[Haskell-cafe] Slow IO?

[Gwern Branwen]

On Sun, Aug 30, 2009 at 6:14 AM, Steve<stevech1097 at yahoo.com.au> wrote:
> 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

Did you try readInt?
http://hackage.haskell.org/packages/archive/bytestring/0.9.1.4/doc/html/Data-ByteString-Char8.html#23

-- 
gwern