[Haskell-cafe] Beginner's speed problem

[Don Stewart]

aditya87:
> Hi,
> 
> I am trying to solve this problem: https://www.spoj.pl/problems/LASTDIG/
> It is very simple. Given a and b, return the last digit of a^b. b
> could be large, so I used logarithmic exponentiation and
> wrote/submitted the code below for this problem:
> 
> 
> ----------------------------------------------------------------------
> lastdigit :: Int -> Int -> Int -> Int
> lastdigit 0 0 _ = 1
> lastdigit a b c | even b = lastdigit ( (a*a) `rem` 10 ) (b `quot` 2) c
>                 | b == 1 = (a*c) `rem` 10
>                 | otherwise = lastdigit ( (a*a) `rem` 10 ) (b `quot` 2) (a*c)
> 
> doit :: [Char] -> Int
> doit line = lastdigit (read $ head $ words line) (read $ last $ words line) 1
> 
> main = do
>   n <- getLine
>   inputs <- sequence $ take (read n) $ repeat getLine
>   let slist = map doit inputs
>   mapM_ (putStrLn.show) slist
> -------------------------------------------------------------------

I notice an unnec. lazy 'c' argument to lastdigit,


    {-# LANGUAGE BangPatterns #-}

    lastdigit :: Int -> Int -> Int -> Int
    lastdigit 0 0 _  = 1
    lastdigit a b !c | even b    = lastdigit ( (a*a) `rem` 10 ) (b `quot` 2) c
                     | b == 1    = (a*c) `rem` 10
                     | otherwise = lastdigit ( (a*a) `rem` 10 ) (b `quot` 2) (a*c)

    doit :: [Char] -> Int
    doit line = lastdigit (read $ head $ words line) (read $ last $ words line) 1

    main = do
      n <- getLine
      inputs <- sequence $ take (read n) $ repeat getLine
      let slist = map doit inputs
      mapM_ (putStrLn.show) slist

Would generate better code for lastdigit.