{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeFamilies #-}

-- | Helper to read a directory of files, and observe it for changes.
--
-- Use @new@ in conjunction with @observe@ in your @runEma@ function call.
module Ema.Helper.FileSystem
  ( filesMatching,
    onChange,
    FileAction (..),
  )
where

import Control.Concurrent (threadDelay)
import Control.Exception (finally)
import Control.Monad.Logger
import System.Directory (canonicalizePath)
import System.FSNotify
  ( ActionPredicate,
    Event (..),
    StopListening,
    WatchManager,
    watchTree,
    withManager,
  )
import System.FilePath (makeRelative)
import System.FilePattern (FilePattern)
import System.FilePattern.Directory (getDirectoryFiles)
import UnliftIO (MonadUnliftIO, withRunInIO)

type FolderPath = FilePath

log :: MonadLogger m => LogLevel -> Text -> m ()
log = logWithoutLoc "Helper.FileSystem"

filesMatching :: (MonadIO m, MonadLogger m) => FolderPath -> [FilePattern] -> m [FilePath]
filesMatching parent' pats = do
  parent <- liftIO $ canonicalizePath parent'
  log LevelInfo $ toText $ "Traversing " <> parent <> " for files matching " <> show pats
  liftIO $ getDirectoryFiles parent pats

data FileAction = Update | Delete
  deriving (Eq, Show)

onChange ::
  forall m.
  (MonadIO m, MonadLogger m, MonadUnliftIO m) =>
  FolderPath ->
  (FilePath -> FileAction -> m ()) ->
  m ()
onChange parent' f = do
  -- NOTE: It is important to use canonical path, because this will allow us to
  -- transform fsnotify event's (absolute) path into one that is relative to
  -- @parent'@ (as passed by user), which is what @f@ will expect.
  parent <- liftIO $ canonicalizePath parent'
  withManagerM $ \mgr -> do
    log LevelInfo $ toText $ "Monitoring " <> parent <> " for changes"
    stop <- watchTreeM mgr parent (const True) $ \event -> do
      log LevelDebug $ show event
      let rel = makeRelative parent
      case event of
        Added (rel -> fp) _ _ -> f fp Update
        Modified (rel -> fp) _ _ -> f fp Update
        Removed (rel -> fp) _ _ -> f fp Delete
        Unknown (rel -> fp) _ _ -> f fp Delete
    liftIO $ threadDelay maxBound `finally` stop

withManagerM ::
  (MonadIO m, MonadUnliftIO m) =>
  (WatchManager -> m a) ->
  m a
withManagerM f = do
  withRunInIO $ \run ->
    withManager $ \mgr -> run (f mgr)

watchTreeM ::
  forall m.
  (MonadIO m, MonadUnliftIO m) =>
  WatchManager ->
  FilePath ->
  ActionPredicate ->
  (Event -> m ()) ->
  m StopListening
watchTreeM wm fp pr f =
  withRunInIO $ \run ->
    watchTree wm fp pr $ \evt -> run (f evt)