code: def longest_dir_prefix(path, prefixes):
    
    longest_match, longest_prefix = 0, None
    for prefix in prefixes:
      if fast_relpath_optional(path, prefix) is not None and len(prefix) > longest_match:
        longest_match, longest_prefix = len(prefix), prefix
  
    return longest_prefix
docs: Given a list of prefixes, return the one that is the longest prefix to the given path.
  
    Returns None if there are no matches.