apfelmus wrote:
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 I'd know...)
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 where 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. :-}