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+{-
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+The prime factors of 13195 are 5, 7, 13 and 29.
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+What is the largest prime factor of the number 600851475143?
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+-}
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+
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+import Debug.Trace
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+
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+is_prime :: Int -> Bool
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+is_prime x = go' x 2
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+ where
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+ go' 0 y = True
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+ go' 1 y = True
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+ go' x 2 = case (mod x 2) of
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+ 0 -> False
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+ _ -> go' x 3
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+ go' x y
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+ | x == y = True
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+ | otherwise = case (mod x y) of
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+ 0 -> False
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+ _ -> go' x (y + 2)
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+
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+solution :: Int
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+solution = go' 1 0 where
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+ target = 600851475143
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+ go' x y
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+ | x == target = y
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+ | otherwise = case (mod target x) of
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+ 0 -> case (is_prime x) of
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+ True -> go' (x + 2) x
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+ False -> go' (x + 2) y
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+ _ -> go' (x + 2) y
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+
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+-- This takes about 1hr to end
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+main :: IO ()
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+main = putStrLn ("Solution: " ++ show solution)
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