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Posted 29 Oct 2007

, 29 Oct 2007
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Presents a C# implementation of Adler-32 checksum calculation for use in the .NET Framework

## Introduction

Checksums are a common way to ensure data integrity. Adler-32 checksum may be used for revealing the damaged files or to compare two files for identity.

## Background (from en.wikipedia.org)

Adler-32 is a checksum algorithm which was invented by Mark Adler. It is almost as reliable as a 32-bit cyclic redundancy check for protecting against accidental modification of data, such as distortions occurring during a transmission.

An Adler-32 checksum is obtained by calculating two 16-bit checksums A and B and concatenating their bits into a 32-bit integer. A is the sum of all bytes in the string; B is the sum of the individual values of A from each step. At the beginning of an Adler-32 run, A is initialized to 1, B to 0. The sums are done modulo 65521 (the largest prime number smaller than 216). The bytes are stored in network order (big endian), B occupying the two most significant bytes. The function may be expressed as A = 1 + D1 + D2 + ... + DN (mod 65521) B = (1 + D1) + (1 + D1 + D2) + ... + (1 + D1 + D2 + ... + DN) (mod 65521) = N×D1 + (N-1)×D2 + (N-2)×D3 + ... + DN + N (mod 65521) Adler-32(D) = B * 65536 + A where D is the string of bytes for which the checksum is to be calculated, and N is the length of D.

## Using the Code

• Add the source code AdlerChecksum.cs to your project and add to your source file `using FileHelper;`
• Create an instance of the `AdlerChecksum`` `class.
• Call `MakeForFile` method for checksum calculation.
• Get Adler-32 checksum value by` ChecksumValue` property. For printing method `ToString` can be used.

See example code below:

```// Adler-32 checksum using
if (acs.MakeForFile(textPath.Text))
textVal.Text = acs.ToString(); //success
else
textVal.Text = "Unable to get checksum!"; //failure```

## Conclusion

I hope this code will be useful! Thanks all for the comments and critique. New suggestions and ideas are welcome.

## History

• Version 1.0.0.0, 29th October 2007 - First initial release

## Share

 Software Developer (Senior) Elmo Motion Control Israel
Software developer since 1992

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 Accuracy seandillon12-Nov-07 2:47 seandillon 12-Nov-07 2:47
 Apalling. GZero5-Nov-07 2:01 GZero 5-Nov-07 2:01
 Re: Apalling. merlin9817-Nov-07 5:37 merlin981 7-Nov-07 5:37
 First, this is a good idea, but it was implemented a little poorly. There are few comments (only XML function comments). The reason the file checksum was performing so badly is that he is reading the file one byte at a time! I rewrote the file checksum function (see below). The only problem with this rewrite is handling larger files, as reading all the bytes will cause your app to use that much memory (a 50M file will cause your app to use and additional 50M during this call). For those large files, I included a GarbageCollection in the finally block, to clear up the used memory. The buffer will almost always be in Gen 0 (sometimes Gen 1), so GC should happen very fast. ```
public bool MakeForFile(string sPath)
{
// holds the file buffer
byte[] bytesBuff = null;

try
{
// exit if no file
if (!File.Exists(sPath))
{
m_unChecksumValue = 0;
return false;
}

// get all the file bytes into the buffer

// if nothing loaded, then exit
if ((bytesBuff == null) || (bytesBuff.Length == 0))
{
m_unChecksumValue = 0;
return false;
}

// get the checksum
}

catch
{
m_unChecksumValue = 0;
return false;
}

finally
{
try
{
// do garbage collection on the array if it is too large (> 5M)
bool DoGC = bytesBuff.Length > 5000000;

if (DoGC)
{
// find out what generation the array lives in
int Gen = GC.GetGeneration(bytesBuff);

// garbage collect on bytesBuff generation
GC.Collect(Gen);
}
}

catch (Exception excep)
{
// garbage collection error handling here
}

finally
{
// clear the buffer
if (bytesBuff != null)
bytesBuff = null;
}
}

return true;
}
``` ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Rhabot - World of Warcraft Bot Long URL's get neat with NeatURL.net ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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