From 6f3f4e8fb85be5dd0874af3f956f6b6f3b47a402 Mon Sep 17 00:00:00 2001
From: John Benediktsson <mrjbq7@gmail.com>
Date: Tue, 28 Apr 2015 19:28:14 -0700
Subject: [PATCH] math.transforms.bwt: faster versions of Burrows-Wheeler
 Transform.

---
 extra/math/transforms/bwt/bwt-tests.factor |  4 ++--
 extra/math/transforms/bwt/bwt.factor       | 19 ++++++++++---------
 2 files changed, 12 insertions(+), 11 deletions(-)

diff --git a/extra/math/transforms/bwt/bwt-tests.factor b/extra/math/transforms/bwt/bwt-tests.factor
index a5b9856bca..9491bf644e 100644
--- a/extra/math/transforms/bwt/bwt-tests.factor
+++ b/extra/math/transforms/bwt/bwt-tests.factor
@@ -1,6 +1,6 @@
 ! Copyright (C) 2012 John Benediktsson
 ! See http://factorcode.org/license.txt for BSD license
-USING: strings tools.test ;
+USING: tools.test ;
 IN: math.transforms.bwt
 
-{ "asdf" } [ "asdf" bwt ibwt >string ] unit-test
+{ "asdf" } [ "asdf" bwt ibwt ] unit-test
diff --git a/extra/math/transforms/bwt/bwt.factor b/extra/math/transforms/bwt/bwt.factor
index b7054c9d3e..35844580e5 100644
--- a/extra/math/transforms/bwt/bwt.factor
+++ b/extra/math/transforms/bwt/bwt.factor
@@ -1,15 +1,16 @@
 ! Copyright (C) 2012 John Benediktsson
 ! See http://factorcode.org/license.txt for BSD license
-USING: arrays fry kernel math sequences sequences.extras
-sorting ;
+USING: assocs fry kernel sequences sequences.rotated sorting ;
 IN: math.transforms.bwt
 
-! Inefficient versions of Burrows-Wheeler Transform
+! Semi-efficient versions of Burrows-Wheeler Transform
 
-: bwt ( seq -- newseq )
-    0 suffix all-rotations natural-sort [ last ] map ;
+: bwt ( seq -- i newseq )
+    dup all-rotations natural-sort
+    [ [ sequence= ] with find drop ]
+    [ [ last ] rot map-as ] 2bi ;
 
-: ibwt ( newseq -- seq )
-    [ length [ { } <array> ] keep ] keep
-    '[ _ [ prefix ] 2map natural-sort ] times
-    [ last 0 = ] find nip but-last ;
+: ibwt ( i newseq -- seq )
+    [ length ]
+    [ <enum> sort-values '[ _ nth first2 ] ]
+    [ replicate-as ] tri nip ;