AoC/2019/day10/day10.hs

import Data.List as L
import Data.Maybe
import Data.Ord
import qualified Data.Vector as V

data Asteroid =
  Asteroid Int Int
  deriving (Show)

instance Eq Asteroid where
  Asteroid a b == Asteroid c d = phi a b == phi c d

instance Ord Asteroid where
  Asteroid a b `compare` Asteroid c d = phi a b `compare` phi c d

phi :: Int -> Int -> Double
phi xi yi
 | x >= 0 = atan2 x (-y)
 | otherwise = 2 * pi + atan2 (-x) y
    where x = fromIntegral xi
          y = fromIntegral yi

(-|) :: Asteroid -> Asteroid -> Asteroid
(-|) (Asteroid a b) (Asteroid c d) = Asteroid (a - c) (b - d)

neg :: Asteroid -> Asteroid
neg (Asteroid a b) = Asteroid (-a) (-b)

dist :: Asteroid -> Int
dist (Asteroid x y) = abs x + abs y

main :: IO ()
main = do
  content <- lines <$> readFile "input"
  let (aMax,sMax) = maxInSight $ parse content
  print sMax
  let newlist =
        listCycle $
        sort $
        sortOn (dist . fromJust) $
        snd $ transformCoordinates (parse content) aMax
  let newnewlist = snd $ transformCoordinates newlist (neg aMax)
  print $ newnewlist !! 199

listCycle :: [Maybe Asteroid] -> [Maybe Asteroid]
listCycle [] = []
listCycle xs = nub xs ++ listCycle (xs \\ nub xs)

parse :: [String] -> [Maybe Asteroid]
parse =
  concat .
  V.toList . V.map V.toList . toAsteroids . V.map V.fromList . V.fromList
  where
    toAsteroids = V.imap g
    g x = V.imap (f x)
    f i j a =
      case a of
        '.' -> Nothing
        _ -> Just $ Asteroid j i

transformCoordinates ::
     [Maybe Asteroid] -> Asteroid -> (Asteroid, [Maybe Asteroid])
transformCoordinates xs a = (a, [Just $ x -| a | Just x <- xs, x /= a])

maxInSight :: [Maybe Asteroid] -> (Asteroid, Int)
maxInSight xs =
  maximumBy (comparing snd) $ map (f . transformCoordinates xs) (catMaybes xs)
  where
    f (x, y) = (x, length . nub $ y)