Eugene Kirpichov ekirpichov at gmail.com
Sun Aug 30 10:52:21 EDT 2009

```Bang patterns are easy: they make a part of the pattern strict by seq'ing it.

f (x, !y) = ...
~
f (x, y) = y `seq` ...

2009/8/30 Steve <stevech1097 at yahoo.com.au>:
> On Sun, 2009-08-30 at 16:34 +0400, Eugene Kirpichov wrote:
>> Here's my version that works in 0.7s for me for a file with 10^7
>> "999999999"'s but for some reason gets a 'wrong answer' at SPOJ :)
>>
>> {-# LANGUAGE BangPatterns #-}
>> module Main where
>>
>> import qualified Data.ByteString.Lazy as B
>> import Data.Word
>>
>> answer :: Int -> B.ByteString -> Int
>> answer k = fst . B.foldl' f (0, 0)
>>   where f :: (Int,Int) -> Word8 -> (Int,Int)
>>         f (!countSoFar, !x) 10
>>           | x`mod`k==0 = (countSoFar+1, 0)
>>           | otherwise  = (countSoFar,   0)
>>         f (!countSoFar, !x) c = (countSoFar, 10*x+(fromIntegral c)-48)
>>
>> readInt :: B.ByteString -> Int
>> readInt = B.foldl' (\x c -> 10*x+fromIntegral c-48) 0
>>
>> main = do
>>   (line, rest) <- B.break (==10) `fmap` B.getContents
>>   let [n, k] = map readInt . B.split 32 \$ line
>>   putStrLn . show \$ answer k rest - 1
>
> Eugene,
> I ran your code on one of my test files and it gave the same answer as
> my code. So I submitted it and it was accepted. Its fast - twice as fast
> as my solution, using much less memory. Overall its the 4th fastest
> Haskell solution. (but its still 10 * slower than C/C++)
> I'll have to read up on BangPatterns to try to understand what its
> doing!
>
> I submitted it as:
>
> {-# LANGUAGE BangPatterns #-}
> module Main where
>
> import qualified Data.ByteString.Lazy as B
> import qualified Data.Word            as DW
>
> answer :: Int -> B.ByteString -> Int
> answer k = fst . B.foldl' f (0, 0)
>  where f :: (Int,Int) -> DW.Word8 -> (Int,Int)
>        f (!countSoFar, !x) 10
>          | x`mod`k==0 = (countSoFar+1, 0)
>          | otherwise  = (countSoFar,   0)
>        f (!countSoFar, !x) c = (countSoFar, 10*x+(fromIntegral c)-48)
>
> readInt :: B.ByteString -> Int
> readInt = B.foldl' (\x c -> 10*x+fromIntegral c-48) 0
>
> main :: IO ()
> main = do
>  (line, rest) <- B.break (==10) `fmap` B.getContents
>  let [_, k] = map readInt . B.split 32 \$ line
>  putStrLn . show \$ answer k rest - 1
>
>
>

--
Eugene Kirpichov
Web IR developer, market.yandex.ru
```