2024/11.hs


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import qualified Data.IntMap.Strict as Map

main :: IO ()
main = do
  s0 <- map (read @Int) . words <$> getContents
  let splitHalf n =
        let nd = length (show n)
        in if even nd then Just $ n `quotRem` (10 ^ (nd `quot` 2))
                      else Nothing
  let blink =
        Map.fromListWith (+)
        . concatMap (\(n, c) ->
            if n == 0 then [(1, c)]
            else case splitHalf n of
                   Just (n1, n2) -> [(n1, c), (n2, c)]
                   Nothing -> [(n * 2024, c)])
        . Map.assocs
  print $ sum . Map.elems $ iterate blink (Map.fromList (map (,1::Int) s0)) !! 25
  print $ sum . Map.elems $ iterate blink (Map.fromList (map (,1) s0)) !! 75