1 files changed, 13 insertions(+), 10 deletions(-)

M 2023/14.hs

@@ 19,13 19,13 @@ import qualified Data.Sequence as Seq
 import qualified Data.Set as Set
 
 data Rock = Round | Cube
-  deriving ( Show, Eq )
+  deriving ( Show, Eq, Ord )
 
 type Pos = (Int,Int)
 type Bounds = (Pos, Pos)
 
 data Grid a = Grid (Map.Map Pos a) Bounds
-  deriving ( Show, Eq )
+  deriving ( Show, Eq, Ord )
 
 parseGrid :: Parser (Maybe a) -> Parser (Grid a)
 parseGrid p = do


@@ 109,17 109,20 @@ partA :: Parse -> Int
 partA = load . tilt north
 
 partB :: Parse -> Int
-partB = f 0
-  where r Nothing = '.'
-        r (Just Cube) = '#'
-        r (Just Round) = 'O'
-        f n g | n == 1000000000 = load g
-        f n g | otherwise = f (succ n) (cycle g)
+partB g =
+  let (a, b) = f Map.empty 0 g in
+  let i = b + ((1_000_000_000 - b) `mod` (a-b)) in
+  load (idx i)
+  where idx i = flip (!!) i $ iterate cycle g
+        f vs n g =
+          case Map.lookup g vs of
+            Just n' -> (n, n')
+            Nothing -> f (Map.insert g n vs) (succ n) (cycle g)
         cycle = tilt east . tilt south . tilt west . tilt north
 
 main :: IO ()
 main = with parser $ do
   run' "14.example" "a example" partA 136
   run' "14.input" "a input" partA 106378
-  run "14.example" "b example" partB
-  --run "14.input" "b input" partB
+  run' "14.example" "b example" partB 64
+  run' "14.input" "b input" partB 90795