Update the namespace declaration of a Clojure buffer according to its pathname.

gistfile1.el

(defun clojure-correct-ns
  ()
  "Returns the namespace name that the file should have."
 (let* ((nsname ())
        (dirs (reverse (split-string (buffer-file-name) "/")))
        (aftersrc nil))
   (dolist (dir dirs)
     (when (not aftersrc)
       (if (or (string= dir "src") (string= dir "test"))
           (setq aftersrc t)
         (setq nsname (append nsname (list dir "."))))))
   (when nsname
     (replace-regexp-in-string "_" "-" (substring (apply 'concat (reverse nsname))  1 -4)))))

(defun clojure-update-ns
  ()
  "Updates the namespace of the current buffer. Useful if a file has been renamed."
  (interactive)
  (let ((nsname (clojure-correct-ns)))
    (when nsname
      (clojure-find-ns) ;; function defined in clojure-mode
      (replace-match nsname nil nil nil 4))))