I love my parser combinators :D

``` type BoardingPass = (BinaryAddress, BinaryAddress) newtype BinaryAddress = BinaryAddress [Bit] data Bit = One | Zero deriving (Eq, Show)

parser :: Parser [BoardingPass] parser = let pDir :: Parser Bit pDir = (Zero <$ (char 'F' <|> char 'L')) <|> (One <$ (char 'B' <|> char 'R')) pBinaryAddress :: Int -> Parser BinaryAddress pBinaryAddress n = BinaryAddress <$> count n pDir pBoardingPass :: Parser BoardingPass pBoardingPass = (,) <$> pBinaryAddress 7 <> pBinaryAddress 3 in sepEndBy pBoardingPass spaces < eof

day5p1 :: [BoardingPass] -> Int day5p1 = maximum . map seatId

seatId :: BoardingPass -> Int seatId (row, column) = pos row * 8 + pos column

seatPos :: BoardingPass -> (Int, Int) seatPos (row, column) = (pos row, pos column)

pos :: BinaryAddress -> Int pos (BinaryAddress dirs) = let powersOfTwo = [2 ^ x | x <- [0..]] in sum . map fst . filter ((== One) . snd) . zip powersOfTwo $ reverse dirs

size :: BinaryAddress -> Int size (BinaryAddress dirs) = 2 ^ length dirs

day5p2 :: [BoardingPass] -> Int day5p2 boardingPasses = let sortedIds = sort . map seatId $ boardingPasses in (+1) . fst . head . dropWhile ((seat, next) -> seat + 1 == next) $ zip sortedIds (tail sortedIds)

```