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-- requires cabal install --lib megaparsec parser-combinators
module Main (main) where
import Control.Applicative.Permutations
import Control.Monad (void, when)
import Data.Char qualified as C
import Data.Either
import Data.Functor
import Data.List qualified as L
import Data.Map qualified as M
import Data.Maybe
import Data.Set qualified as S
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
import Debug.Trace
exampleExpectedOutput = 6
data Direction = L | R deriving (Eq, Show)
type Branch = (String, String)
type Map = M.Map String Branch
data Input = Input [Direction] Map deriving Show
type Parser = Parsec Void String
parseDirection :: Parser Direction
parseDirection = char 'L' $> L
<|> char 'R' $> R
parseNode :: Parser String
parseNode = some alphaNumChar
parseBranch :: Parser Branch
parseBranch = (,) <$> (char '(' *> parseNode)
<*> (string ", " *> parseNode <* char ')')
parseMapElt :: Parser (String, Branch)
parseMapElt = (,) <$> (parseNode <* string " = ")
<*> (parseBranch <* eol)
parseInput' :: Parser Input
parseInput' = Input <$> some parseDirection <* eol <* eol
<*> (M.fromList <$> some parseMapElt <* eof)
parseInput :: String -> IO Input
parseInput filename = do
input <- readFile filename
case runParser parseInput' filename input of
Left bundle -> error $ errorBundlePretty bundle
Right input' -> return input'
compute :: Input -> Int
compute (Input directions m) = L.foldr (lcm . pathLength 0) 1 startingNodes
where
startingNodes = L.filter ((==) 'A' . last) $ M.keys m
pathLength :: Int -> String -> Int
pathLength i node | last node == 'Z' = i
| otherwise = pathLength (i+1) (nextOne node)
where
nextOne :: String -> String
nextOne = next . (m M.!)
next :: Branch -> String
next | directions L.!! (i `mod` l) == L = fst
| otherwise = snd
l = length directions
main :: IO ()
main = do
example <- parseInput "example2"
let exampleOutput = compute example
when (exampleOutput /= exampleExpectedOutput) (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
input <- parseInput "input"
print $ compute input
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