[Haskell-cafe] bytea decoding

[Adam Langley]

2008/1/20 Bryan Green <dbryan.green at gmail.com>:
> Does anyone know of a library that will handle bytea encodings from
> postgres?  The bytea field that I need to access contains a jpg file.  I
> want to retrieve it from the database and write it out for an image display
> program.

I'd love to see Don give the correct ByteString solution, but here's a
slow version I knocked up (probably buggy):

import qualified Data.ByteString as B
import Data.ByteString.Internal (c2w)
import Data.Maybe (catMaybes)
import Data.List (mapAccumL)

byteaDecode :: B.ByteString -> B.ByteString
byteaDecode = B.pack . catMaybes . snd . mapAccumL f initState . B.unpack where
  initState = (0, 0)
  f (0, _) 92 = ((1, 0), Nothing)
  f (0, _) x = ((0, 0), Just x)
  f (1, _) 92 = ((0, 0), Just 92)
  f (3, n) x = ((0, 0), Just (n * 8 + (x - 48)))
  f (c, n) x = ((c + 1, n * 8 + (x - 48)), Nothing)


AGL

-- 
Adam Langley                                      agl at imperialviolet.org
http://www.imperialviolet.org                       650-283-9641