[Haskell-cafe] Slow IO?

Steve stevech1097 at yahoo.com.au
Sun Aug 30 06:14:15 EDT 2009

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.


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)
    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?


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