AoC/2021/day03/day03.hs

import Data.Bits
import Data.Char
import Data.List

main :: IO ()
main = do
  input <- lines <$> readFile "input"
  let rating = take (length $ head input) $ transpose (map tails input)
  let [gamma, epsilon] = [toDec $ map (commonBit f) rating | f <- [(<=), (>)]]
  print $ gamma * epsilon
  let part2 = [genRating x input [] | x <- [(<=), (>)]]
  print $ product $ map toDec part2

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