module StreamPatch.Stream where

import           StreamPatch.Patch

import           Util

import           Data.Vinyl
import           GHC.Natural
import           Data.Kind
import qualified Data.ByteString            as BS
import qualified Data.ByteString.Builder    as BB
import qualified Data.ByteString.Lazy       as BL
import           Control.Monad.State
import           Control.Monad.Reader
import           System.IO                  ( Handle, SeekMode(..), hSeek )

import qualified Data.List                  as List

-- TODO also require reporting cursor position (for error reporting)
class Monad m => MonadFwdStream m where
    type Chunk m :: Type

    -- | Read a number of bytes without advancing the cursor.
    readahead :: Natural -> m (Chunk m)

    -- | Advance cursor without reading.
    advance :: Natural -> m ()

    -- | Insert bytes into the stream at the cursor position, overwriting
    --   existing bytes.
    overwrite :: Chunk m -> m ()

-- TODO Need MonoTraversable to define for Text, ByteString etc easily. Bleh. I
-- think Snoyman's advice is to reimplement. Also bleh.
instance Monad m => MonadFwdStream (StateT ([a], [a]) m) where
    type Chunk (StateT ([a], [a]) m) = [a]
    readahead n = List.take (fromIntegral n) <$> gets fst
    advance n = do
        (src, out) <- get
        let (bs, src') = List.splitAt (fromIntegral n) src
        put (src', out <> bs)
    overwrite bs = do
        (src, out) <- get
        let (_, src') = List.splitAt (List.length bs) src
        put (src', out <> bs)

instance MonadIO m => MonadFwdStream (ReaderT Handle m) where
    type Chunk (ReaderT Handle m) = BS.ByteString
    readahead n = do
        hdl <- ask
        bs <- liftIO $ BS.hGet hdl (fromIntegral n)
        liftIO $ hSeek hdl RelativeSeek (- fromIntegral n)
        return bs
    advance n = do
        hdl <- ask
        liftIO $ hSeek hdl RelativeSeek (fromIntegral n)
    overwrite bs = do
        hdl <- ask
        liftIO $ BS.hPut hdl bs

instance Monad m => MonadFwdStream (StateT (BS.ByteString, BB.Builder) m) where
    type Chunk (StateT (BS.ByteString, BB.Builder) m) = BS.ByteString
    readahead n = BS.take (fromIntegral n) <$> gets fst
    advance n = do
        (src, out) <- get
        let (bs, src') = BS.splitAt (fromIntegral n) src
        put (src', out <> BB.byteString bs)
    overwrite bs = do
        (src, out) <- get
        let (_, src') = BS.splitAt (BS.length bs) src
        put (src', out <> BB.byteString bs)

-- | A forward stream, but backward too.
class MonadFwdStream m => MonadCursorStream m where
    -- | Move cursor.
    move :: Integer -> m ()

instance MonadIO m => MonadCursorStream (ReaderT Handle m) where
    move n = do
        hdl <- ask
        liftIO $ hSeek hdl RelativeSeek (fromIntegral n)