Day 15: Lens Library

Megathread guidelines

  • Keep top level comments as only solutions, if you want to say something other than a solution put it in a new post. (replies to comments can be whatever)
  • Code block support is not fully rolled out yet but likely will be in the middle of the event. Try to share solutions as both code blocks and using something such as https://topaz.github.io/paste/ , pastebin, or github (code blocks to future proof it for when 0.19 comes out and since code blocks currently function in some apps and some instances as well if they are running a 0.19 beta)

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  • @LeixB
    2•1 year ago

    Haskell

    import Data.Array
import qualified Data.ByteString.Char8 as BS
import Data.Char (isAlpha, isDigit)
import Relude
import qualified Relude.Unsafe as Unsafe
import Text.ParserCombinators.ReadP hiding (get)

hash :: String -> Int
hash = foldl' (\a x -> (a + x) * 17 `mod` 256) 0 . fmap ord

part1 :: ByteString -> Int
part1 = sum . fmap (hash . BS.unpack) . BS.split ',' . BS.dropEnd 1

-- Part 2

type Problem = [Operation]

type S = Array Int [(String, Int)]

data Operation = Set String Int | Remove String deriving (Show)

parse :: BS.ByteString -> Maybe Problem
parse = fmap fst . viaNonEmpty last . readP_to_S parse' . BS.unpack
  where
    parse' = sepBy parseOperation (char ',') <* char '\n' <* eof
    parseOperation =
      munch1 isAlpha
        >>= \label -> (Remove label <$ char '-') +++ (Set label . Unsafe.read <$> (char '=' *> munch1 isDigit))

liftOp :: Operation -> Endo S
liftOp (Set label v) = Endo $ \s ->
  let (b, a) = second (drop 1) $ span ((/= label) . fst) (s ! hash label)
   in s // [(hash label, b <> [(label, v)] <> a)]
liftOp (Remove l) = Endo $ \s -> s // [(hash l, filter ((/= l) . fst) (s ! hash l))]

score :: S -> Int
score m = sum $ join [(* (i + 1)) <$> zipWith (*) [1 ..] (snd <$> (m ! i)) | i <- [0 .. 255]]

part2 :: ByteString -> Maybe Int
part2 input = do
  ops <- appEndo . foldMap liftOp . reverse <$> parse input
  pure . score . ops . listArray (0, 255) $ repeat []