{-# LANGUAGE ForeignFunctionInterface #-}
{-# OPTIONS -Wall -O #-}

module PixbufExtras (pixbufGetSizeInfoFile) where

import Control.Applicative (Applicative, liftA, pure)
import Data.Word
import Foreign.C
import Foreign.Marshal.Alloc
import Foreign.Ptr
import Foreign.Storable
import Graphics.UI.Gtk.Gdk.Pixbuf ()

-- | Read file metadata and return image size
pixbufGetSizeInfoFile :: FilePath -> IO (Maybe (Int, Int))
pixbufGetSizeInfoFile fp =
    withCString fp $ \cfp ->
        alloca $ \wp -> alloca $ \hp -> pixbufGetFileInfo cfp wp hp
                        >>= ptrMaybeA (\_ -> peek wp >>= \w
                                          -> peek hp >>= \h
                                          -> return (w, h))

-- FIXME: the return value shall not be a Ptr to a Word.
foreign import ccall "static gdk/gdkpixbuf.h gdk_pixbuf_get_file_info"
    pixbufGetFileInfo :: CString -> Ptr Int -> Ptr Int -> IO (Ptr Word)

-- | Apply function if the pointer is not NULL
ptrMaybeA :: Applicative m => (Ptr a -> m b) -> Ptr a -> m (Maybe b)
ptrMaybeA f p | p == nullPtr = pure  Nothing
              | otherwise    = liftA Just $ f p