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Common Lisp implementation running on the JVM
There is a newer version: 1.9.2
;;; enough-namestring.lisp
;;;
;;; Copyright (C) 2004-2005 Peter Graves
;;; $Id$
;;;
;;; This program is free software; you can redistribute it and/or
;;; modify it under the terms of the GNU General Public License
;;; as published by the Free Software Foundation; either version 2
;;; of the License, or (at your option) any later version.
;;;
;;; This program is distributed in the hope that it will be useful,
;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;;; GNU General Public License for more details.
;;;
;;; You should have received a copy of the GNU General Public License
;;; along with this program; if not, write to the Free Software
;;; Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
;;;
;;; As a special exception, the copyright holders of this library give you
;;; permission to link this library with independent modules to produce an
;;; executable, regardless of the license terms of these independent
;;; modules, and to copy and distribute the resulting executable under
;;; terms of your choice, provided that you also meet, for each linked
;;; independent module, the terms and conditions of the license of that
;;; module.  An independent module is a module which is not derived from
;;; or based on this library.  If you modify this library, you may extend
;;; this exception to your version of the library, but you are not
;;; obligated to do so.  If you do not wish to do so, delete this
;;; exception statement from your version.

;;; Adapted from SBCL.

(in-package #:system)

(declaim (inline equal-components-p))
(defun equal-components-p (component1 component2)
  #+win32 (equalp component1 component2)
  #-win32 (equal component1 component2))

(defun enough-namestring (pathname
                          &optional
                          (defaults *default-pathname-defaults*))
  (unless (equal (pathname-host pathname) (pathname-host defaults))
    (return-from enough-namestring (namestring pathname)))
  (let ((pathname-directory (pathname-directory pathname)))
    (if pathname-directory
        (let* ((defaults-directory (pathname-directory defaults))
               (prefix-len (length defaults-directory))
               (result-directory
                (cond ((and (>= prefix-len 1)
                            (>= (length pathname-directory) prefix-len)
                            (equal-components-p (subseq pathname-directory 0 prefix-len)
                                                defaults-directory))
                       (cons :relative (nthcdr prefix-len pathname-directory)))
                      ((eq (car pathname-directory) :absolute)
                       pathname-directory)
                      (t
                       (return-from enough-namestring (namestring pathname))))))
          (if (equal result-directory '(:relative))
              (file-namestring pathname)
              (concatenate 'simple-string
                           (directory-namestring (make-pathname :directory result-directory))
                           (file-namestring pathname))))
        (file-namestring pathname))))