+findRepeat :: Grid -> Grid -> (Grid, Cache, Int)
+findRepeat rGrid cGrid = head $ dropWhile test $ iterate go (rGrid, M.empty, 0)
+ where test (g, c, _) = g `M.notMember` c
+ go (g, c, i) = (rollCycle g cGrid, M.insert g i c, (i + 1))
+
+rollNorth, rollCycle :: Grid -> Grid -> Grid
+rollNorth rGrid cGrid = roll (V2 0 0) (V2 0 1) (V2 1 0) cGrid rGrid
+
+rollCycle rGrid cGrid = foldl' go rGrid [ (V2 0 0, V2 0 1, V2 1 0)
+ , (V2 0 0, V2 1 0, V2 0 1)
+ , (V2 r 0, V2 0 1, V2 -1 0)
+ , (V2 0 c, V2 1 0, V2 0 -1)
+ ]
+ where (_, V2 r c) = U.bounds rGrid
+ go g (start, majorStep, minorStep) =
+ roll start majorStep minorStep cGrid g
+
+roll :: Position -> Position -> Position -> Grid -> Grid -> Grid
+roll start majorStep minorStep cGrid rGrid =
+ A.runSTUArray $
+ do grid <- A.thaw rGrid
+ holes <- newSTRef Q.Empty
+ forM_ (takeWhile (inBounds rGrid) $ iterate (^+^ majorStep) start) $ \maj ->
+ do writeSTRef holes Q.Empty
+ forM_ (takeWhile (inBounds rGrid) $ iterate (^+^ minorStep) maj) $ \here ->
+ rollPosition grid cGrid holes here
+ return grid
+
+rollPosition :: (MGrid s) -> Grid -> (Gaps s) -> Position -> ST s ()
+rollPosition grid cGrid holes here
+ | cGrid U.! here = writeSTRef holes Q.Empty
+ | otherwise = do roundHere <- A.readArray grid here
+ holesVal <- readSTRef holes
+ if roundHere then
+ case holesVal of
+ Q.Empty -> return ()
+ (h :<| hs) -> do A.writeArray grid h True
+ A.writeArray grid here False
+ writeSTRef holes (hs :|> here)
+ else modifySTRef holes (:|> here)
+
+inBounds :: Grid -> Position -> Bool
+inBounds g h = inRange (U.bounds g) h
+
+getLoad :: Grid -> Int
+getLoad grid = sum columnLoads
+ where (_, V2 rMax cMax) = U.bounds grid
+ columnLoads = [getColLoad c | c <- [0..cMax]]
+ getColLoad c = sum [(rMax - r + 1) | r <- [0..rMax], grid U.! (V2 r c)]
+
+readGrids :: String -> (Grid, Grid)
+readGrids text = (rGrid, cGrid)
+ where rows = lines text
+ r = length rows - 1
+ c = (length $ head rows) - 1
+ rGrid = U.listArray ((V2 0 0), (V2 r c)) $ fmap (== 'O') $ concat rows
+ cGrid = U.listArray ((V2 0 0), (V2 r c)) $ fmap (== '#') $ concat rows
+
+-- readElem :: Char -> Element
+-- readElem '.' = Empty
+-- readElem '#' = Cube
+-- readElem 'O' = Round
+
+showGrid :: Grid -> Grid -> String
+showGrid rGrid cGrid = unlines rows
+ where (_, V2 rMax cMax) = U.bounds rGrid
+ rows = [showRow r | r <- [0..rMax]]
+ showRow r = [showElem r c | c <- [0..cMax]]
+ showElem r c = let isR = rGrid U.! (V2 r c)
+ isC = cGrid U.! (V2 r c)
+ in if | isR && isC -> 'X'
+ | isR -> 'O'
+ | isC -> '#'
+ | otherwise -> '.'