module Engine.ReactiveBanana
  ( eventHandler
  , timer
  , observe

  , allocateActuated
  , allocatePaused

  , pushWorkerInput
  , pushWorkerInputJust

  , pushWorkerOutput
  , pushWorkerOutputJust

  , reactimateDebugShow

  , debounce
  ) where

import RIO

import Engine.Worker qualified as Worker
import GHC.Stack (withFrozenCallStack)
import Reactive.Banana qualified as RB
import Reactive.Banana.Frameworks qualified as RBF
import Resource.Region qualified as Region
import UnliftIO.Resource (ResourceT)
import UnliftIO.Resource qualified as Resource

eventHandler
  :: (Resource.MonadResource m, MonadIO io)
  => ((a -> io ()) -> m Resource.ReleaseKey)
  -> ResourceT m (RBF.MomentIO (RB.Event a))
eventHandler action = do
  (addHandler, fire) <- liftIO RBF.newAddHandler
  Region.local_ $ action (liftIO . fire)
  pure $ RBF.fromAddHandler addHandler

timer
  :: (MonadUnliftIO m)
  => Int
  -> ResourceT m (RBF.MomentIO (RB.Event Double))
timer delayMS = do
  (addHandler, fire) <- liftIO RBF.newAddHandler
  ticker <- async do
    begin <- getMonotonicTime
    threadDelay delayMS
    forever do
      before <- getMonotonicTime
      liftIO $ fire before
      after <- getMonotonicTime
      let
        tickNum       = (after - begin) * 1e6 / fromIntegral delayMS :: Double
        intTick       = truncate tickNum :: Integer
        driftTicks    = tickNum - fromInteger intTick :: Double
        driftMS       = driftTicks * fromIntegral delayMS :: Double
        adjustedDelay = max 0 $ delayMS - ceiling driftMS :: Int
      -- when (driftTicks > 0.01) $
      --   -- traceShowM driftTicks
      --   traceShowM (delayMS, (tickNum, intTick, driftTicks), driftMS, adjustedDelay)
      threadDelay adjustedDelay

  Region.attachAsync ticker
  pure $ RBF.fromAddHandler addHandler

observe
  :: (MonadUnliftIO m)
  => Worker.Var a
  -> ResourceT m (RBF.MomentIO (RB.Event a))
observe var = do
  (addHandler, fire) <- liftIO RBF.newAddHandler

  initial <- readTVarIO var

  tracker <- async $
    go fire (Worker.vVersion initial)
  Region.attachAsync tracker

  liftIO $ fire (Worker.vData initial) -- XXX: the network isn't compiled yet!
  pure $ RBF.fromAddHandler addHandler
  where
    go fire oldVersion = do
      Worker.Versioned{..} <- atomically do
        next <- readTVar var
        if Worker.vVersion next > oldVersion then
          pure next
        else
          retrySTM
      liftIO $ fire vData
      go fire vVersion

allocateActuated
  :: MonadUnliftIO m
  => (UnliftIO m -> RB.Event () -> RBF.MomentIO ())
  -> ResourceT m RBF.EventNetwork
allocateActuated builder = do
  (ah, fire) <- liftIO RBF.newAddHandler

  network <- allocatePaused \unlift -> do
    started <- RBF.fromAddHandler ah
    builder unlift started

  liftIO do
    RBF.actuate network
    fire ()
    pure network

allocatePaused
  :: MonadUnliftIO m
  => (UnliftIO m -> RBF.MomentIO ())
  -> ResourceT m RBF.EventNetwork
allocatePaused builder = do
  unlift <- lift askUnliftIO
  fmap snd $
    Resource.allocate
      (RBF.compile $ builder unlift)
      RBF.pause

pushWorkerInput
  :: Worker.HasInput var
  => var
  -> RB.Event (Worker.GetInput var)
  -> RBF.MomentIO ()
pushWorkerInput p = RBF.reactimate . fmap (Worker.pushInput p . const)

pushWorkerInputJust
  :: Worker.HasInput var
  => var
  -> RB.Event (Maybe (Worker.GetInput var))
  -> RBF.MomentIO ()
pushWorkerInputJust p = RBF.reactimate . fmap (traverse_ $ Worker.pushInput p . const)

pushWorkerOutput
  :: Worker.HasOutput var
  => var
  -> RB.Event (Worker.GetOutput var)
  -> RBF.MomentIO ()
pushWorkerOutput p = RBF.reactimate . fmap (Worker.pushOutput p . const)

pushWorkerOutputJust
  :: Worker.HasOutput var
  => var
  -> RB.Event (Maybe (Worker.GetOutput var))
  -> RBF.MomentIO ()
pushWorkerOutputJust p = RBF.reactimate . fmap (traverse_ $ Worker.pushOutput p . const)

reactimateDebugShow
  :: (Show a, MonadIO m, MonadReader env m, HasLogFunc env, HasCallStack)
  => (m () -> IO ())
  -> RB.Event a
  -> RBF.MomentIO ()
reactimateDebugShow unlift =
  RBF.reactimate . fmap (unlift . withFrozenCallStack logDebug . displayShow)

debounce :: Eq a => a -> RB.Event a -> RBF.MomentIO (RB.Event a)
debounce initial spamUpdates = do
  (e, fire) <- RBF.newEvent
  oldVar <- newIORef initial
  RBF.reactimate $
    spamUpdates <&> \new -> do
      changed <- atomicModifyIORef' oldVar \old ->
        (new, old /= new)
      when changed $
        fire new
  pure e