math.transforms.bwt: slightly faster, remove suffixes because it seems buggy.

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

@@ -1,8 +1,13 @@
 ! Copyright (C) 2012 John Benediktsson
 ! See http://factorcode.org/license.txt for BSD license
-USING: kernel tools.test ;
+USING: tools.test ;
 IN: math.transforms.bwt
 
 { "asdf" } [ "asdf" bwt ibwt ] unit-test
 
-{ t } [ "hello" [ bwt nip ] [ bwt* ] bi = ] unit-test
+{
+    29
+    "TEXYDST.E.IXIXIXXSSMPPS.B..E.S.EUSFXDIIOIIIT"
+} [
+    "SIX.MIXED.PIXIES.SIFT.SIXTY.PIXIE.DUST.BOXES" bwt
+] unit-test

extra/math/transforms/bwt/bwt.factor

@@ -1,21 +1,15 @@
 ! Copyright (C) 2012 John Benediktsson
 ! See http://factorcode.org/license.txt for BSD license
-USING: accessors assocs fry kernel math sequences
-sequences.rotated sorting suffix-arrays.private ;
+USING: accessors assocs fry kernel locals sequences
+sequences.rotated sorting ;
 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 ]
-    [ [ last ] rot map-as ] 2bi ;
+:: bwt ( seq -- i newseq )
+    seq all-rotations natural-sort
+    [ [ n>> 0 = ] find drop ] keep
+    [ last ] seq map-as ;
 
 : ibwt ( i newseq -- seq )
     [ length ]