math.transforms.bwt: faster version of bwt if only needing transform.

extra/math/transforms/bwt/bwt-tests.factor

@@ -1,6 +1,8 @@
 ! Copyright (C) 2012 John Benediktsson
 ! See http://factorcode.org/license.txt for BSD license
-USING: tools.test ;
+USING: kernel tools.test ;
 IN: math.transforms.bwt
 
 { "asdf" } [ "asdf" bwt ibwt ] unit-test
+
+{ t } [ "hello" [ bwt nip ] [ bwt* ] bi = ] unit-test

extra/math/transforms/bwt/bwt.factor

@@ -1,10 +1,17 @@
 ! Copyright (C) 2012 John Benediktsson
 ! See http://factorcode.org/license.txt for BSD license
-USING: assocs fry kernel sequences sequences.rotated sorting ;
+USING: accessors assocs fry kernel math sequences
+sequences.rotated sorting suffix-arrays.private ;
 IN: math.transforms.bwt
 
 ! Semi-efficient versions of Burrows-Wheeler Transform
 
+: bwt* ( seq -- newseq )
+    [
+        dup suffixes natural-sort
+        [ dup from>> [ to>> ] [ nip ] if-zero 1 - over nth ]
+    ] [ map-as ] bi nip ;
+
 : bwt ( seq -- i newseq )
     dup all-rotations natural-sort
     [ [ sequence= ] with find drop ]