perceptual-hash-0.1.1.0: src/ForeignHash.hs

-- N.B. this is a good deal faster than the Haskell code
module ForeignHash ( foreignFileHash ) where

import           Data.Word             (Word64)
import           Foreign.C.String      (CString, withCString)
import           Foreign.C.Types       (CInt (..), CULLong)
import           Foreign.Marshal.Alloc (alloca)
import           Foreign.Ptr           (Ptr)
import           Foreign.Storable      (peek)

foreign import ccall ph_dct_imagehash :: CString -> Ptr CULLong -> IO CInt

-- | Doesn't work with @.gif@ files
foreignFileHash :: FilePath -> IO Word64
foreignFileHash fp = withCString fp $ \cstr ->
    alloca $ \hashPtr -> do
        res <- ph_dct_imagehash cstr hashPtr
        check res
        fromIntegral <$> peek hashPtr

    where check (-1) = error ("Hash of file " ++ fp ++ " failed.")
          check 0    = pure ()
          check _    = error "This should never happen."