{-# LANGUAGE ScopedTypeVariables #-}
module CrazyIO (module CrazyIO, mmapFileByteString) where

import qualified Data.Vector.Storable      as V
import qualified Data.ByteString as BS
import qualified Data.ByteString.Internal as BS
import           Foreign
import           System.IO.MMap

crazyLoad :: forall a. Storable a => FilePath -> Maybe (Int64, Int) -> IO (V.Vector a)
crazyLoad name range =
  do (ptr,offset,size) <- mmapFileForeignPtr name ReadOnly range
     return $
       V.unsafeFromForeignPtr
         ptr
         offset
         (size `div`
          sizeOf (undefined :: a))

crazyLoadReinterpret :: Storable a => FilePath -> Maybe (Int64, Int) -> (BS.ByteString -> IO BS.ByteString) -> IO (V.Vector a)
crazyLoadReinterpret name range f = do
  -- It would sometimes be useful to load a lazy bytestring here. However
  -- in testing it proved to just result in ERROR being thrown somewhere.
  v <- mmapFileByteString name range
  x <- f v
  return $ byteStringToVector x

byteStringToVector :: (Storable a) => BS.ByteString -> V.Vector a
byteStringToVector bs = vec where
    vec = V.unsafeFromForeignPtr (castForeignPtr fptr) (scale off) (scale len)
    (fptr, off, len) = BS.toForeignPtr bs
    scale = (`div` sizeOfElem vec)

sizeOfElem :: (Storable a) => V.Vector a -> Int
sizeOfElem vec = sizeOf (undefined `asTypeOf` V.head vec)