module JPEGparser where
import Data.ByteString.Lazy.Char8(unpack)

-- | Parse just enough of a JPEG file to obtain the size
-- (width and height in pixels) of the JPEG image.
sizeOfJPEG = sizeOfJPEG' . unpack

-- Based on https://web.archive.org/web/20131016210645/http://www.64lines.com/jpeg-width-height
sizeOfJPEG' = size0 . drop 4
  where
    size0 (hi:lo:s) = size (drop (be hi lo-2) s)
    size ('\xff':'\xc0':_:_:_:h1:h2:w1:w2:_) = Just (be w1 w2,be h1 h2)
    size ('\xff':'\xc2':_:_:_:h1:h2:w1:w2:_) = Just (be w1 w2,be h1 h2)
    size ('\xff':_:s) = size0 s
    size _ = Nothing

    be hi lo = 256*fromEnum hi+fromEnum lo