advent19.hs

   1 import Prelude hiding (length, take, drop)
   2 import Data.Sequence
   3
   4 -- input = 5
   5 input = 3012210
   6
   7 main :: IO ()
   8 main = do
   9     part1
  10     part2
  11
  12 part1 :: IO ()
  13 part1 = print $ 2 * (input - 2 ^ (toInteger (floor $ logBase 2 (fromIntegral input)))) + 1
  14
  15 part2 :: IO ()
  16 part2 = print $ flip index 0 $ presentSteps initial
  17
  18 presentSteps :: Seq Int -> Seq Int
  19 presentSteps elves
  20     | isFinished elves = elves
  21     | otherwise = presentSteps $ next elves
  22
  23 initial :: Seq Int
  24 initial = fromList [1..input]
  25
  26 isFinished :: Seq Int -> Bool
  27 isFinished elves = length elves == 1
  28
  29 next :: Seq Int -> Seq Int
  30 next elves = prefix >< (midfix |> suffix)
  31     where
  32         target = length elves `quot` 2
  33         prefix = drop 1 $ take target elves
  34         midfix = drop (target+1) elves
  35         suffix = index elves 0