module Day12 where
import Data.HashSet (HashSet, member, insert, fromList)
import Data.Map ( Map, fromList, unionWith, empty, (!) )
import Data.List.Split (splitOn)
import qualified Data.Char

type Path = (String,String)
type System = Map String [String]

readPath :: String -> Path
readPath s = (from,to) where [from,to] = splitOn "-" s

addPath :: Path -> System -> System
addPath (from,to) = unionWith (++) (Data.Map.fromList [(from,[to]), (to,[from])])

isUpper :: String -> Bool
isUpper = Data.Char.isUpper . head

pathFinder :: System -> [String] -> HashSet String -> Int
pathFinder _ [] _ = error "no starting point!"
pathFinder sys p@("end":xs) _ = 1
pathFinder sys p@(x:xs) visited = sum $ map (\next -> pathFinder sys (next:p) (next `insert` visited)) elig
    where elig = filter (\cave -> isUpper cave || not (cave `member` visited)) (sys ! x)

pathFinder2 :: System -> [String] -> HashSet String -> Int
pathFinder2 _ [] _ = error "no starting point!"
pathFinder2 sys p@("end":xs) _ = 1
pathFinder2 sys p@(x:xs) visited = sum $ map (\next -> f next sys (next:p) (next `insert` visited)) elig
    where
        f cave = if isUpper cave || not (cave `member` visited) then pathFinder2 else pathFinder
        elig = filter (/= "start") (sys ! x)

main :: IO ()
main = do
    input <- map readPath . lines <$> getContents
    let system = foldr addPath Data.Map.empty input

    putStrLn "part 1: "
    print $ pathFinder system ["start"] (Data.HashSet.fromList ["start"])

    putStrLn "part 2: "
    print $ pathFinder2 system ["start"] (Data.HashSet.fromList ["start"])