Advent2025-1.hs

data Dial = Dial Int deriving (Show, Eq)

dialMod :: Num a => a
dialMod = 100

instance Num Dial where
  (Dial x) + (Dial y) = Dial ((dialMod + x + y) `mod` dialMod)
  negate (Dial x) = Dial ((dialMod - x) `mod` dialMod)
  fromInteger x = Dial (fromInteger (x `mod` dialMod))
  -- other Num methods skipped for brevity

parseDial :: String -> Dial
parseDial cs =
  case cs of
    'L' : xs -> negate $ val xs
    'R' : xs -> val xs
  where
    val xs = Dial (read xs)

data PasswordState = PasswordState
  { getDial :: Dial
  , getPassword :: Int
  }
  deriving (Show, Eq)

processDials :: [Dial] -> Int
processDials = getPassword . foldl' handleDial initialState
  where
    initialState = PasswordState (Dial 50) 0

    handleDial ps d = updatePassword (applyDial d ps)

    applyDial d ps = ps { getDial = getDial ps + d }

    updatePassword (PasswordState 0 p) = PasswordState 0 (p + 1)
    updatePassword ps = ps

main :: IO ()
main = interact $ (++"\n") . show . processDials . map parseDial . lines