C[omp]ute

From: "andrew cooke" <andrew@...>

Date: Fri, 17 Feb 2006 09:26:10 -0300 (CLST)

This is very neat.  A reversible transform that makes data trivial to
compress.

The Transform:

Take the text, of length n, and generate n different texts from the
rotations available.  Note that the last character of each line is the
prefix to the first.  Then sort and note the index of the "correct"
(unrotated) line.

The transform is the index and the final characters of the sorted list.

The Inverse:

The transform characters have all the charcters in the text.  So the
leading character column can be constructed by sorting.  This gives the
first and last character of each rotated string.  Which is a sequence of
(character, next character pairs).  Which can be chained together to give
the original text (except for an arbitrary rotation, which is corrected by
the index).

The compression:

The first characters of each line are sorted.  The transform characters
are their prefixes.  So the transform will be strongly ordered if
character pairs are strongly correlated.


Wikipedia explanation -
http://en.wikipedia.org/wiki/Burrows-Wheeler_transform

Article - http://dogma.net/markn/articles/bwt/bwt.htm

Paper - http://citeseer.ist.psu.edu/76182.html

(This is the bzip/bzip2 compression method; note that it requires the
whole input - or at least a large block size - and that decompression is
faster than compression).

From http://pmd.sourceforge.net/cpd.html
From someone on pragprog (sorry, lost reference)

Andrew