2023 day 12: star 2

Oh my god

2023/12/Main2.hs

@@ -0,0 +1,48 @@
+import           Data.List.Split (splitOn)
+import           Data.Maybe (fromMaybe)
+import qualified Data.Vector as V
+import           Data.Vector (Vector)
+import           Control.Monad (guard)
+
+nFolds :: Int
+nFolds = 5
+
+solve :: (Vector Int, Vector Char) -> Int
+solve (records, fences) = getAt 0 0
+  where
+    v = V.generate (V.length records + 1)
+      $ V.generate (V.length fences + 1) . getAt'
+
+    getAt r f = v V.! r V.! f
+
+    getAt' r f = maybe noFencesLeft getWithFence $ fences V.!? f
+      where
+        noFencesLeft
+          | r == V.length records = 1
+          | otherwise = 0
+
+        getWithFence thisFence = worksHere thisFence + startBreakHere thisFence
+
+        worksHere '#' = 0
+        worksHere _ = getAt r (f + 1)
+
+        startBreakHere '.' = 0
+        startBreakHere _ = fromMaybe 0
+          $ do
+            thisRecord <- records V.!? r
+            let remainingFences = V.drop f fences
+            fenceAfter <- remainingFences V.!? thisRecord
+            guard ('.' `notElem` V.take thisRecord remainingFences)
+            guard (fenceAfter /= '#')
+            return $ getAt (r + 1) (f + thisRecord + 1)
+
+parseLine :: String -> (Vector Int, Vector Char)
+parseLine l = ( unfoldRecords . map read . splitOn "," $ y
+              , (`V.snoc` '.') . V.tail . unfoldRecords $ '?':x)
+  where
+    (x:y:_) = splitOn " " l
+
+    unfoldRecords = V.concat . replicate nFolds . V.fromList
+
+main :: IO ()
+main = print . sum . map (solve . parseLine) . lines =<< readFile "data.txt"