{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE ScopedTypeVariables #-}

-- | Main module of the library
-- Contains all the entrypoints
module Discord
  ( runDiscord
  , restCall
  , sendCommand
  , readCache
  , stopDiscord
  , getGatewayLatency
  , measureLatency

  , DiscordHandler

  , DiscordHandle
  , Cache(..)
  , RestCallErrorCode(..)
  , RunDiscordOpts(..)
  , FromJSON
  , Request
  , def
  ) where

import Prelude hiding (log)
import Control.Exception (Exception)
import Control.Monad.Reader (ReaderT, runReaderT, void, ask, liftIO, forever, asks)
import Data.Aeson (FromJSON)
import Data.Default (Default, def)
import Data.IORef (writeIORef)
import qualified Data.Text as T
import qualified Data.Text.Encoding as TE
import Data.Time (NominalDiffTime, diffUTCTime, getCurrentTime)

import UnliftIO (race, try, finally, SomeException, IOException, readIORef)
import UnliftIO.Concurrent

import Discord.Handle
import Discord.Internal.Rest
import Discord.Internal.Rest.User (UserRequest(GetCurrentUser))
import Discord.Internal.Gateway

-- | A `ReaderT` wrapper around `DiscordHandle` and `IO`. Most functions act in
-- this monad
type DiscordHandler = ReaderT DiscordHandle IO

-- | Options for the connection. 
data RunDiscordOpts = RunDiscordOpts
  { -- | Token for the discord API
    discordToken :: T.Text
  , -- | Actions executed right after a connexion to discord's API is
    -- established
    discordOnStart :: DiscordHandler ()
  , -- | Actions executed at termination.
    --
    -- Note that this runs in plain `IO` and not in `DiscordHandler` as the
    -- connexion has been closed before this runs.
    --
    -- Useful for cleaning up.
    discordOnEnd :: IO ()
  , -- | Actions run upon the reception of an `Event`. This is here most of the
    -- code of the bot may get dispatched from.
    discordOnEvent :: Event -> DiscordHandler ()
  , -- | Dispatching on internal logs
    discordOnLog :: T.Text -> IO ()
  , -- | Fork a thread for every `Event` recived
    discordForkThreadForEvents :: Bool
  , -- | The gateway intents the bot is asking for
    discordGatewayIntent :: GatewayIntent
  , -- | Whether to use the cache (may use a lot of memory, only enable if it will be used!)
    discordEnableCache :: Bool
  }

-- | Default values for `RunDiscordOpts`
instance Default RunDiscordOpts where
  def = RunDiscordOpts { discordToken = ""
                       , discordOnStart = pure ()
                       , discordOnEnd = pure ()
                       , discordOnEvent = \_ -> pure ()
                       , discordOnLog = \_ -> pure ()
                       , discordForkThreadForEvents = True
                       , discordGatewayIntent = def
                       , discordEnableCache = False
                       }

-- | Entrypoint to the library 
runDiscord :: RunDiscordOpts -> IO T.Text
runDiscord opts = do
  log <- newChan
  logId <- liftIO $ startLogger (discordOnLog opts) log
  (cache, cacheId) <- liftIO $ startCacheThread (discordEnableCache opts) log
  (rest, restId) <- liftIO $ startRestThread (Auth (discordToken opts)) log
  (gate, gateId) <- liftIO $ startGatewayThread (Auth (discordToken opts)) (discordGatewayIntent opts) cache log

  libE <- newEmptyMVar

  let handle = DiscordHandle { discordHandleRestChan = rest
                             , discordHandleGateway = gate
                             , discordHandleCache = cache
                             , discordHandleLog = log
                             , discordHandleLibraryError = libE
                             , discordHandleThreads =
                                 [ HandleThreadIdLogger logId
                                 , HandleThreadIdRest restId
                                 , HandleThreadIdCache cacheId
                                 , HandleThreadIdGateway gateId
                                 ]
                             }

  finally (runDiscordLoop handle opts)
          (discordOnEnd opts >> runReaderT stopDiscord handle)

-- | Runs the main loop 
runDiscordLoop :: DiscordHandle -> RunDiscordOpts -> IO T.Text
runDiscordLoop handle opts = do
  resp <- liftIO $ writeRestCall (discordHandleRestChan handle) GetCurrentUser
  case resp of
    Left (RestCallInternalErrorCode c e1 e2) -> libError $
             "HTTP Error Code " <> T.pack (show c) <> " " <> TE.decodeUtf8 e1
                                                   <> " " <> TE.decodeUtf8 e2
    Left (RestCallInternalHttpException e) -> libError ("HTTP Exception -  " <> T.pack (show e))
    Left (RestCallInternalNoParse _ _) -> libError "Couldn't parse GetCurrentUser"
    _ -> do me <- liftIO . runReaderT (try $ discordOnStart opts) $ handle
            case me of
              Left (e :: SomeException) -> libError ("discordOnStart handler stopped on an exception:\n\n" <> T.pack (show e))
              Right _ -> loop
 where
   libError :: T.Text -> IO T.Text
   libError msg = tryPutMVar (discordHandleLibraryError handle) msg >> pure msg

   loop :: IO T.Text
   loop = do next <- race (readMVar (discordHandleLibraryError handle))
                          (readChan (gatewayHandleEvents (discordHandleGateway handle)))
             case next of
               Left err -> libError err
               Right (Left err) -> libError (T.pack (show err))
               Right (Right event) -> do
                 let userEvent = userFacingEvent event
                 let action = if discordForkThreadForEvents opts then void . forkIO
                                                                 else id
                 action $ do me <- liftIO . runReaderT (try $ discordOnEvent opts userEvent) $ handle
                             case me of
                               Left (e :: SomeException) -> writeChan (discordHandleLog handle)
                                         ("eventhandler - crashed on [" <> T.pack (show userEvent) <> "] "
                                          <> "          with error: "  <> T.pack (show e))
                               Right _ -> pure ()
                 loop

-- | A Error code following a rest call
data RestCallErrorCode = RestCallErrorCode Int T.Text T.Text
  deriving (Show, Read, Eq, Ord)

instance Exception RestCallErrorCode

-- | Execute one http request and get a response
restCall :: (Request (r a), FromJSON a) => r a -> DiscordHandler (Either RestCallErrorCode a)
restCall r = do h <- ask
                empty <- isEmptyMVar (discordHandleLibraryError h)
                if not empty
                then pure (Left (RestCallErrorCode 400 "Library Stopped Working" ""))
                else do
                    resp <- liftIO $ writeRestCall (discordHandleRestChan h) r
                    case resp of
                      Right x -> pure (Right x)
                      Left (RestCallInternalErrorCode c e1 e2) -> do
                        pure (Left (RestCallErrorCode c (TE.decodeUtf8 e1) (TE.decodeUtf8 e2)))
                      Left (RestCallInternalHttpException _) ->
                        threadDelay (10 * 10^(6 :: Int)) >> restCall r
                      Left (RestCallInternalNoParse err dat) -> do
                        let formaterr = T.pack ("restcall - parse exception [" <> err <> "]"
                                              <> " while handling" <> show dat)
                        writeChan (discordHandleLog h) formaterr
                        pure (Left (RestCallErrorCode 400 "Library Parse Exception" formaterr))

-- | Send a user GatewaySendable
sendCommand :: GatewaySendable -> DiscordHandler ()
sendCommand e = do
  h <- ask
  writeChan (gatewayHandleUserSendables (discordHandleGateway h)) e
  case e of
    UpdateStatus opts -> liftIO $ writeIORef (gatewayHandleLastStatus (discordHandleGateway h)) (Just opts)
    _ -> pure ()

-- | Access the current state of the gateway cache
readCache :: DiscordHandler Cache
readCache = do
  h <- ask
  merr <- readMVar (cacheHandleCache (discordHandleCache h))
  case merr of
    Left (c, _) -> pure c
    Right c -> pure c


-- | Stop all the background threads
stopDiscord :: DiscordHandler ()
stopDiscord = do h <- ask
                 _ <- tryPutMVar (discordHandleLibraryError h) "Library has closed"
                 threadDelay (10^(6 :: Int) `div` 10)
                 mapM_ (killThread . toId) (discordHandleThreads h)
  where toId t = case t of
                   HandleThreadIdRest a -> a
                   HandleThreadIdGateway a -> a
                   HandleThreadIdCache a -> a
                   HandleThreadIdLogger a -> a

-- | Starts the internal logger
startLogger :: (T.Text -> IO ()) -> Chan T.Text -> IO ThreadId
startLogger handle logC = forkIO $ forever $
  do me <- try $ readChan logC >>= handle
     case me of
       Right _ -> pure ()
       Left (_ :: IOException) ->
         -- writeChan logC "Log handler failed"
         pure ()

-- | Read the gateway latency from the last time we sent and received a 
-- Heartbeat. From Europe tends to give ~110ms
getGatewayLatency :: DiscordHandler NominalDiffTime
getGatewayLatency = do
  gw <- asks discordHandleGateway
  (send1, send2) <- readIORef (gatewayHandleHeartbeatTimes gw)

  ack <- readIORef (gatewayHandleHeartbeatAckTimes gw)

  pure . diffUTCTime ack $ 
    if ack > send1 -- if the ack is before the send just gone, use the previous send
      then send1
      else send2

-- | Measure the current latency by making a request and measuring the time 
-- taken. From Europe tends to give 200ms-800ms.
--
-- The request is getting the bot's user, which requires the `identify` scope.
measureLatency :: DiscordHandler NominalDiffTime
measureLatency = do
  startTime <- liftIO getCurrentTime
  _ <- restCall GetCurrentUser
  endTime <- liftIO getCurrentTime
  pure $ diffUTCTime endTime startTime

-- internal note: it seems bad that it's taking 2x-8x as much time to perform 
-- this specific request, considering that the latency we expect is much less.
-- might be worth looking into efficiencies or a better event to use.