import pandas as pd

class GTFS:
    def __init__(self, folder_path):
        self.folder_path = folder_path
        self.agency = self.Agency(self.folder_path)
        self.stops = self.Stops(self.folder_path)
        self.routes = self.Routes(self.folder_path)
        self.trips = self.Trips(self.folder_path)
        self.stop_times = self.StopTimes(self.folder_path)
        self.calendar = self.Calendar(self.folder_path)
        self.calendar_dates = self.CalendarDates(self.folder_path)
        self.fare_attributes = self.FareAttributes(self.folder_path)
        self.fare_rules = self.FareRules(self.folder_path)
        self.shapes = self.Shapes(self.folder_path)
        self.frequencies = self.Frequencies(self.folder_path)
        self.transfers = self.Transfers(self.folder_path)
        self.feed_info = self.FeedInfo(self.folder_path)
        self.errors = []
    
    class GTFSFile:
        def __init__(self, folder_path, file_name):
            self.file_path = f"{folder_path}/{file_name}.txt"
            self.data = self.load_data()

        def load_data(self):
            try:
                return pd.read_csv(self.file_path)
            except FileNotFoundError:
                return None

    class Agency(GTFSFile):
        def __init__(self, folder_path):
            super().__init__(folder_path, 'agency')

    class Stops(GTFSFile):
        def __init__(self, folder_path):
            super().__init__(folder_path, 'stops')

    class Routes(GTFSFile):
        def __init__(self, folder_path):
            super().__init__(folder_path, 'routes')

    class Trips(GTFSFile):
        def __init__(self, folder_path):
            super().__init__(folder_path, 'trips')

    class StopTimes(GTFSFile):
        def __init__(self, folder_path):
            super().__init__(folder_path, 'stop_times')

    class Calendar(GTFSFile):
        def __init__(self, folder_path):
            super().__init__(folder_path, 'calendar')

    class CalendarDates(GTFSFile):
        def __init__(self, folder_path):
            super().__init__(folder_path, 'calendar_dates')

    class FareAttributes(GTFSFile):
        def __init__(self, folder_path):
            super().__init__(folder_path, 'fare_attributes')

    class FareRules(GTFSFile):
        def __init__(self, folder_path):
            super().__init__(folder_path, 'fare_rules')

    class Shapes(GTFSFile):
        def __init__(self, folder_path):
            super().__init__(folder_path, 'shapes')

    class Frequencies(GTFSFile):
        def __init__(self, folder_path):
            super().__init__(folder_path, 'frequencies')

    class Transfers(GTFSFile):
        def __init__(self, folder_path):
            super().__init__(folder_path, 'transfers')

    class FeedInfo(GTFSFile):
        def __init__(self, folder_path):
            super().__init__(folder_path, 'feed_info')

    def validate(self):
        self.validate_agency()
        self.validate_stops()
        self.validate_routes()
        self.validate_trips()
        self.validate_stop_times()
        self.validate_calendar()
        self.validate_cross_references()
        
        if not self.errors:
            return None
        else:
            return self.errors

    def validate_agency(self):
        required_fields = ["agency_id", "agency_name", "agency_url", "agency_timezone"]
        self.validate_required_fields(self.agency.data, required_fields, "agency.txt")

    def validate_stops(self):
        required_fields = ["stop_id", "stop_name", "stop_lat", "stop_lon"]
        self.validate_required_fields(self.stops.data, required_fields, "stops.txt")
        self.validate_lat_lon(self.stops.data)

    def validate_routes(self):
        required_fields = ["route_id", "route_short_name", "route_long_name", "route_type"]
        self.validate_required_fields(self.routes.data, required_fields, "routes.txt")

    def validate_trips(self):
        required_fields = ["route_id", "service_id", "trip_id"]
        self.validate_required_fields(self.trips.data, required_fields, "trips.txt")

    def validate_stop_times(self):
        required_fields = ["trip_id", "arrival_time", "departure_time", "stop_id", "stop_sequence"]
        self.validate_required_fields(self.stop_times.data, required_fields, "stop_times.txt")

    def validate_calendar(self):
        required_fields = ["service_id", "monday", "tuesday", "wednesday", "thursday", "friday", "saturday", "sunday", "start_date", "end_date"]
        self.validate_required_fields(self.calendar.data, required_fields, "calendar.txt")

    def validate_required_fields(self, data, required_fields, filename):
        for field in required_fields:
            if field not in data.columns:
                self.errors.append(f"Error: {filename} missing required field: {field}")

    def validate_lat_lon(self, data):
        for index, row in data.iterrows():
            if not (-90 <= row['stop_lat'] <= 90):
                self.errors.append(f"Error: stops.txt invalid latitude at row {index}: {row['stop_lat']}")
            if not (-180 <= row['stop_lon'] <= 180):
                self.errors.append(f"Error: stops.txt invalid longitude at row {index}: {row['stop_lon']}")

    def validate_cross_references(self):
        # Validate that trip_ids in stop_times.txt exist in trips.txt
        stop_times_trip_ids = set(self.stop_times.data['trip_id'])
        trips_trip_ids = set(self.trips.data['trip_id'])
        missing_trip_ids = stop_times_trip_ids - trips_trip_ids
        for trip_id in missing_trip_ids:
            self.errors.append(f"Error: trip_id {trip_id} in stop_times.txt does not exist in trips.txt")
        
        # Validate that stop_ids in stop_times.txt exist in stops.txt
        stop_times_stop_ids = set(self.stop_times.data['stop_id'])
        stops_stop_ids = set(self.stops.data['stop_id'])
        missing_stop_ids = stop_times_stop_ids - stops_stop_ids
        for stop_id in missing_stop_ids:
            self.errors.append(f"Error: stop_id {stop_id} in stop_times.txt does not exist in stops.txt")