Day 14: Restroom Redoubt
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Haskell. For part 2 I just wrote 10000 text files and went through them by hand. I quickly noticed that every 103 seconds, an image started to form, so it didn’t take that long to find the tree.
Code
import Data.Maybe
import Text.ParserCombinators.ReadP
import qualified Data.Map.Strict as M
type Coord = (Int, Int)
type Robot = (Coord, Coord)
int :: ReadP Int
int = fmap read $ many1 $ choice $ map char $ '-' : ['0' .. '9']
coord :: ReadP Coord
coord = (,) <$> int <*> (char ',' *> int)
robot :: ReadP Robot
robot = (,) <$> (string "p=" *> coord) <*> (string " v=" *> coord)
robots :: ReadP [Robot]
robots = sepBy robot (char '\n')
simulate :: Coord -> Int -> Robot -> Coord
simulate (x0, y0) t ((x, y), (vx, vy)) =
((x + t * vx) `mod` x0, (y + t * vy) `mod` y0)
quadrant :: Coord -> Coord -> Maybe Int
quadrant (x0, y0) (x, y) = case (compare (2*x + 1) x0, compare (2*y + 1) y0) of
(LT, LT) -> Just 0
(LT, GT) -> Just 1
(GT, LT) -> Just 2
(GT, GT) -> Just 3
_ -> Nothing
freqs :: (Foldable t, Ord a) => t a -> M.Map a Int
freqs = foldr (\x -> M.insertWith (+) x 1) M.empty
solve :: Coord -> Int -> [Robot] -> Int
solve grid t = product . freqs . catMaybes . map (quadrant grid . simulate grid t)
showGrid :: Coord -> [Coord] -> String
showGrid (x0, y0) cs = unlines
[ [if (x, y) `M.member` m then '#' else ' ' | x <- [0 .. x0]]
| let m = M.fromList [(c, ()) | c <- cs]
, y <- [0 .. y0]
]
main :: IO ()
main = do
rs <- fst . last . readP_to_S robots <$> getContents
let g = (101, 103)
print $ solve g 100 rs
sequence_
[ writeFile ("tree_" ++ show t) $ showGrid g $ map (simulate g t) rs
| t <- [0 .. 10000]
]
