Advent of Code 2020 day 5

    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. B and R represented the digit 1; F and L represented 0.

    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.


    You can find the code here or on Gitlab.

    Neil Smith

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