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Thread: identity of obscure algorithms

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    Member asmodean's Avatar
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    identity of obscure algorithms

    Occasionally while reverse engineering compression schemes I run across techniques I don't recognize even after constructing a working re-implementation in C. Exotic algorithms or common ones obfuscated by optimized implementation.

    I would like to identify those algorithms so that I can study the theory behind them. Maybe some of the compression enthusiasts here are interested to take a look sometimes?

    For example, here is something I worked on recently:
    http://asmodean.reverse.net/pages/expcf.html

    The first stage of decompression (unwtf() in that code) reads a table of character weights(?) and then fills a string replicating each character by the weight. Then it loops doing some divisions to compute indexes that I don't understand enough to describe in words.

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    Programmer Bulat Ziganshin's Avatar
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    >reads a table of character weights(?) and then fills a string replicating each character by the weight. Then it loops doing some divisions to compute indexes that I don't understand enough to describe in words.

    looks like entropy decoder

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    Member asmodean's Avatar
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    Quote Originally Posted by Bulat Ziganshin View Post
    >looks like entropy decoder
    Yeah, Shelwein also identified it as range encoding on IRC. Clearly this was too easy.

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