AoC/2021/day03/day03.hs

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import Data.Bits
import Data.Char
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import Data.List
main :: IO ()
main = do
input <- lines <$> readFile "input"
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let rating = take (length $ head input) $ transpose (map tails input)
let [gamma, epsilon] = [toDec $ map (commonBit f) rating | f <- [(<=), (>)]]
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print $ gamma * epsilon
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let part2 = [genRating x input [] | x <- [(<=), (>)]]
print $ product $ map toDec part2
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commonBit :: (Int -> Int -> Bool) -> [String] -> Int
commonBit f =
(\[x, y] -> fromEnum (x `f` y)) .
map (length) . group . sort . head . transpose
genRating :: (Int -> Int -> Bool) -> [String] -> [Int] -> [Int]
genRating _ (x:[]) acc = acc ++ (map digitToInt x)
genRating f input acc =
genRating
f
(map tail $ filter (isPrefixOf (show $ commonBit f input)) input)
(acc ++ [commonBit f input])
toDec :: [Int] -> Int
toDec = sum . map (uncurry (*)) . zip (map bit [0 ..]) . reverse