Day8.hs

module Day8 where

import qualified Data.Text as T
import qualified Data.Map.Strict as M
import Data.Monoid
import Data.List
import Data.Ord (comparing)
import Data.Char (digitToInt)
import Data.Maybe (fromMaybe, catMaybes)

import Util

count :: (Ord k, Num a) => [k] -> M.Map k a
count xs = M.fromListWith (+) $ zip xs (repeat 1)

run :: IO ()
run = do
  inp <- map (map digitToInt . T.unpack) <$> readInput "day8" (T.chunksOf (25 * 6))
  print $ part1 inp
  mapM_ print $ part2 inp
 where
   part1 :: [[Int]] -> Int
   part1 i =
     let m = minimumBy (comparing (fetch 0)) $ map count i
         (ones, twos) = (fetch 1 m, fetch 2 m)
     in ones * twos

   fetch :: Ord k => k -> M.Map k Int -> Int
   fetch k m = fromMaybe 0 $ M.lookup k m

   part2 i =
     map catMaybes
     . chunk 25
     . map (getFirst . mconcat)
     . transpose
     $ map (map frobnicate) i
   frobnicate 2 = First Nothing
   frobnicate x = First $ Just x