# Copyright (C) 2010 Chris Jerdonek (cjerdonek@webkit.org)

#

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# 1.  Redistributions of source code must retain the above copyright

#     notice, this list of conditions and the following disclaimer.

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#

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"""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