{-# LANGUAGE DerivingStrategies    #-}
{-# LANGUAGE ExplicitNamespaces    #-}
{-# LANGUAGE FlexibleContexts      #-}
{-# LANGUAGE FlexibleInstances     #-}
{-# LANGUAGE LambdaCase            #-}
{-# LANGUAGE OverloadedLists       #-}
{-# LANGUAGE PartialTypeSignatures #-}
{-# LANGUAGE RecordWildCards       #-}
{-# LANGUAGE TypeApplications      #-}


-- Implementation of the API. This module is the main point of the program.
module Server (application) where
import           Control.Concurrent.STM      (TQueue, TVar, atomically,
                                              newTQueue, newTVar, readTQueue,
                                              readTVar, writeTQueue, writeTVar)
import           Control.Monad               (forever, unless, void, when)
import           Control.Monad.Catch         (handle)
import           Control.Monad.Extra         (ifM, maybeM, unlessM, whenJust,
                                              whenM)
import           Control.Monad.IO.Class      (MonadIO (liftIO))
import           Control.Monad.Logger        (LoggingT, logWarnN)
import           Control.Monad.Reader        (forM)
import           Control.Monad.Trans         (lift)
import qualified Data.Aeson                  as A
import qualified Data.ByteString.Char8       as C8
import           Data.Coerce                 (coerce)
import           Data.Functor                ((<&>))
import qualified Data.Map                    as M
import           Data.Pool                   (Pool)
import           Data.Proxy                  (Proxy (Proxy))
import           Data.Swagger                (toSchema)
import           Data.Text                   (Text)
import           Data.Text.Encoding          (decodeUtf8)
import           Data.Time                   (NominalDiffTime,
                                              UTCTime (utctDay), addUTCTime,
                                              diffUTCTime, getCurrentTime,
                                              nominalDay)
import qualified Data.Vector                 as V
import           Database.Persist
import           Database.Persist.Postgresql (SqlBackend, runMigration)
import           Fmt                         ((+|), (|+))
import qualified Network.WebSockets          as WS
import           Servant                     (Application,
                                              ServerError (errBody), err401,
                                              err404, serve, throwError)
import           Servant.API                 (NoContent (..), (:<|>) (..))
import           Servant.Server              (Handler, hoistServer)
import           Servant.Swagger             (toSwagger)

import           API
import           GTFS
import           Persist
import           Server.ControlRoom
import           Server.GTFS_RT              (gtfsRealtimeServer)
import           Server.Util                 (Service, ServiceM, runService,
                                              sendErrorMsg)
import           Yesod                       (toWaiAppPlain)

import           Extrapolation               (Extrapolator (..),
                                              LinearExtrapolator (..))
import           System.IO.Unsafe

import           Config                      (ServerConfig)
import           Data.ByteString             (ByteString)
import           Data.ByteString.Lazy        (toStrict)
import           Prometheus
import           Prometheus.Metric.GHC

application :: GTFS -> Pool SqlBackend -> ServerConfig -> IO Application
application gtfs dbpool settings = do
  doMigration dbpool
  metrics <- Metrics
    <$> register (gauge (Info "ws_connections" "Number of WS Connections"))
  register ghcMetrics
  subscribers <- atomically $ newTVar mempty
  pure $ serve (Proxy @CompleteAPI) $ hoistServer (Proxy @CompleteAPI) runService $ server gtfs metrics subscribers dbpool settings

-- databaseMigration :: ConnectionString -> IO ()
doMigration pool = runSql pool $
  -- TODO: before that, check if the uuid module is enabled
  -- in sql: check if SELECT * FROM pg_extension WHERE extname = 'uuid-ossp';
  -- returns an empty list
  runMigration migrateAll

server :: GTFS -> Metrics -> TVar (M.Map TripID ([TQueue (Maybe TrainPing)])) -> Pool SqlBackend -> ServerConfig -> Service CompleteAPI
server gtfs@GTFS{..} Metrics{..} subscribers dbpool settings = handleDebugAPI
  :<|> (handleStations :<|> handleTimetable :<|> handleTrip
        :<|> handleRegister :<|> handleTrainPing (throwError err401) :<|> handleWS
        :<|> handleSubscribe :<|> handleDebugState :<|> handleDebugTrain
        :<|> handleDebugRegister :<|> gtfsRealtimeServer gtfs dbpool)
  :<|> metrics
  :<|> pure (unsafePerformIO (toWaiAppPlain (ControlRoom gtfs dbpool settings)))
  where handleStations = pure stations
        handleTimetable station maybeDay = do
          -- TODO: resolve "overlay" trips (perhaps just additional CalendarDates?)
          day <- liftIO $ maybeM (getCurrentTime <&> utctDay) pure (pure maybeDay)
          pure
            -- don't send stations ending at this station
            . M.filter ((==) station . stationId . stopStation . V.last . tripStops)
            $ tripsOnDay gtfs day
        handleTrip trip = case M.lookup trip trips of
          Just res -> pure res
          Nothing  -> throwError err404
        handleRegister tripID RegisterJson{..} = do
          today <- liftIO getCurrentTime <&> utctDay
          unless (runsOnDay gtfs tripID today)
            $ sendErrorMsg "this trip does not run today."
          expires <- liftIO $ getCurrentTime <&> addUTCTime validityPeriod
          RunningKey token <- runSql dbpool $ insert (Running expires False tripID today Nothing registerAgent)
          pure token
        handleDebugRegister tripID day = do
          expires <- liftIO $ getCurrentTime <&> addUTCTime validityPeriod
          RunningKey token <- runSql dbpool $ insert (Running expires False tripID day Nothing "debug key")
          pure token
        handleTrainPing onError ping = isTokenValid dbpool (coerce $ trainPingToken ping) >>= \case
          Nothing -> do
            onError
            pure Nothing
          Just running@Running{..} -> do
            let anchor = extrapolateAnchorFromPing LinearExtrapolator gtfs running ping
            -- TODO: are these always inserted in order?
            runSql dbpool $ do
              insert ping
              last <- selectFirst
                [TrainAnchorTrip ==. runningTrip, TrainAnchorDay ==. runningDay]
                [Desc TrainAnchorWhen]
              -- only insert new estimates if they've actually changed anything
              when (fmap (trainAnchorDelay . entityVal) last /= Just (trainAnchorDelay anchor))
                $ void $ insert anchor
            queues <- liftIO $ atomically $ do
              queues <- readTVar subscribers <&> M.lookup runningTrip
              whenJust queues $
                mapM_ (\q -> writeTQueue q (Just ping))
              pure queues
            pure (Just anchor)
        handleWS conn = do
          liftIO $ WS.forkPingThread conn 30
          incGauge metricsWSGauge
          handle (\(e :: WS.ConnectionException) -> decGauge metricsWSGauge) $ forever $ do
            msg <- liftIO $ WS.receiveData conn
            case A.eitherDecode msg of
              Left err -> do
                logWarnN ("stray websocket message: "+|show msg|+" (could not decode: "+|err|+")")
                liftIO $ WS.sendClose conn (C8.pack err)
                -- TODO: send a close msg (Nothing) to the subscribed queues?               decGauge metricsWSGauge
              Right ping ->
                -- if invalid token, send a "polite" close request. Note that the client may
                -- ignore this and continue sending messages, which will continue to be handled.
                liftIO $ handleTrainPing (WS.sendClose conn ("" :: ByteString)) ping >>= \case
                  Just anchor -> WS.sendTextData conn (A.encode anchor)
                  Nothing     -> pure ()
        handleSubscribe tripId day conn = liftIO $ WS.withPingThread conn 30 (pure ()) $ do
          queue <- atomically $ do
            queue <- newTQueue
            qs <- readTVar subscribers
            writeTVar subscribers
              $ M.insertWith (<>) tripId [queue] qs
            pure queue
          -- send most recent ping, if any (so we won't have to wait for movement)
          lastPing <- runSql dbpool $ do
            tokens <- selectList [RunningDay ==. day, RunningTrip ==. tripId] []
              <&> fmap entityKey
            selectFirst [TrainPingToken <-. tokens] [Desc TrainPingTimestamp]
              <&> fmap entityVal
          whenJust lastPing $ \ping ->
            WS.sendTextData conn (A.encode lastPing)
          handle (\(e :: WS.ConnectionException) -> removeSubscriber queue) $ forever $ do
            res <- atomically $ readTQueue queue
            case res of
              Just ping -> WS.sendTextData conn (A.encode ping)
              Nothing -> do
                removeSubscriber queue
                WS.sendClose conn (C8.pack "train ended")
            where removeSubscriber queue = atomically $ do
                    qs <- readTVar subscribers
                    writeTVar subscribers
                     $ M.adjust (filter (/= queue)) tripId qs
        handleDebugState = do
          now <- liftIO getCurrentTime
          runSql dbpool $ do
           running <- selectList [RunningBlocked ==. False, RunningExpires >=. now] []
           pairs <- forM running $ \(Entity token@(RunningKey uuid) _) -> do
             entities <- selectList [TrainPingToken ==. token] []
             pure (uuid, fmap entityVal entities)
           pure (M.fromList pairs)
        handleDebugTrain tripId day = do
          unless (runsOnDay gtfs tripId day)
            $ sendErrorMsg ("this trip does not run on "+|day|+".")
          runSql dbpool $ do
            tokens <- selectList [RunningTrip ==. tripId, RunningDay ==. day] []
            pings <- forM tokens $ \(Entity token _) -> do
              selectList [TrainPingToken ==. token] [] <&> fmap entityVal
            pure (concat pings)
        handleDebugAPI = pure $ toSwagger (Proxy @API)
        metrics = exportMetricsAsText <&> (decodeUtf8 . toStrict)


-- TODO: proper debug logging for expired tokens
isTokenValid :: MonadIO m => Pool SqlBackend -> Token -> m (Maybe Running)
isTokenValid dbpool token = runSql dbpool $ get (coerce token) >>= \case
  Just trip | not (runningBlocked trip) -> do
    ifM (hasExpired (runningExpires trip))
      (pure Nothing)
      (pure (Just trip))
  _ -> pure Nothing

hasExpired :: MonadIO m => UTCTime -> m Bool
hasExpired limit = do
  now <- liftIO getCurrentTime
  pure (now > limit)

validityPeriod :: NominalDiffTime
validityPeriod = nominalDay