Auto-insert " * " in block comments in Emacs

block-comments.el

(defun elmord-block-comment-auto-prefix ()
  (when (eq last-command-event ?\n)
    (let* ((last-comment-start
            (save-excursion (search-backward "/*" nil t)))
           (last-comment-end
            (save-excursion (search-backward "*/" last-comment-start t))))
      (when (and last-comment-start (not last-comment-end))
        (insert "* ")
        (indent-for-tab-command)))))

(defun elmord-block-comment-auto-prefix-enable-in-buffer ()
  (add-hook 'post-self-insert-hook
            'elmord-block-comment-auto-prefix
            nil t))

(add-hook 'scala-mode-hook 'elmord-block-comment-auto-prefix-enable-in-buffer)