Advent of Code 2019 day 8

    After the complexity of yesterday, Day 8 was some light relief. It also uses quite a few of Haskell's generic data manipulation functions from the standard library.

    For once, I didn't use Megaparsec for parsing the data file: just read the file and map digitToInt over it to convert the characters into digits. The built-in chunksOf splits the raw string into layers.

    Part 1 becomes clear through the use of the count utility function, and the built-in comparing function to find the layer with the most zeroes.

    In part 2, I use transpose to convert the data from being a layer upon layer of pixels, to being a list of "pixels", but each "pixel" is in fact the value of that pixel in all the layers in the image. I can find the colour of each pixel by dropping all the leading transparent pixels and picking the first of what's left.

    I then convert these visible pixels into the strings that will represent them on the screen. concatMap does the conversion and represents the image as a single one-dimensional string; the chunksOf / unlines pair reshapes that row into the grid.


    Here's the entirety of the code (and Github).

    import Data.List
    import Data.List.Split
    import Data.Char
    import Data.Ord
    main :: IO ()
    main = do 
            text <- readFile "data/advent08.txt"
            let digits = successfulParse text
            let layers = chunksOf (imageWidth * imageHeight) digits
            print $ part1 layers
            putStrLn $ part2 layers
    imageWidth = 25
    imageHeight = 6
    part1 layers = (count 1 target) * (count 2 target)
        where target = minimumBy (comparing (count 0)) layers
    part2 layers = unlines rows
        where pixelLayers = transpose layers
              pixels = map firstVisible pixelLayers
              image = concatMap showPixel pixels
              rows = chunksOf imageWidth image
    firstVisible = head . dropWhile (== 2)
    showPixel 0 = " "
    showPixel 1 = "\x2588"
    count n = length . filter (== n)
    successfulParse :: String -> [Int]
    successfulParse input = map digitToInt input

    Neil Smith

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