Day 8: Resonant Collinearity
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Haskell
import Control.Arrow
import Control.Monad
import Data.List
import Data.Map qualified as M
type Pos = [Int]
parse :: String -> (Pos, [(Char, Pos)])
parse s = ([n, m], [(c, [i, j]) | i <- [0 .. n], j <- [0 .. m], c <- [l !! i !! j], c /= '.'])
where
l = lines s
n = pred $ length $ head l
m = pred $ length l
buildMap :: [(Char, Pos)] -> M.Map Char [Pos]
buildMap = M.fromListWith (++) . fmap (second pure)
allPairs :: [Pos] -> [(Pos, Pos)]
allPairs l = [(x, y) | (x : xs) <- tails l, y <- xs]
add = zipWith (+)
sub = zipWith (-)
antinodes :: Pos -> Pos -> [Pos]
antinodes a b = [a `sub` ab, b `add` ab]
where
ab = b `sub` a
inBounds [x', y'] [x, y] = x >= 0 && y >= 0 && x <= x' && y <= y'
antinodes' :: Pos -> Pos -> Pos -> [Pos]
antinodes' l a b = al ++ bl
where
ab = b `sub` a
al = takeWhile (inBounds l) $ iterate (`sub` ab) a
bl = takeWhile (inBounds l) $ iterate (`add` ab) b
part1 l = length . nub . filter (inBounds l) . concat . M.elems . fmap (allPairs >=> uncurry antinodes)
part2 l = length . nub . concat . M.elems . fmap (allPairs >=> uncurry (antinodes' l))
main = getContents >>= print . (uncurry part1 &&& uncurry part2) . second buildMap . parse
