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I am reading about many compression and archiving techniques.. I am not able to find the compression algorithms that are used by rar. Also I would like to know if rar compresses each file separately or as a group to exploit more redundancies?

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closed as too localized by Xavierjazz, Dave, Brad Patton, 8088, Karan May 1 '13 at 21:19

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The answer to your second question depends entirely on whether solid archiving was used or not. Read the Wikipedia article on solid compression. –  Karan May 1 '13 at 21:17

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up vote 3 down vote accepted

We do not know the exact compression algorith for RAR archives, because it is a proprietary archive file format/software.

The RAR compression utility is proprietary software, to which Alexander L. Roshal, the elder brother of Eugene Roshal, owns the copyright. (Source: Wikipedia)

Although we don't know the compression algorithm, we know the decompression one. RARLAB (home of WinrRAR) provides addons (source code, plugins, DLL-s) for decompressing archives made by RAR. So this way you can learn a bit about the compression algorithm, but keep in mind that these addons are not free softwares and as the RAR EULA states:

Neither RAR binary code, WinRAR binary code, UnRAR source or UnRAR binary code may be used or reverse engineered to re-create the RAR compression algorithm, which is proprietary, without written permission.

Wikipedia also tell us that version 3 of RAR is based on Lempel-Ziv (LZSS) and prediction by partial matching (PPM) compression, specifically the PPMd implementation of PPMII by Dmitry Shkarin.

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