[Haskell-cafe] Re: Need for speed: the Burrows-Wheeler Transform
andrewcoppin at btinternet.com
Sat Jun 23 05:41:00 EDT 2007
> Note that the one usually adds an "end of string" character $ in the
> Burrows-Wheeler transform for compression such that sorting rotated
> strings becomes sorting suffices.
Yeah, I noticed that the output from by program can never actually be
reverted to its original form. ;-) Maybe it I alter the code to stick a
0-byte in there or something...
(Hmm, I wonder how Mr C++ managed it? Heh. If I could read C++, maybe
> Concerning the algorithm at hand, you can clearly avoid calculating
> Raw.append over and over:
> bwt :: Raw.ByteString -> Raw.ByteString
> bwt xs = Raw.pack . map (Raw.last) . sort $ rotations
> n = length xs
> rotations = take n . map (take n) . tails $ xs `Raw.append` xs
> assuming that take n is O(1).
...which is amusing because that's what the *first* implementation did. ;-)
I was trying to avoid O(n^2) RAM usage. :-}
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