Mastodawn

Haskell

Woo! I got on the leaderboard at last. I don’t think I’ve seen a problem like this one before, but fortunately it wasn’t as tricky as it seemed at first glance.

import Control.Monad  
import Data.List  
import Data.List.Split  
import Data.Tuple  

readInput :: String -> [(Int, Int)]  
readInput = map fixOrder . (zip <*> tail) . map read . splitOn ","  
  where  
    fixOrder (x, y)  
      | x > y = (y, x)  
      | otherwise = (x, y)  

crosses (a, b) (c, d) =  
  not (a == c || a == d || b == c || b == d)  
    && ((a < c && c < b) /= (a < d && d < b))  

part1 n = length . filter ((== n `quot` 2) . uncurry (-) . swap)  

part2 n = sum . (zipWith countKnots <*> inits)  
  where  
    countKnots x strings = length $ filter (crosses x) strings  

part3 n strings =  
  maximum [countCuts (a, b) | a <- [1 .. n - 1], b <- [a + 1 .. n]]  
  where  
    countCuts x = length $ filter (\s -> x == s || x `crosses` s) strings  

main =  
  forM_  
    [ ("everybody_codes_e2025_q08_p1.txt", part1 32),  
      ("everybody_codes_e2025_q08_p2.txt", part2 256),  
      ("everybody_codes_e2025_q08_p3.txt", part3 256)  
    ]  
    $ \(input, solve) -> readFile input >>= print . solve . readInput

🦆 Everybody.Codes 2025 Quest 8 Solutions 🦆 - programming.dev