(defvarl eof "\xDC00")

(defun bwt (str)
  (if (contains eof str)
    (error "~s: input may not contain ~a" %fun% eof))
  (let ((seof `@str@eof`))
    (flow 0..(len seof) (mapcar (op rot seof)) sort (mappend last))))

(defun ibwt (str)
  (let* ((ch (tuples 1 str))
         (row (sort ch)))
    (dotimes (i (pred (len str)))
      (upd row (mapcar append ch) nsort))
    [(find-if (op ends-with eof) row) 0..-1]))