[Haskell-cafe] Fast code question

4 Jun 2009

      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?

{code}
module Main 
where 

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 ";"
{code}

I knew that csv output had to be 14 fields per line.

Best,
Bartek

[Haskell-cafe] Fast code question

Bartosz Wójcik