advent-of-code/2023/8/Main2.hs

import qualified Data.HashMap.Strict as HMS
import Data.List (foldl1')

type DataMap = HMS.HashMap String (String, String)

getDirection :: Char -> (a, a) -> a
getDirection 'L' = fst
getDirection 'R' = snd

countTimes :: String -> DataMap -> String -> Int
countTimes directions m start = countTimes' directions start 0
  where
    countTimes' (d:ds) ss@(s:_)
      | s == 'Z' = id
      | otherwise = countTimes' ds (getDirection d $ m HMS.! ss) . succ

solve :: String -> DataMap -> Int
solve directions m = foldl1' lcm
  . map (countTimes directions m)
  . filter ((== 'A') . head)
  . HMS.keys
  $ m

getDataMap :: [String] -> DataMap
getDataMap = HMS.fromList . map parseLine
  where
    get3At x = reverse . take 3 . drop x
    parseLine l = (get3At 0 l, (get3At 7 l, get3At 12 l))

main :: IO ()
main = do
  allLines <- lines <$> readFile "data.txt"
  let directions = cycle . head $ allLines
  print . solve directions . getDataMap . drop 2 $ allLines