Calculate servicepattern for the gtfs_obj
set_servicepattern(gtfs_obj, hash_algo = "md5", id_prefix = "s_",
hash_length = 7)
gtfs feed
hashing algorithm used by digest
all ids start with this string
length the hash should be cut to with substr(). Use -1 if the full hash should be used
modified gtfs_obj with added servicepattern list and a table linking trips and pattern (trip_servicepatterns)