> Now MP3 does that to sound files, but it does that by DELETING DATA. The
> compressed file has lower fidelity than the original. It's not that
> surprising that you can make a file smaller by removing data from it.
> But LOSSLESS compression algorithms do something seemingly impossible:
> they take a file, somehow make it smaller, and yet the process is
> EXACTLY REVERSIBLE. When you decompress the file again, it is IDENTICAL
> to the original. How is that even possible?

It's not.  The resulting decompressed file is a good* approximation of 
the original, but it's not the same.

*For some arbitrary definition of good.

>
> Well, it ISN'T always possible. Sometimes when you zip a file it gets
> tiny, and sometimes it doesn't get that much smaller. You may not have
> experienced this, but occasionally a file will get LARGER. For example,
> zip a file - any file - and then go zip the zip file. It WILL NOT get
> any smaller. I promise. (Think about it; otherwise you could take an
> arbitrary file and zip it over and over until it gets arbitrarily small.
> But that really IS impossible!)

That's normal.  If the space saved by compressing the file is smaller 
than the dictionary, then you end up with a file that's going to be 
larger than the original.

[Rest of discussion snipped]

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