    {-# LANGUAGE DeriveDataTypeable #-}

    -- |
    -- Module: Data.Knob
    -- Copyright: 2011 John Millikin
    -- License: MIT
    --
    -- Maintainer: jmillikin@gmail.com
    -- Portability: GHC only
    --
    -- Create memory&#8208;backed 'IO.Handle's, referencing virtual files. This is
    -- mostly useful for testing 'IO.Handle'&#8208;based APIs without having to
    -- interact with the filesystem.
    --
    -- > import Data.ByteString (pack)
    -- > import Data.Knob
    -- > import System.IO
    -- >
    -- > main = do
    -- >     knob <- newKnob (pack [])
    -- >     h <- newFileHandle knob "test.txt" WriteMode
    -- >     hPutStrLn h "Hello world!"
    -- >     hClose h
    -- >     bytes <- Data.Knob.getContents knob
    -- >     putStrLn ("Wrote bytes: " ++ show bytes)
    module Data.Knob
            ( Knob
            , newKnob
            , Data.Knob.getContents
            , setContents

            , newFileHandle
            , withFileHandle
            ) where

    import qualified Control.Concurrent.MVar as MVar
    import           Control.Exception (bracket, throwIO)
    import           Control.Monad (when)
    import           Control.Monad.IO.Class (MonadIO, liftIO)
    import qualified Data.ByteString
    import           Data.ByteString (ByteString)
    import           Data.ByteString.Unsafe (unsafeUseAsCStringLen)
    import           Data.Typeable (Typeable)
    import qualified Foreign
    import qualified GHC.IO.Buffer as IO
    import qualified GHC.IO.BufferedIO as IO
    import qualified GHC.IO.Device as IO
    import qualified GHC.IO.Exception as IO
    import qualified GHC.IO.Handle as IO
    import qualified System.IO as IO

    -- | A knob is a basic virtual file, which contains a byte buffer. A knob can
    -- have multiple 'IO.Handle's open to it, each of which behaves like a standard
    -- file handle.
    --
    -- Use 'getContents' and 'setContents' to inspect and modify the knob&#8217;s
    -- byte buffer.
    newtype Knob = Knob (MVar.MVar ByteString)

    data Device = Device IO.IOMode (MVar.MVar ByteString) (MVar.MVar Int)
            deriving (Typeable)

    instance IO.IODevice Device where
            ready _ _ _ = return True
            close _ = return ()
            isTerminal _ = return False
            isSeekable _ = return True

            seek (Device _ _ var) IO.AbsoluteSeek off = do
                    checkOffset off
                    MVar.modifyMVar_ var (\_ -> return (fromInteger off))

            seek (Device _ _ var) IO.RelativeSeek off = do
                    MVar.modifyMVar_ var (\old_off -> do
                            let new_off = toInteger old_off + off
                            checkOffset new_off
                            return (fromInteger new_off))

            seek dev@(Device _ _ off_var) IO.SeekFromEnd off = do
                    MVar.modifyMVar_ off_var (\_ -> do
                            size <- IO.getSize dev
                            let new_off = size + off
                            checkOffset new_off
                            return (fromInteger new_off))

            tell (Device _ _ var) = fmap toInteger (MVar.readMVar var)
            getSize (Device _ var _) = do
                    bytes <- MVar.readMVar var
                    return (toInteger (Data.ByteString.length bytes))
            setSize dev size = setDeviceSize dev size
            devType _ = return IO.RegularFile

    checkOffset :: Integer -> IO ()
    checkOffset off = when (toInteger (maxBound :: Int) < off) (throwIO err) where
            err = IO.IOError Nothing IO.InvalidArgument "" "offset > (maxBound :: Int)" Nothing Nothing

    setDeviceSize :: Device -> Integer -> IO ()
    setDeviceSize (Device mode bytes_var _) size = checkSize >> setBytes where
            intSize :: Int
            intSize = fromInteger size

            checkSize = when (size > toInteger (maxBound :: Int)) $ do
                    throwIO (IO.IOError Nothing IO.InvalidArgument "" "size > (maxBound :: Int)" Nothing Nothing)

            setBytes = MVar.modifyMVar_ bytes_var $ \bytes -> case mode of
                    IO.ReadMode -> throwIO (IO.IOError Nothing IO.IllegalOperation "" "handle in ReadMode" Nothing Nothing)
                    IO.WriteMode -> return (Data.ByteString.replicate intSize 0)
                    IO.ReadWriteMode -> return (clip bytes)
                    IO.AppendMode -> return (clip bytes)

            clip bytes = case intSize - Data.ByteString.length bytes of
                    padLen | padLen > 0 -> Data.ByteString.append bytes (Data.ByteString.replicate padLen 0)
                    _ -> Data.ByteString.take intSize bytes

    instance IO.BufferedIO Device where
            newBuffer _ = IO.newByteBuffer 4096

            fillReadBuffer dev buf = do
                    (numRead, newBuf) <- IO.fillReadBuffer0 dev buf
                    return (maybe 0 id numRead, newBuf)

            fillReadBuffer0 (Device _ bytes_var pos_var) buf = do
                    MVar.withMVar bytes_var $ \bytes -> do
                            MVar.modifyMVar pos_var $ \pos -> do
                                    if pos >= Data.ByteString.length bytes
                                            then return (pos, (Nothing, buf))
                                            else do
                                                    let chunk = Data.ByteString.take (IO.bufSize buf) (Data.ByteString.drop pos bytes)
                                                    unsafeUseAsCStringLen chunk $ \(chunkPtr, chunkLen) -> do
                                                            Foreign.withForeignPtr (IO.bufRaw buf) $ \ptr -> do
                                                                    Foreign.copyArray ptr (Foreign.castPtr chunkPtr) chunkLen
                                                            return (pos + chunkLen, (Just chunkLen, (buf { IO.bufL = 0, IO.bufR = chunkLen })))

            flushWriteBuffer (Device _ bytes_var pos_var) buf = do
                    MVar.modifyMVar_ bytes_var $ \bytes -> do
                            MVar.modifyMVar pos_var $ \pos -> do
                                    let (before, after) = Data.ByteString.splitAt pos bytes
                                    let padding = Data.ByteString.replicate (pos - Data.ByteString.length before) 0

                                    let bufStart ptr = Foreign.castPtr (Foreign.plusPtr ptr (IO.bufL buf))
                                    let bufLen = IO.bufR buf - IO.bufL buf
                                    bufBytes <- Foreign.withForeignPtr (IO.bufRaw buf) (\ptr ->
                                            Data.ByteString.packCStringLen (bufStart ptr, bufLen))
                                    let newBytes = Data.ByteString.concat [before, padding, bufBytes, Data.ByteString.drop bufLen after]
                                    return (pos + bufLen, newBytes)
                    return (buf { IO.bufL = 0, IO.bufR = 0 })

            flushWriteBuffer0 dev buf = do
                    newBuf <- IO.flushWriteBuffer dev buf
                    return (IO.bufR buf - IO.bufL buf, newBuf)

    newKnob :: MonadIO m => ByteString -> m Knob
    newKnob bytes = do
            var <- liftIO (MVar.newMVar bytes)
            return (Knob var)

    getContents :: MonadIO m => Knob -> m ByteString
    getContents (Knob var) = liftIO (MVar.readMVar var)

    setContents :: MonadIO m => Knob -> ByteString -> m ()
    setContents (Knob var) bytes = liftIO (MVar.modifyMVar_ var (\_ -> return bytes))

    -- | Create a new 'IO.Handle' pointing to a 'Knob'. This handle behaves like
    -- a file-backed handle for most purposes.
    newFileHandle :: MonadIO m
                  => Knob
                  -> String -- ^ Filename shown in error messages
                  -> IO.IOMode -> m IO.Handle
    newFileHandle (Knob var) name mode = liftIO $ do
            startPosition <- MVar.withMVar var $ \bytes -> return $ case mode of
                    IO.AppendMode -> Data.ByteString.length bytes
                    _ -> 0
            posVar <- MVar.newMVar startPosition
            IO.mkFileHandle (Device mode var posVar) name mode Nothing IO.noNewlineTranslation

    -- | See 'newFileHandle'.
    withFileHandle :: MonadIO m
                   => Knob
                   -> String -- ^ Filename shown in error messages.
                   -> IO.IOMode -> (IO.Handle -> IO a) -> m a
    withFileHandle knob name mode io = liftIO (bracket (newFileHandle knob name mode) IO.hClose io)