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tinderbox
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Physical-pathname-directory-separator
Monday, February 6, 2006

I couldn't immediately think of an existing way to determine the physical pathname directory separator in Lisp so I wrote this:

(defun physical-pathname-directory-separator ()
  (let* ((directory-1 "foo")
         (directory-2 "bar")
         (pn (namestring
              (translate-logical-pathname
               (make-pathname
                :host nil
                :directory `(:absolute ,directory-1 ,directory-2)
                :name nil
                :type nil))))
         (foo-pos (search directory-1 pn :test #'char-equal))
         (bar-pos (search directory-2 pn :test #'char-equal)))
    (subseq pn (+ foo-pos (length directory-1)) bar-pos)))

This works on my Mac under Digitool's MCL, SBCL, CLISP and Allegro. I still don't have a Windows box to test upon but maybe some happy reader can contribute! Alternatively, some unhappy reader can tell me the 100-ways in which the code above is wrong <smile>.

(BTW, I assume in the code that the delimiter could be a string with length greater than one. It's probably always going to be a single character which would make my life slightly more easy... Does anyone know of a file system that uses multi-character delimiters between directories? How about something that would disallow such a scheme?).

Update: Peter Seibel reminds me that I should probably be using #'char-equal, instead of #string-equal in my calls to search. Also, I have reports of success under SBCL and Windows 2000. I'll take that as a success.


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