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+# Copyright (C) 2010 Chris Jerdonek (cjerdonek@webkit.org)
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions
+# are met:
+# 1.  Redistributions of source code must retain the above copyright
+#     notice, this list of conditions and the following disclaimer.
+# 2.  Redistributions in binary form must reproduce the above copyright
+#     notice, this list of conditions and the following disclaimer in the
+#     documentation and/or other materials provided with the distribution.
+#
+# THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS'' AND
+# ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+# WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+# DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS BE LIABLE FOR
+# ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+# SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+# CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+# OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+"""Contains a substitute for Python 2.6's os.path.relpath()."""
+
+import os
+
+
+# This function is a replacement for os.path.relpath(), which is only
+# available in Python 2.6:
+#
+# http://docs.python.org/library/os.path.html#os.path.relpath
+#
+# It should behave essentially the same as os.path.relpath(), except for
+# returning None on paths not contained in abs_start_path.
+def relpath(path, start_path, os_path_abspath=None):
+    """Return a path relative to the given start path, or None.
+
+    Returns None if the path is not contained in the directory start_path.
+
+    Args:
+      path: An absolute or relative path to convert to a relative path.
+      start_path: The path relative to which the given path should be
+                  converted.
+      os_path_abspath: A replacement function for unit testing.  This
+                       function should strip trailing slashes just like
+                       os.path.abspath().  Defaults to os.path.abspath.
+
+    """
+    if os_path_abspath is None:
+        os_path_abspath = os.path.abspath
+
+    # Since os_path_abspath() calls os.path.normpath()--
+    #
+    # (see http://docs.python.org/library/os.path.html#os.path.abspath )
+    #
+    # it also removes trailing slashes and converts forward and backward
+    # slashes to the preferred slash os.sep.
+    start_path = os_path_abspath(start_path)
+    path = os_path_abspath(path)
+
+    if not path.lower().startswith(start_path.lower()):
+        # Then path is outside the directory given by start_path.
+        return None
+
+    rel_path = path[len(start_path):]
+
+    if not rel_path:
+        # Then the paths are the same.
+        pass
+    elif rel_path[0] == os.sep:
+        # It is probably sufficient to remove just the first character
+        # since os.path.normpath() collapses separators, but we use
+        # lstrip() just to be sure.
+        rel_path = rel_path.lstrip(os.sep)
+    else:
+        # We are in the case typified by the following example:
+        #
+        # start_path = "/tmp/foo"
+        # path = "/tmp/foobar"
+        # rel_path = "bar"
+        return None
+
+    return rel_path