Re: [Haskell-cafe] Slow IO?

30 Aug 2009

      On Sun, 2009-08-30 at 14:40 +0400, Eugene Kirpichov wrote:
Thanks, that works nicely too. However, I believe its not a standard
package, so I don't think it can be used for Sphere Online problems.

I timed a test run on a 10MB file and its a little slower than my
solution with the ByteString readInt improvement.

Steve
...
module Main where
import qualified Data.ByteString.Lazy as B
import Data.ByteString.Nums.Careless -- from bytestring-nums package
bint :: B.ByteString -> Int
bint = int
main = do
  line : rest <- B.split 10 `fmap` B.getContents
  let [n, k] = map int . B.split 32 $ line
  putStrLn . show . length . tail . filter ((==0).(`mod`k).bint)  $ rest
This does a 100MB file in 2.7s (probably because the file is cached by
the filesystem).
2009/8/30 Steve :
...
Hi,
I'm tackling a Sphere Online Judge tutorial question where it tests how
fast you can process input data. You need to achieve at least 2.5MB of
input data per second at runtime (on an old machine running ghc 6.6.1).
This is probably close to the limit of Haskell's ability.
https://www.spoj.pl/problems/INTEST/
I can see that 24 haskell programmers have solved it, but most are very
close to the 8 secs limit (and 6/24 are even over the limit!).
Here's my code. It fails with a "time limit exceeded" error. (I think it
would calculate the correct result, eventually).
module Main where
import qualified Data.List as DLi
import qualified System.IO as SIO
main :: IO ()
main = do
 line1 <- SIO.hGetLine SIO.stdin
 let k = read $ words line1 !! 1
 s <- SIO.hGetContents SIO.stdin
 print $ count s k
count :: String -> Int -> Int
count s k = DLi.foldl' foldFunc 0 (map read $ words s)
 where
   foldFunc :: Int -> Int -> Int
   foldFunc a b
     | mod b k == 0  = a+1
     | otherwise     = a
I tried using Data.ByteString but then found that 'read' needs a String,
not a ByteString.
I tried using buffered IO, but it did not make any difference.
Any suggestions on how to speed it up?
Regards,
Steve
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