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module Main where
import Control.Monad
import Control.Monad.ST
import Data.Array.ST
import Data.Array.Unboxed
soeST :: forall s. Int -> ST s (STUArray s Int Bool)
soeST n = do
arr <- newArray (0, n) True
mapM_ (\i -> writeArray arr i False) [0, 1]
let n2 = n `div` 2
let loop :: Int -> ST s ()
loop i | i > n2 = return ()
loop i = do
b <- readArray arr i
let reset :: Int -> ST s ()
reset j | j > n = return ()
reset j = writeArray arr j False >> reset (j + i)
when b (reset (2*i))
loop (succ i)
loop 2
return arr
soeA :: Int -> UArray Int Bool
soeA n = runST (soeST n >>= freeze)
soe :: Int -> [Int]
soe = map fst . filter snd . assocs . soeA
soeCount :: Int -> Int
soeCount = length . filter id . elems . soeA
main :: IO ()
main = print (soeCount (10^8)) |