module Engine.StageSwitch
  ( StageSwitchVar
  , newStageSwitchVar

  , StageSwitch(..)
  , trySwitchStage
  , trySwitchStageSTM
  , getNextStage
  ) where

import RIO

import RIO.App (appEnv)

import Engine.Types (NextStage, StageRIO, StageSwitch(..), StageSwitchVar)
import Engine.Types qualified as Engine

newStageSwitchVar :: MonadIO m => m StageSwitchVar
newStageSwitchVar = newEmptyTMVarIO

trySwitchStage :: NextStage -> StageRIO rs Bool
trySwitchStage nextStage = do
  var <- asks $ Engine.ghStageSwitch . appEnv
  atomically $ trySwitchStageSTM var nextStage

trySwitchStageSTM :: StageSwitchVar -> NextStage -> STM Bool
trySwitchStageSTM switchVar = tryPutTMVar switchVar . StageSwitchPending

getNextStage :: StageRIO rs (Maybe NextStage)
getNextStage = do
  var <- asks $ Engine.ghStageSwitch . appEnv
  atomically do
    noSwitch <- isEmptyTMVar var
    if noSwitch then
      pure Nothing
    else
      takeTMVar var >>= \case
        StageSwitchPending nextStage -> do
          putTMVar var StageSwitchHandled
          pure $ Just nextStage
        StageSwitchHandled ->
          pure Nothing