import System.IO (hPutStrLn, stderr)
import System.Posix.Files (getFileStatus, isDirectory)
import System.Environment (getArgs, getProgName)
import System.Directory (canonicalizePath)
import Filesystem.Path (directory)
import Data.String (fromString)
import System.FSNotify (Event (..), WatchManager, startManager, stopManager, watchTree, watchDir)
import System.Exit (ExitCode (..), exitSuccess, exitFailure)
import System.Process (createProcess, proc, waitForProcess)
import Control.Monad (void, when)
import System.Posix.Signals (installHandler, Handler(Catch), sigINT, sigTERM)
import Control.Concurrent (forkIO, killThread)
import Control.Concurrent.MVar

data FileType = File | Directory deriving Eq

-- watches a file or directory and whenever a “modified” event is registered
-- puts a () in the “run” trigger. `tryPutMVar` is used to avoid re-running the
-- command many times if the file/dir is changed > 1 time while the command is
-- running.
watch :: FileType -> WatchManager -> String -> MVar () -> IO ()
watch filetype m path trigger =
  let watchFun = case filetype of
                   Directory -> watchTree m (fromString path) (const True)
                   File      -> watchDir  m (directory $ fromString path) isThisFile
   in watchFun (\_ -> void $ tryPutMVar trigger ())

  where isThisFile (Modified p _) = p == fromString path
        isThisFile _              = False

runCmd :: String -> [String] -> MVar () -> IO ()
runCmd cmd args trigger = do
  _ <- takeMVar trigger
  putStrLn $ "Running " ++ cmd ++ " " ++ unwords args ++  "..."
  (_, _, _, ph) <- createProcess (proc cmd args)
  exitCode <- waitForProcess ph
  hPutStrLn stderr $ case exitCode of
                       ExitSuccess   -> "Process completed successfully"
                       ExitFailure n -> "Process completed with exitcode " ++ show n
  runCmd cmd args trigger

main :: IO ()
main = do
  argv <- getArgs
  when (length argv < 2) $ getProgName >>= usage >> exitFailure

  let [path,cmd]  = take 2 argv
  let args = drop 2 argv

  m <- startManager

  -- Create an empty MVar and install INT/TERM handlers that will fill it.
  -- We will wait for one of these signals before cleaning up and exiting.
  interrupted <- newEmptyMVar
  _ <- installHandler sigINT  (Catch $ putMVar interrupted ()) Nothing
  _ <- installHandler sigTERM (Catch $ putMVar interrupted ()) Nothing

  canonicalPath <- canonicalizePath path

  -- check if path is a file or directory
  s <- getFileStatus canonicalPath
  let filetype = if isDirectory s then Directory else File

  runTrigger <- newEmptyMVar
  runThread <- forkIO $ runCmd cmd args runTrigger
  watch filetype m canonicalPath runTrigger
  putStr $ "Started to watch " ++ path
  putStrLn $ if canonicalPath == path then "" else " [→ " ++ canonicalPath ++ "]"

  _ <- readMVar interrupted
  putStrLn "\nStopping."
  stopManager m
  killThread runThread
  exitSuccess
    where usage n = hPutStrLn stderr $ "Usage: " ++ n
                             ++ " <file/directory to watch>"
                             ++ " <command to run> [arguments for command]"