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Aziz ur Rahman

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Compression in Dot Net (Zip format).

Like many of you out there, I have also used #ZipLib (pronounced as SharpZipLib) to do compression programmatically. For those who do not know about #ZipLib, it is an open source compression library for zip formats, written in C#. The code is available at their website.

I was thinking that how it is possible that Microsoft did not provide any type of compression APIs in Dot Net Framework. After googling for some time, I came to know that there is a way to do that in dot net. But first let me give you some historical background.

Like many other compression formats e.g. tar.gz, tar.bz2; zip format is one of most popular formats used for compression. Its popularity comes from the fact (beside good compression) that the Zip data format is open and not subject to patents or other legal issues. Developers are free to create applications that manipulate Zip files and to use the low-level Zip compression algorithms. The authors of the Zip , Jean-loup Gailly (compression) and Mark Adler (decompression),  made the compression and decompression algorithms available to developers in a library named ZLib. This library was adopted by the Java platform in version 1.1 of the Java Development Kit (JDK) to form the basis of the Java Archive (JAR) file format. You can find these classes under the java.util.zip namespace.

Now here comes the dot net. When Microsoft "transforms"  all of the Java APIs to J#, zip API in java is also inherited in J#. so the zip API is not in the dot net framework but it is present in J# APIs. Just import the assembly and start zipping.

The java.util.zip namespace is implemented in the vjslib.dll assembly. This assembly can be found in the C:\WINNT\Microsoft.NET\Framework\v1.0.4205\ directory (you will need to replace WINNT with your actual Windows directoroy).

Check out this article describing all of this in detail. Actually I was just reading different compression algorithms like Huffman coding, run length encoding , arithmetic coding,  lz-77 encoding etc when suddenly a question came to my mind that what are the algorithms used in zip format compression. The deflation algorithm used by gzip (also zip and zlib) is a variation of Huffman coding and LZ77 compression. One can find more details at their website.

A day away from database oriented applications, a day for just algorithms. :)

Posted: Saturday, July 09, 2005 2:33 PM by aziz
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