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+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE TypeFamilies #-}
+{-# LANGUAGE KindSignatures #-}
+{-# LANGUAGE StandaloneKindSignatures #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE StandaloneDeriving #-}
+{-# LANGUAGE TypeApplications #-}
+{-# LANGUAGE LambdaCase #-}
+{-# LANGUAGE RecordWildCards #-}
+
+
+module Main where
+
+
+import qualified Data.ByteString.Lazy as LB
+import qualified Data.ByteString as BS
+import qualified Data.Csv as CSV
+import Data.Csv ((.:))
+import qualified Codec.Archive.Zip as Zip
+import qualified Data.Vector as V
+import Data.Vector (Vector)
+import Text.Regex.TDFA ( (=~) )
+import Data.Text (Text)
+import Fmt ( (+|), (|+) )
+import Data.Kind (Type)
+import Data.Maybe (fromMaybe, fromJust)
+import Data.Functor ((<&>))
+import qualified Data.Time.Calendar.OrdinalDate as Day
+import Data.Time.Calendar.OrdinalDate (Day)
+import Data.Time.Calendar.MonthDay (monthAndDayToDayOfYearValid)
+import Data.Time (getCurrentTime, UTCTime (utctDay), dayOfWeek)
+import Data.Time.Calendar.WeekDate (DayOfWeek(..))
+
+
+newtype Time = Time { toSeconds :: Int }
+
+instance CSV.FromField Time where
+  parseField f = do
+    text :: String <- CSV.parseField f
+    let (_,_,_,subs) = text =~ ("([0-9][0-9]?):([0-9][0-9]?):([0-9][0-9]?)" :: Text)
+          :: (String, String, String, [String])
+    case subs of
+      [hh,mm,ss] -> pure $ Time $ read hh * 3600 + read mm * 60 + read ss
+      _ -> fail $ "encountered an invalid date: " <> text
+
+instance Show Time where
+  show (Time seconds) = ""
+    +|seconds `div` 3600|+":"
+    +|(seconds `mod` 3600) `div` 60|+":"
+    +|seconds `mod` 60|+""
+
+instance CSV.FromField Day where
+  parseField f = do
+    text :: String <- CSV.parseField f
+    let (_,_,_,subs) = text =~ ("([0-9][0-9][0-9][0-9])([0-9][0-9])([0-9][0-9])" :: Text)
+          :: (String, String, String, [String])
+    case subs of
+      [yyyy,mm,dd] -> do
+        let Just dayOfYear = monthAndDayToDayOfYearValid (Day.isLeapYear (read yyyy)) (read mm) (read dd)
+        pure $ Day.fromOrdinalDate (read yyyy) dayOfYear
+      _ -> fail $ "invalid date encountered: " <> show f
+
+
+
+data Depth = Shallow | Deep
+type Switch :: Depth -> Type -> Type -> Type
+type family Switch c a b where
+  Switch Deep a b = a
+  Switch Shallow a b = b
+
+type StationID = Text
+
+-- | This is what's called a Stop in GTFS
+data Station = Station
+  { stationId :: StationID
+  , stationName :: Text
+  , stationLat :: Float
+  , stationLon :: Float
+  } deriving Show
+
+-- | This is what's called a stop time in GTFS
+data Stop (deep :: Depth) = Stop
+  { stopTrip :: Int
+  , stopArrival :: Time
+  , stopDeparture :: Time
+  , stopStation:: Switch deep Station StationID
+  , stopSequence :: Int
+  }
+
+deriving instance Show (Stop 'Shallow)
+deriving instance Show (Stop 'Deep)
+
+
+data Calendar = Calendar
+  { calServiceId :: Text
+  , calMonday :: Bool
+  , calTuesday :: Bool
+  , calWednesday :: Bool
+  , calThursday :: Bool
+  , calFriday :: Bool
+  , calSaturday :: Bool
+  , calSunday :: Bool
+  , calStartDate :: Day
+  , calEndDate :: Day
+  } deriving Show
+
+data CalendarExceptionType = ServiceAdded | ServiceRemoved
+  deriving (Show, Eq)
+
+data CalendarDate = CalendarDate
+  { caldateServiceId :: Text
+  , caldateDate :: Day
+  , caldateExceptionType :: CalendarExceptionType
+  } deriving Show
+
+type TripID = Text
+type ServiceID = Text
+
+data Trip = Trip
+  { tripRoute :: Text
+  , tripTripID :: TripID
+  , tripHeadsign :: Maybe Text
+  , tripShortName :: Maybe Text
+  , tripDirection :: Maybe Bool
+  -- NOTE: there's also block_id, which we're unlikely to ever need
+  , tripServiceId :: Text
+  -- , tripWheelchairAccessible :: Bool
+  -- , tripBikesAllowed :: Bool
+  , tripShapeId :: Text
+  } deriving Show
+
+-- | helper function to find things in Vectors of things
+tableLookup :: Eq key => (a -> key) -> key -> Vector a -> Maybe a
+tableLookup proj key = V.find (\a -> proj a == key)
+
+instance CSV.FromNamedRecord Station where
+  parseNamedRecord r = Station
+    <$> r .: "stop_id"
+    <*> r .: "stop_name"
+    <*> r .: "stop_lat"
+    <*> r .: "stop_lon"
+
+instance CSV.FromNamedRecord (Stop 'Shallow) where
+  parseNamedRecord r = Stop
+    <$> r .: "trip_id"
+    <*> r .: "arrival_time"
+    <*> r .: "departure_time"
+    <*> r .: "stop_id"
+    <*> r .: "stop_sequence"
+
+instance CSV.FromNamedRecord Calendar where
+  parseNamedRecord r = Calendar
+    <$> r .: "service_id"
+    <*> intAsBool' r "monday"
+    <*> intAsBool' r "tuesday"
+    <*> intAsBool' r "wednesday"
+    <*> intAsBool' r "thursday"
+    <*> intAsBool' r "friday"
+    <*> intAsBool' r "saturday"
+    <*> intAsBool' r "sunday"
+    <*> r .: "start_date"
+    <*> r .: "end_date"
+
+intAsBool :: CSV.NamedRecord -> BS.ByteString -> CSV.Parser (Maybe Bool)
+intAsBool r field = do
+  int <- r .: field
+  pure $ case int :: Int of
+    1 -> Just True
+    0 -> Just False
+    _ -> Nothing
+
+intAsBool' :: CSV.NamedRecord -> BS.ByteString -> CSV.Parser Bool
+intAsBool' r field = intAsBool r field >>= maybe
+  (fail "unexpected value for a boolean.")
+  pure
+
+
+instance CSV.FromNamedRecord CalendarDate where
+  parseNamedRecord r = CalendarDate
+    <$> r .: "service_id"
+    <*> r .: "date"
+    <*> do
+      int <- r .: "exception_type"
+      case int :: Int of
+        1 -> pure ServiceAdded
+        2 -> pure ServiceRemoved
+        _ -> fail $ "unexpected value in exception_type: "+|int|+"."
+
+
+instance CSV.FromNamedRecord Trip where
+  parseNamedRecord r = Trip
+    <$> r .: "route_id"
+    <*> r .: "trip_id"
+    <*> r .: "trip_headsign"
+    <*> r .: "trip_short_name"
+    <*> intAsBool r "direction_id"
+    <*> r .: "service_id"
+    -- NOTE: these aren't booleans but triple-values
+    -- <*> intAsBool r "wheelchair_accessible"
+    -- <*> intAsBool r "bikes_allowed"
+    <*> r .: "shape_id"
+
+data GTFS (depth :: Depth) = GTFS
+  { stations :: Vector Station
+  , stops :: Vector (Stop depth)
+  , trips :: Vector Trip
+  , calendar :: Maybe (Vector Calendar)
+  , calendarDates :: Maybe (Vector CalendarDate)
+  }
+
+deriving instance Show (GTFS Shallow)
+deriving instance Show (GTFS Deep)
+
+class Loadable depth where
+  loadGtfs :: FilePath -> IO (GTFS depth)
+
+instance Loadable Shallow where
+  loadGtfs path = do
+    zip <- Zip.toArchive <$> LB.readFile "./gtfs.zip"
+    GTFS <$> decodeTable' "stops.txt" zip
+         <*> decodeTable' "stop_times.txt" zip
+         <*> decodeTable' "trips.txt" zip
+         <*> decodeTable "calendar.txt" zip
+         <*> decodeTable "calendar_dates.txt" zip
+    where
+      decodeTable :: CSV.FromNamedRecord a => FilePath -> Zip.Archive -> IO (Maybe (Vector a))
+      decodeTable path zip = do
+        case Zip.findEntryByPath path zip of
+          Nothing -> pure Nothing
+          Just csv -> case CSV.decodeByName (Zip.fromEntry csv) of
+            Left err -> error "blah"
+            Right (_,v :: a) -> pure (Just v)
+      decodeTable' path zip =
+        decodeTable path zip >>= \case
+          Nothing -> fail $ "required file "+|path|+" not found in gtfs.zip"
+          Just a -> pure a
+
+instance Loadable Deep where
+  loadGtfs path = do
+    shallow <- loadGtfs @Shallow path
+    stops' <- V.mapM (pushStop (stations shallow)) (stops shallow)
+    pure $ shallow { stops = stops' }
+    where
+      pushStop :: Vector Station -> Stop Shallow -> IO (Stop Deep)
+      pushStop stations stop = do
+        station <- case tableLookup stationId (stopStation stop) stations of
+          Just a -> pure a
+          Nothing -> fail $ "station with id "+|stopStation stop|+"is mentioned but not defined."
+        pure $ stop { stopStation = station }
+
+
+
+servicesOnDay :: GTFS Deep -> Day -> Vector ServiceID
+servicesOnDay GTFS{..} day =
+  fmap caldateServiceId added <> V.filter notCancelled regular
+  where (added,removed) = case calendarDates of
+          Nothing -> (mempty,mempty)
+          Just exs ->
+            V.partition (\cd -> caldateExceptionType cd == ServiceAdded)
+            $ V.filter (\cd -> caldateDate cd == day) exs
+        regular = case calendar of
+          Nothing -> mempty
+          Just cs -> V.mapMaybe (\cal -> if isRunning cal then Just (calServiceId cal) else Nothing) cs
+          where isRunning Calendar{..} =
+                  day >= calStartDate &&
+                  day <= calEndDate &&
+                  case weekday of
+                    Monday -> calMonday
+                    Tuesday -> calTuesday
+                    Wednesday -> calWednesday
+                    Thursday -> calThursday
+                    Friday -> calFriday
+                    Saturday -> calSaturday
+                    Sunday -> calSunday
+        weekday = dayOfWeek day
+        notCancelled serviceID =
+          null (tableLookup caldateServiceId serviceID removed)
+
+tripsOfService :: GTFS Deep -> ServiceID -> Vector Trip
+tripsOfService GTFS{..} serviceId =
+  V.filter (\trip -> tripServiceId trip == serviceId ) trips
+
+
+main :: IO ()
+main = do
+  gtfs <- loadGtfs @Deep "./gtfs.zip"
+
+  -- today <- getCurrentTime <&> utctDay
+  -- print (calendar gtfs)
+  let today = Day.fromOrdinalDate 2022 (fromJust $ monthAndDayToDayOfYearValid False 6 6)
+  print today
+
+  putStrLn "trips today:"
+  print (fmap (tripsOfService gtfs)  (servicesOnDay gtfs today))
+
+
+{-
+TODO:
+there should be a basic API allowing the questions:
+ - what are the next trips leaving from $station? (or $geolocation?)
+ - all stops of a given tripID
+
+then the "ingress" API:
+ - train ping (location, estimated delay, etc.)
+ - cancel trip
+ - add trip?
+
+-}