Discussion on: Advent of Code 2020 Solution Megathread - Day 5: Binary Boarding

[Benedict Gaster]

After discussing with my partner I thought I should do the binary version too, so here is a slightly modified version, but rather than using Haskell binary stuff, just used fold to convert the binary string to an int.

-- we need to accumulate, rather than simply fold
foldl' f z []     = z
foldl' f z (x:xs) = let z' = z `f` x 
                    in seq z' $ foldl' f z' xs

-- calculate our half way range
half = (\ranges m ->  if m == 'F' || m == 'L'
                       then fst ranges 
                       else snd ranges) . ranges
    where
        ranges (min, max) = let mp = (min + max) `div` 2
                            in ((min, mp), (mp+1, max))

-- calculate a seat number
seat = toDec . map (\x -> if x == 'F' || x == 'L'
                      then '0'
                      else '1')
    where
        toDec = foldl' (\acc x -> acc * 2 + digitToInt x) 0

main = do xs <- readFile "day5_input" <&> lines
          let seats = map seat xs
          print (maximum (map seat xs))
          print $ fst $ head $ dropWhile snd 
                             $ dropWhile (not . snd)  
                             $ zip [0..] (foldr update emptySeats seats)
    where
        emptySeats = map (const False) [0..1023]
        update s = element s .~ True