aoc2021/app/D3P1.hs

module Main where

import Data.List (transpose)
import Data.Bits (shift, complement, (.&.))

main :: IO ()
main = interact solution

solution :: String -> String
solution = show
         . (\x -> x * (complement x .&. 4095))
         . bits_to_number
         . map mostcommon
         . transpose
         . map (map ((read :: String -> Int) . (pure :: a -> [a])))
         . lines
  where
    mostcommon :: [Int] -> Int
    mostcommon a = if count 0 a > count 1 a then 0 else 1;

    count :: Eq a => a -> [a] -> Int
    count e [] = 0
    count e (a:r) = count e r + if a == e then 1 else 0

    bits_to_number :: [Int] -> Int
    bits_to_number = btn' . reverse
      where
        btn' :: [Int] -> Int
        btn' [] = 0
        btn' (a:r) = shift (btn' r) 1 + a