import Data.List (sort)

main :: IO ()
main = do
  raw <- readFile "day5.txt"
  let valid = foldr (&&) True $ map validate $ lines raw
      ls = if valid
           then map parse $ lines raw
           else error "invalid input"
      ansA = maximum ls
      ansB = findGap $ sort ls
   in do 
     putStrLn $ "day5a: " ++ (show ansA)
     putStrLn $ "day5b: " ++ (show ansB)

validate :: String -> Bool
validate s =
    length s == 10
 && all (`elem` ['F', 'B']) (take 7 s)
 && all (`elem` ['L', 'R']) (drop 7 s)

parse :: String -> Int
parse s = row * 8 + column
  where
    rowString = (take 7 s)
    columnString = (drop 7 s)
    row = fromBinaryString 'B' rowString
    column = fromBinaryString 'R' columnString

fromBinaryString :: Char -> String -> Int
fromBinaryString t s =
  foldr (+) 0 $ map snd $ filter (\x -> fst x == t) $ zip (reverse s) powList 

findGap :: [Int] -> Int
findGap [] = error "empty list"
findGap [x] = error "short list"
findGap (x:y:xs) =
  if y == succ x
  then findGap (y:xs)
  else succ x

powList :: [Int]
powList = 1 : map (* 2) powList