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)
Is there a leaderboard for the community?: We have a programming.dev leaderboard with the info on how to join in this post: https://programming.dev/post/6631465
π Thread is locked until there's at least 100 2 star entries on the global leaderboard
{-# LANGUAGE NumericUnderscores #-}
import qualified Data.ByteString.Char8 as BS
import qualified Data.Map as M
import Relude
type Problem = [ByteString]
-- We apply rotation so that north is to the right, this makes
-- all computations easier since we can just sort the rows.
parse :: ByteString -> Problem
parse = rotate . BS.split '\n'
count :: Problem -> [[Int]]
count = fmap (fmap succ . BS.elemIndices 'O')
rotate, move, rotMov, doCycle :: Problem -> Problem
rotate = fmap BS.reverse . BS.transpose
move = fmap (BS.intercalate "#" . fmap BS.sort . BS.split '#')
rotMov = rotate . move
doCycle = rotMov . rotMov . rotMov . rotMov
doNcycles :: Int -> Problem -> Problem
doNcycles n = foldl' (.) id (replicate n doCycle)
findCycle :: Problem -> (Int, Int)
findCycle = go 0 M.empty
where
go :: Int -> M.Map Problem Int -> Problem -> (Int, Int)
go n m p =
let p' = doCycle p
in case M.lookup p' m of
Just n' -> (n', n + 1)
Nothing -> go (n + 1) (M.insert p' n m) p'
part1, part2 :: ByteString -> Int
part1 = sum . join . count . move . parse
part2 input =
let n = 1_000_000_000
p = parse input
(s, r) = findCycle p
numRots = s + ((n - s) `mod` (r - s - 1))
in sum . join . count $ doNcycles numRots p