at line 149 of file Adler32.cs.

void ICSharpCode::SharpZipLib::Checksums::Adler32::Update int  bval  )  [inline]
 

Updates the checksum with the byte b.

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

Implements ICSharpCode::SharpZipLib::Checksums::IChecksum.

Definition at line 130 of file Adler32.cs.


Member Data Documentation

readonly uint ICSharpCode::SharpZipLib::Checksums::Adler32::BASE = 65521 [static, private]
 

largest prime smaller than 65536

Definition at line 94 of file Adler32.cs.

uint ICSharpCode::SharpZipLib::Checksums::Adler32::checksum [private]
 

Definition at line 96 of file Adler32.cs.


Property Documentation

long ICSharpCode::SharpZipLib::Checksums::Adler32::Value [get]
 

Returns the Adler32 data checksum computed so far.

Implements ICSharpCode::SharpZipLib::Checksums::IChecksum.

Definition at line 101 of file Adler32.cs.


The documentation for this class was generated from the following file:
Generated on Fri Jun 23 21:50:05 2006 for OblivionModTranslator by  doxygen 1.4.6-NO