Day 5 was one of a complex description hiding a simple problem. All the guff about boarding numbers and the like obscured the fact that we were dealing with numbers written in base 2: binary.
R represented the digit
Haskell already has a function for converting strings of arbitrary "digits" in arbitrary bases into numbers:
readInt. That needs a couple of supplementary functions for converting characters into digits, and it produces more output than we need, but the conversion is fairly painless.
directionToInt :: Char -> Int directionToInt dir = if dir `elem` "BR" then 1 else 0 convert :: String -> Int convert = fst . head . readInt 2 (`elem` "FBLR") directionToInt
Now I have essentially a list of numbers as input, finding the largest is trivial.
part1 = maximum . map convert
For part 2, finding the gap, I created the set of numbers I expected (in the range lowest to highest), and the set of numbers I had, subtracted one from the other, and that was the answer.
part2 passes = head $ expecteds \\ ns where ns = map convert passes highest = maximum ns lowest = minimum ns expecteds = [lowest..highest]
I could have used a
Set, but the set-like operations in
Data.List were sufficient for this case.