{-# LANGUAGE ForeignFunctionInterface #-}

module System.Terminal
( getTerminalWidth
) where

import           Foreign
import           Foreign.C.Types

#include <sys/ioctl.h>
#include <unistd.h>

newtype WindowWidth = WindowWidth CUShort
    deriving (Eq, Ord, Show)

instance Storable WindowWidth where
  sizeOf _ = (#size struct winsize)
  alignment _ = (#alignment struct winsize)
  peek p = WindowWidth <$> (#peek struct winsize, ws_col) p
  poke p (WindowWidth w) = do
    (#poke struct winsize, ws_col) p w

foreign import ccall "sys/ioctl.h ioctl"
  ioctl :: CInt -> CInt -> Ptr WindowWidth -> IO CInt

-- | Get the width, in columns, of the terminal if we can.
getTerminalWidth :: IO (Maybe Int)
getTerminalWidth =
  alloca $ \p -> do
    errno <- ioctl (#const STDOUT_FILENO) (#const TIOCGWINSZ) p
    if errno < 0
      then return Nothing
      else do
        WindowWidth w <- peek p
        return . Just . fromIntegral $ w