[Haskell-cafe] Fast code question
dons at galois.com
Thu Jun 4 18:21:21 EDT 2009
Can you use the bytestring csv parser (or convert it into a pretty
> Hi Folks,
> I had to transform Packed Decimal file into csv format (does anybody here know
> what this Mainframe format is?). Because of the file size I could not do
> this on mainframe directly. So I've created simply program using ByteString.
> Generally I'm happy with my solution: pgm processes arroud 2MB/s on my pc, so
> my 3GB file was transformed in reasonable 30 min time.
> My question is: how to do this faster?
> module Main
> import qualified Data.ByteString.Lazy as B
> main = B.getContents >>= myPrint . myConcat . B.unpack
That looks bad.
> myConcat = myConcat' 0
> myConcat' :: (Integral a) => Integer -> [a] -> [Integer]
> myConcat' _  = 
> myConcat' acc (x:xs) = case x `mod` 16 of
> 12 -> (10*acc + (restOf . fromIntegral) x) : myConcat' 0 xs
> 13 -> ((-10)*acc + (restOf . fromIntegral) x) : myConcat' 0 xs
> 15 -> (10*acc + (restOf . fromIntegral) x) : myConcat' 0 xs
> 10 -> fail $ show acc
> 11 -> fail $ show acc
> 14 -> fail $ show acc
> v -> myConcat' (100*acc + (numberOf . fromIntegral) x) xs
> where restOf n = (n - 12) `div` 16
> numberOf n = n - 6 * (n `div` 16)
> myPrint  = return ()
> myPrint xs = mapM_ myShow (take 14 xs) >> putStrLn "" >> myPrint (drop 14 xs)
> myShow x = (putStr . show) x >> putStr ";"
> I knew that csv output had to be 14 fields per line.
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