code: def get_closest_mac_host_platform_pair(darwin_version_upper_bound,
                                         platform_name_map=SUPPORTED_PLATFORM_NORMALIZED_NAMES):
    
    darwin_versions = [int(x[1]) for x in platform_name_map if x[0] == 'darwin']
    bounded_darwin_versions = [v for v in darwin_versions if v <= int(darwin_version_upper_bound)]
    if not bounded_darwin_versions:
      return None, None
    max_darwin_version = str(max(bounded_darwin_versions))
    return platform_name_map[('darwin', max_darwin_version)]
docs: Return the (host, platform) pair for the highest known darwin version less than the bound.