run.hs

import Data.Bits (xor)
import Data.Ord (comparing)
import Data.Maybe
import Data.List

parseAll = lines

binary :: Int -> String -> Int
binary n = fst . foldl go (0, n)
  where go (l, h) v
          | v == 'F' || v == 'L' = (l, (l + h) `div` 2)
          | otherwise            = ((l + h + 1) `div` 2, h)

findSeat :: String -> (Int, Int)
findSeat spec =
  let (rspec, cspec) = splitAt 7 spec
  in (binary 127 rspec, binary 7 cspec)

seatId :: (Int, Int) -> Int
seatId (r, c) = r * 8 + c

posFromId :: Int -> (Int, Int)
posFromId = (`divMod` 8)

part1 :: [String] -> Int
part1 = maximum
  . map seatId
  . map findSeat

allIds :: [Int]
allIds = [0..127*8]

missing :: [Int] -> [Int]
missing = (allIds \\)

identifySeat :: [Int] -> Int
identifySeat xs =
  head $ [m | (b:m:a:_) <- tails xs
            , b + 1 /= m
            , m + 1 /= a]

part2 = identifySeat . missing . map (seatId . findSeat)

example = "FBFBBFFRLR"

main = do
   input <- parseAll <$> readFile "input.txt"
   print (part1 input)
   print (part2 input)