adventofcode16/app/advent19.hs

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