import posixpath


def shift_path_info(environ):
    """Shift a name from PATH_INFO to SCRIPT_NAME, returning it

    If there are no remaining path segments in PATH_INFO, return None.
    Note: 'environ' is modified in-place; use a copy if you need to keep
    the original PATH_INFO or SCRIPT_NAME.

    Note: when PATH_INFO is just a '/', this returns '' and appends a trailing
    '/' to SCRIPT_NAME, even though empty path segments are normally ignored,
    and SCRIPT_NAME doesn't normally end in a '/'.  This is intentional
    behavior, to ensure that an application can tell the difference between
    '/x' and '/x/' when traversing to objects.
    """
    path_info = environ.get('PATH_INFO','')
    if not path_info:
        return None

    path_parts = path_info.split('/')
    path_parts[1:-1] = [p for p in path_parts[1:-1] if p and p != '.']
    name = path_parts[1]
    del path_parts[1]

    script_name = environ.get('SCRIPT_NAME','')
    script_name = posixpath.normpath(script_name+'/'+name)
    if script_name.endswith('/'):
        script_name = script_name[:-1]
    if not name and not script_name.endswith('/'):
        script_name += '/'

    environ['SCRIPT_NAME'] = script_name
    environ['PATH_INFO']   = '/'.join(path_parts)

    # Special case: '/.' on PATH_INFO doesn't get stripped,
    # because we don't strip the last element of PATH_INFO
    # if there's only one path part left.  Instead of fixing this
    # above, we fix it here so that PATH_INFO gets normalized to
    # an empty string in the environ.
    if name=='.':
        name = None
    return name