[Haskell-cafe] Fast code question
bartek at sudety.it
Thu Jun 4 19:31:26 EDT 2009
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?
import qualified Data.ByteString.Lazy as B
main = B.getContents >>= myPrint . myConcat . B.unpack
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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