module Engine.Worker
  ( Versioned(..)
  , Var
  , newVar
  , readVar
  , stateVar
  , stateVarMap

  , HasInput(..)
  , pushInput
  , pushInputSTM
  , updateInput
  , updateInputSTM
  , getInputData
  , getInputDataSTM

  , HasConfig(..)
  , modifyConfig
  , modifyConfigSTM

  , HasOutput(..)
  , pushOutput
  , pushOutputSTM
  , updateOutput
  , updateOutputSTM
  , getOutputData
  , getOutputDataSTM

  , Cell
  , spawnCell

  , Timed(..)
  , spawnTimed
  , spawnTimed_

  , Merge(..)
  , spawnMerge1
  , spawnMerge2
  , spawnMerge3
  , spawnMerge4
  , spawnMergeT

  , ObserverIO
  , newObserverIO
  , observeIO
  , observeIO_
  , readObservedIO

  , Source
  , newSource
  , pubSource
  , subSource
  ) where

import RIO

import Control.Concurrent.Chan.Unagi.Unboxed (UnagiPrim)
import Control.Concurrent.Chan.Unagi.Unboxed qualified as UnagiPrim
import Data.Vector.Unboxed qualified as Unboxed
import UnliftIO.Concurrent (forkIO, killThread)
import UnliftIO.Resource (MonadResource)
import UnliftIO.Resource qualified as Resource
import Data.StateVar qualified as StateVar

data Versioned a = Versioned
  { vVersion :: Unboxed.Vector Word64
  , vData    :: a
  }
  deriving (Show, Functor, Foldable, Traversable)

instance Eq (Versioned a) where
  a == b = vVersion a == vVersion b

instance Ord (Versioned a) where
  compare a b = compare (vVersion a) (vVersion b)

type Var a = TVar (Versioned a)

newVar :: MonadUnliftIO m => a -> m (Var a)
newVar initial = newTVarIO Versioned
  { vVersion = Unboxed.singleton 0
  , vData    = initial
  }

stateVar :: HasInput var => var -> StateVar.StateVar (GetInput var)
stateVar var = StateVar.makeStateVar
  (getInputData var)
  (\new -> pushInput var \_old -> new)

stateVarMap
  :: HasInput var
  => (GetInput var -> a)
  -> (a -> GetInput var -> GetInput var)
  -> var
  -> StateVar.StateVar a
stateVarMap mapGet mapSet var = StateVar.makeStateVar
  (fmap mapGet $ getInputData var)
  (\new -> pushInput var (mapSet new))

{-# INLINEABLE readVar #-}
readVar :: (MonadUnliftIO m) => Var a -> m a
readVar = fmap vData . readTVarIO

{-# INLINEABLE pushVarSTM #-}
pushVarSTM :: Var a -> (a -> a) -> STM ()
pushVarSTM var f =
  modifyTVar' var \Versioned{vVersion, vData} ->
    Versioned
      { vVersion = Unboxed.map (+ 1) vVersion
      , vData = f vData
      }

{-# INLINEABLE updateVarSTM #-}
updateVarSTM :: Var a -> (a -> Maybe a) -> STM ()
updateVarSTM var f = do
  current <- readTVar var
  let
    oldData = vData current
  case f oldData of
    Nothing ->
      pure ()
    Just newData ->
      writeTVar var Versioned
        { vVersion = Unboxed.map (+1) (vVersion current)
        , vData    = newData
        }

class HasInput a where
  type GetInput a
  getInput :: a -> Var (GetInput a)

instance HasInput (Var a) where
  type GetInput (Var a) = a
  getInput = id

instance HasInput a => HasInput (a, b) where
  type GetInput (a, b) = GetInput a
  getInput = getInput . fst

{-# INLINEABLE pushInput #-}
pushInput
  :: (MonadIO m, HasInput var)
  => var
  -> (GetInput var -> GetInput var)
  -> m ()
pushInput input =
  atomically . pushInputSTM input

{-# INLINEABLE pushInputSTM #-}
pushInputSTM
  :: HasInput var
  => var
  -> (GetInput var -> GetInput var)
  -> STM ()
pushInputSTM input = pushVarSTM (getInput input)

{-# INLINEABLE updateInput #-}
updateInput
  :: ( MonadIO m
     , HasInput var
     )
  => var
  -> (GetInput var -> Maybe (GetInput var))
  -> m ()
updateInput input =
  atomically . updateInputSTM input

updateInputSTM
  :: HasInput var
  => var
  -> (GetInput var -> Maybe (GetInput var))
  -> STM ()
updateInputSTM input = updateVarSTM (getInput input)

{-# INLINEABLE getInputData #-}
getInputData :: (HasInput worker, MonadIO m) => worker -> m (GetInput worker)
getInputData = fmap vData . readTVarIO . getInput

{-# INLINEABLE getInputDataSTM #-}
getInputDataSTM :: (HasInput worker) => worker -> STM (GetInput worker)
getInputDataSTM = fmap vData . readTVar . getInput

class HasConfig a where
  type GetConfig a
  getConfig :: a -> TVar (GetConfig a)

instance HasConfig (TVar a) where
  type GetConfig (TVar a) = a
  getConfig = id

modifyConfig
  :: (MonadIO m, HasConfig var)
  => var
  -> (GetConfig var -> GetConfig var)
  -> m ()
modifyConfig config =
  atomically . modifyConfigSTM config

modifyConfigSTM
  :: HasConfig var
  => var
  -> (GetConfig var -> GetConfig var)
  -> STM ()
modifyConfigSTM config =
  modifyTVar' (getConfig config)

class HasOutput a where
  type GetOutput a
  getOutput  :: a -> Var (GetOutput a)

instance HasOutput (Var a) where
  type GetOutput (Var a) = a
  getOutput = id

instance HasOutput b => HasOutput (a, b) where
  type GetOutput (a, b) = GetOutput b
  getOutput = getOutput . snd

{-# INLINEABLE pushOutput #-}
pushOutput
  :: (MonadIO m, HasOutput var)
  => var
  -> (GetOutput var -> GetOutput var)
  -> m ()
pushOutput output =
  atomically . pushOutputSTM output

pushOutputSTM
  :: HasOutput var
  => var
  -> (GetOutput var -> GetOutput var)
  -> STM ()
pushOutputSTM output f =
  modifyTVar' (getOutput output) \Versioned{vVersion, vData} ->
    Versioned
      { vVersion = Unboxed.map (+1) vVersion
      , vData    = f vData
      }

{-# INLINEABLE updateOutput #-}
updateOutput
  :: (MonadIO m, HasOutput var)
  => var
  -> (GetOutput var -> Maybe (GetOutput var))
  -> m ()
updateOutput output =
  atomically . updateOutputSTM output

updateOutputSTM
  :: HasOutput var
  => var
  -> (GetOutput var -> Maybe (GetOutput var))
  -> STM ()
updateOutputSTM output f = do
  current <- readTVar outputVar
  let
    oldData = vData current
  case f oldData of
    Nothing ->
      pure ()
    Just newData ->
      writeTVar outputVar Versioned
        { vVersion = Unboxed.map (+1) (vVersion current)
        , vData    = newData
        }
  where
    outputVar = getOutput output

{-# INLINEABLE getOutputData #-}
getOutputData :: (HasOutput worker, MonadIO m) => worker -> m (GetOutput worker)
getOutputData = fmap vData . readTVarIO . getOutput

{-# INLINEABLE getOutputDataSTM #-}
getOutputDataSTM :: (HasOutput worker) => worker -> STM (GetOutput worker)
getOutputDataSTM = fmap vData . readTVar . getOutput

-- * Suppply

-- | Updatable cell for composite input or costly output.
type Cell input output = (Var input, Merge output)

spawnCell
  :: ( MonadUnliftIO m
     , MonadResource m
     )
  => (input -> output)
  -> input
  -> m (Cell input output)
spawnCell f initialInput = do
  input <- newVar initialInput
  fmap (input,) $
    spawnMerge1 f input

-- | Timer-driven stateful producer.
data Timed config output = Timed
  { tWorker :: ThreadId
  , tKey    :: Resource.ReleaseKey
  , tActive :: TVar Bool
  , tConfig :: TVar config
  , tOutput :: Var output
  }

instance HasConfig (Timed config output) where
  type GetConfig (Timed config output) = config
  getConfig = tConfig

instance HasOutput (Timed config output) where
  type GetOutput (Timed config output) = output
  getOutput = tOutput

spawnTimed_
  :: ( MonadUnliftIO m
     , MonadResource m
     )
  => Bool
  -> Int
  -> output
  -> m output
  -> m (Timed () output)
spawnTimed_ startActive dt initialOutput stepF =
  spawnTimed
    startActive
    (Left dt)
    (\() -> pure (initialOutput, ()))
    (\_old () ->
        stepF >>= \res ->
          pure (Just res, ())
    )
    ()

spawnTimed
  :: ( MonadUnliftIO m
     , MonadResource m
     )
  => Bool
  -> Either Int (config -> Int)
  -> (config -> m (output, state))
  -> (state -> config -> m (Maybe output, state))
  -> config
  -> m (Timed config output)
spawnTimed startActive dtF initF stepF initialConfig = do
  tActive <- newTVarIO startActive
  tConfig <- newTVarIO initialConfig
  (initialOutput, initialState) <- initF initialConfig
  tOutput <- newVar initialOutput
  tWorker <- forkIO $
    step tActive tConfig tOutput initialState
  tKey <- Resource.register $ killThread tWorker
  pure Timed{..}
  where
    step activeVar configVar output curState = do
      case dtF of
        Left static ->
          threadDelay static
        Right fromConfig ->
          readTVarIO configVar >>=
            threadDelay . fromConfig

      active <- readTVarIO activeVar
      if active then do
        config <- readTVarIO configVar
        (nextOutput, nextState) <- stepF curState config
        updateOutput output $ const nextOutput
        step activeVar configVar output nextState
      else
        step activeVar configVar output curState

-- | Supply-driven step cell.
data Merge o = Merge
  { mWorker :: ThreadId
  , mKey    :: Resource.ReleaseKey
  , mOutput :: TVar (Versioned o)
  }

instance HasOutput (Merge o) where
  type GetOutput (Merge o) = o
  getOutput = mOutput

spawnMerge1
  :: ( MonadUnliftIO m
     , MonadResource m
     , HasOutput i
     )
  => (GetOutput i -> o)
  -> i
  -> m (Merge o)
spawnMerge1 f i = do
  output <- atomically do
    initial <- readTVar (getOutput i)
    newTVar Versioned
      { vVersion = vVersion initial
      , vData    = f (vData initial)
      }

  worker <- forkIO $
    forever $ atomically do
      next <- readTVar (getOutput i)
      old <- readTVar output

      let nextVersion = vVersion next
      if nextVersion > vVersion old then
        writeTVar output Versioned
          { vVersion = nextVersion
          , vData    = f (vData next)
          }
      else
        retrySTM

  key <- Resource.register $ killThread worker

  pure Merge
    { mWorker = worker
    , mKey    = key
    , mOutput = output
    }

spawnMerge2
  :: ( MonadUnliftIO m
     , MonadResource m
     , HasOutput i1
     , HasOutput i2
     )
  => (GetOutput i1 -> GetOutput i2 -> o)
  -> i1
  -> i2
  -> m (Merge o)
spawnMerge2 f i1 i2 = do
  output <- atomically do
    (initial1, initial2) <- (,)
      <$> readTVar (getOutput i1)
      <*> readTVar (getOutput i2)

    newTVar Versioned
      { vVersion = mkVersion initial1 initial2
      , vData    = f (vData initial1) (vData initial2)
      }

  worker <- forkIO $
    forever $ atomically $ do
      next1 <- readTVar (getOutput i1)
      next2 <- readTVar (getOutput i2)
      old <- readTVar output

      let nextVersion = mkVersion next1 next2

      if nextVersion > vVersion old then
        writeTVar output Versioned
          { vVersion = nextVersion
          , vData    = f (vData next1) (vData next2)
          }
      else
        retrySTM

  key <- Resource.register $ killThread worker

  pure Merge
    { mWorker = worker
    , mKey    = key
    , mOutput = output
    }
  where
    mkVersion a b = vVersion a <> vVersion b

spawnMerge3
  :: ( MonadUnliftIO m
     , MonadResource m
     , HasOutput i1
     , HasOutput i2
     , HasOutput i3
     )
  => (GetOutput i1 -> GetOutput i2 -> GetOutput i3 -> o)
  -> i1
  -> i2
  -> i3
  -> m (Merge o)
spawnMerge3 f i1 i2 i3 = do
  output <- atomically do
    (initial1, initial2, initial3) <- (,,)
      <$> readTVar (getOutput i1)
      <*> readTVar (getOutput i2)
      <*> readTVar (getOutput i3)

    newTVar Versioned
      { vVersion = mkVersion initial1 initial2 initial3
      , vData    = f (vData initial1) (vData initial2) (vData initial3)
      }

  worker <- forkIO $
    forever $ atomically $ do
      next1 <- readTVar (getOutput i1)
      next2 <- readTVar (getOutput i2)
      next3 <- readTVar (getOutput i3)
      old <- readTVar output

      let nextVersion = mkVersion next1 next2 next3

      if nextVersion > vVersion old then
        writeTVar output Versioned
          { vVersion = nextVersion
          , vData    = f (vData next1) (vData next2) (vData next3)
          }
      else
        retrySTM

  key <- Resource.register $ killThread worker

  pure Merge
    { mWorker = worker
    , mKey    = key
    , mOutput = output
    }
  where
    mkVersion a b c = mconcat [vVersion a, vVersion b, vVersion c]

spawnMerge4
  :: ( MonadUnliftIO m
     , MonadResource m
     , HasOutput i1
     , HasOutput i2
     , HasOutput i3
     , HasOutput i4
     )
  => (GetOutput i1 -> GetOutput i2 -> GetOutput i3 -> GetOutput i4 -> o)
  -> i1
  -> i2
  -> i3
  -> i4
  -> m (Merge o)
spawnMerge4 f i1 i2 i3 i4 = do
  output <- atomically do
    (initial1, initial2, initial3, initial4) <- (,,,)
      <$> readTVar (getOutput i1)
      <*> readTVar (getOutput i2)
      <*> readTVar (getOutput i3)
      <*> readTVar (getOutput i4)

    newTVar Versioned
      { vVersion = mkVersion initial1 initial2 initial3 initial4
      , vData    = f (vData initial1) (vData initial2) (vData initial3) (vData initial4)
      }

  worker <- forkIO $
    forever $ atomically $ do
      next1 <- readTVar (getOutput i1)
      next2 <- readTVar (getOutput i2)
      next3 <- readTVar (getOutput i3)
      next4 <- readTVar (getOutput i4)
      old <- readTVar output

      let nextVersion = mkVersion next1 next2 next3 next4

      if nextVersion > vVersion old then
        writeTVar output Versioned
          { vVersion = nextVersion
          , vData    = f (vData next1) (vData next2) (vData next3) (vData next4)
          }
      else
        retrySTM

  key <- Resource.register $ killThread worker

  pure Merge
    { mWorker = worker
    , mKey    = key
    , mOutput = output
    }
  where
    mkVersion a b c d = mconcat [vVersion a, vVersion b, vVersion c, vVersion d]

{- |
  Spawn a merge over a homogeneous traversable collection of processes.

  A merging function will receive a collection of outputs to summarize.
-}
spawnMergeT
  :: ( Traversable t
     , HasOutput input
     , MonadUnliftIO m
     , MonadResource m
     )
  => (t (GetOutput input) -> output)
  -> t input
  -> m (Merge output)
spawnMergeT f inputs = do
  output <- atomically do
    initial <- traverse (readTVar . getOutput) inputs

    newTVar Versioned
      { vVersion = foldMap vVersion initial
      , vData    = f (fmap vData initial)
      }

  worker <- forkIO $
    forever $ atomically do
      next <- traverse (readTVar . getOutput) inputs
      old <- readTVar output

      let nextVersion = foldMap vVersion next

      if nextVersion > vVersion old then
        writeTVar output Versioned
          { vVersion = nextVersion
          , vData    = f (fmap vData next)
          }
      else
        retrySTM

  key <- Resource.register $ killThread worker

  pure Merge
    { mWorker = worker
    , mKey    = key
    , mOutput = output
    }

-- * Demand

type ObserverIO a = IORef (Versioned a)

newObserverIO :: MonadIO m => a -> m (ObserverIO a)
newObserverIO initial = newIORef Versioned
  { vVersion = mempty
  , vData    = initial
  }

observeIO
  :: (MonadUnliftIO m, HasOutput output)
  => output
  -> ObserverIO a
  -> (a -> GetOutput output -> m a)
  -> m a
observeIO output currentRef action = do
  outputV <- readTVarIO (getOutput output)
  currentV <- readIORef currentRef
  if vVersion outputV > vVersion currentV then do
    derived <- action (vData currentV) (vData outputV)
    atomicWriteIORef currentRef Versioned
      { vVersion = vVersion outputV
      , vData    = derived
      }
    pure derived
  else
    pure (vData currentV)

observeIO_
  :: (MonadUnliftIO m, HasOutput output)
  => output
  -> ObserverIO a
  -> (a -> GetOutput output -> m a)
  -> m ()
observeIO_ output currentRef =
  void . observeIO output currentRef

{-# INLINEABLE readObservedIO #-}
readObservedIO :: (MonadUnliftIO m) => IORef (Versioned a) -> m a
readObservedIO = fmap vData . readIORef

-- * PubSub

newtype Source a = Source (UnagiPrim.InChan a)

newSource :: (MonadUnliftIO m, UnagiPrim a) => m (Source a)
newSource = fmap (Source . fst) $ liftIO UnagiPrim.newChan

pubSource :: (MonadUnliftIO m, UnagiPrim a) => Source a -> a -> m ()
pubSource (Source ic) = liftIO . UnagiPrim.writeChan ic

subSource :: MonadUnliftIO m => Source a -> m (UnagiPrim.OutChan a)
subSource (Source ic) = liftIO $ UnagiPrim.dupChan ic