potentially wrong handling of GTFS Time values?

[derhuerst]

GTFS Time is not defined relative to midnight, but relative to noon - 12h. While that makes "writing" GTFS feeds easier, it makes processing a lot harder.

Expected functionality

As explained in my note about GTFS Time values, with the Europe/Berlin time zone (+1h standard time to +2 DST shift occurs at 2021-03-28T02:00+01:00), I expect

• the departure_time of 00:30 of a trip running on 2021-03-28 to happen at 1616884200/2021-03-28T00:30+02:00, not at 1616887800/2021-03-28T00:30+01:00;
• the departure_time of 06:30 of a trip running on 2021-03-28 to happen at 1616905800/2021-03-28T06:30+02:00, not at 1616909400/2021-03-28T06:30+01:00.

Describe the bug

I'm not familiar with this code base, but it seems that fastgtfs is affected by this problem on those days that the DST <-> standard time switch occurs on.

I'm not sure how that actually manifests in fastgtfs's output, but I assume that wrong delays will be calculated, or that realtime data can't be matched against static data.

I tried to find some places in the code base:

• fastgtfs/src/gtfs_data.rs

  Lines 113 to 127 in f77cd57

  	.filter(|t| {
  	t.stop_times_id == stop_times_id
  	&& self.is_trip_active_on_time(t, min_time, None)
  	&& t.start_time + trips_duration >= min_time.since_midnight() as i64
  	})
  	.find_map(|trip| {
  	// this returns the first for which the content is a Some result.
  	inxes_for_stop
  	.iter()
  	.filter(|&&inx| {
  	stop_times[inx].time + trip.start_time > min_time.since_midnight() as i64
  	})
  	.map(|&inx| (trip, inx))
  	.next()
  	})

• fastgtfs/src/gtfs_data.rs

  Lines 164 to 169 in f77cd57

  	/// returns the number of seconds since midnight this trip departs and arrives.
  	pub fn get_trip_departure_arrival_times(&self, trip: &Trip) -> (i64, i64) {
  	let stop_times = self.get_stop_times(trip.stop_times_id);
  	let trip_duration = stop_times.stop_times.last().unwrap().time;
  	(trip.start_time, trip.start_time + trip_duration)
  	}

• fastgtfs/src/gtfs_data.rs

  Lines 182 to 194 in f77cd57

  	let seconds_in_h = 60 * 60;
  	let within_seconds = within_seconds.unwrap_or(24 * seconds_in_h);
  	let target_time = time.since_midnight() as i64;
  	// It starts after our window.
  	if trip.start_time > target_time + within_seconds {
  	return false;
  	}
  	let (_, arrival_time) = self.get_trip_departure_arrival_times(trip);
  	// It finishes before our window.
  	if arrival_time < target_time {
  	return false;
  	}

• fastgtfs/src/gtfs_data.rs

  Lines 319 to 326 in f77cd57

  	let stop_times = &self.get_stop_times(trip.stop_times_id).stop_times;
  	let upper_time = GtfsTime::new_from_timestamp(date.timestamp + within_sec);
  	for &inx in stop_indexes {
  	let time = date.new_replacing_time(stop_times[inx].time + trip.start_time);
  	if date <= time && time <= upper_time {
  	return Some((trip.trip_id, inx));
  	}
  	}

• fastgtfs/src/gtfs_data.rs

  Lines 453 to 464 in f77cd57

  	pub fn new_from_midnight(time: i64) -> GtfsTime {
  	let ts = SystemTime::now()
  	.duration_since(SystemTime::UNIX_EPOCH)
  	.unwrap()
  	.as_secs() as i64;
  	let sec_since_midnight = Utc.timestamp(ts as i64, 0).num_seconds_from_midnight() as i64;
  	let last_midnight = ts - sec_since_midnight;
  	GtfsTime {
  	timestamp: last_midnight + time,
  	}
  	}

• fastgtfs/src/gtfs_data.rs

  Lines 479 to 487 in f77cd57

  	pub fn from_date(yyyymmdd: &str) -> GtfsTime {
  	let date = NaiveDate::parse_from_str(yyyymmdd, "%Y%m%d")
  	.expect(&format!("Invalid date: {}", yyyymmdd));
  	let date = Utc
  	.from_utc_date(&date)
  	.and_time(NaiveTime::from_hms(0, 0, 0))
  	.unwrap();
  	GtfsTime::new_from_timestamp(date.timestamp())
  	}

• fastgtfs/src/gtfs_data.rs

  Lines 501 to 518 in f77cd57

  	pub fn is_same_day(&self, other: &GtfsTime) -> bool {
  	self.date_time().num_days_from_ce() == other.date_time().num_days_from_ce()
  	}
  	pub fn day_of_week(&self) -> u32 {
  	self.date_time().weekday().num_days_from_monday()
  	}
  	pub fn h(&self) -> u32 {
  	self.date_time().hour()
  	}
  	pub fn m(&self) -> u32 {
  	self.date_time().minute()
  	}
  	pub fn s(&self) -> u32 {
  	self.date_time().second()
  	}
  	pub fn since_midnight(&self) -> u64 {
  	self.date_time().num_seconds_from_midnight() as u64
  	}

• fastgtfs/src/realtime_position.rs

  Lines 46 to 50 in f77cd57

  	// seconds after the initial_point
  	let att_time = seconds_since_midnight - self.time_at_shape_point_seconds[start_point];
  	let delta_time_ = self.time_at_shape_point_seconds[start_point + 1]
  	- self.time_at_shape_point_seconds[start_point]
  	+ 1;

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related: google/transit#15