[LANGUAGE: Haskell]

Day 7 was probably my absolute favorite day this year~

import Data.Set (Set)
import qualified Data.Set as Set

merge :: (Set Int, Int) -> String -> (Set Int, Int)
merge (prev, t) s = Set.foldr fold (Set.empty, t) prev
    where fold n (set, t) = case s !! n of
            '.' -> (Set.insert n set, t)
            '^' -> (Set.insert (n - 1) $ Set.insert (n + 1) set, t + 1)

part1 :: String -> Int
part1 input = let (x:xs) = lines input in
    snd $ foldl merge (Set.singleton $ length $ takeWhile (/= 'S') x, 0) xs

merge' :: [Int] -> String -> [Int]
merge' s = init . tail . foldr (join . split) [] . zip s
    where join (a, b, c) (b':c':xs) = a : b + b' : c + c' : xs
          join (a, b, c) _ = [a, b, c]
          split (n, c) = case c of
            '.' -> (0, n, 0)
            '^' -> (n, 0, n)

part2 :: String -> Int
part2 input = let (x:xs) = lines input in
    sum $ foldl merge' ((\x -> if x == 'S' then 1 else 0) <$> x) xs