repack-group-numbers

(defcommand repack-group-numbers (&optional preserved) ()
  "Ensure that used group numbers do not have gaps; ignore PRESERVED group numbers."
  (let ((groups (remove-if
                 (lambda (x) (member (group-number x) preserved))
                 (sort1 (screen-groups (current-screen))
                        #'<
                        :key #'group-number))))
    (labels
        ((repack (&optional items (idx 1))
           (cond
            ((null items)
             nil)

            ((= idx (group-number (car items)))
             (repack (cdr items) (1+ idx)))

            ((member idx (mapcar #'group-number groups))
             (repack items (1+ idx)))

            (t (progn
                 (setf (group-number (car items)) idx)
                 (repack (cdr items) (1+ idx)))))))

      (repack groups))))