1 files changed, 94 insertions, 0 deletions
diff --git a/2024/16-Reindeer_Maze/second.hs b/2024/16-Reindeer_Maze/second.hs
new file mode 100644
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--- /dev/null
+++ b/2024/16-Reindeer_Maze/second.hs
@@ -0,0 +1,94 @@
+-- requires cabal install --lib megaparsec parser-combinators heap vector
+module Main (main) where
+
+import           Control.Monad        (void, when)
+import           Data.Functor
+import qualified Data.Heap            as H
+import qualified Data.List            as L
+import qualified Data.Map             as M
+import qualified Data.Set             as S
+import qualified Data.Vector          as V
+import           Data.Void            (Void)
+import           Text.Megaparsec
+import           Text.Megaparsec.Char
+
+exampleExpectedOutput = 64
+
+data Tile = Wall | Floor | Start | End deriving (Eq, Show)
+type Line = V.Vector Tile
+type Input = V.Vector Line
+
+type Parser = Parsec Void String
+
+parseTile :: Parser Tile
+parseTile = char '#' $> Wall
+        <|> char '.' $> Floor
+        <|> char 'E' $> End
+        <|> char 'S' $> Start
+
+parseLine :: Parser Line
+parseLine = do
+  line <- some parseTile <* eol
+  return $ V.generate (length line) (line !!)
+
+parseInput' :: Parser Input
+parseInput' = do
+  line <- some parseLine <* eof
+  return $ V.generate (length line) (line !!)
+
+parseInput :: String -> IO Input
+parseInput filename = do
+  input <- readFile filename
+  case runParser parseInput' filename input of
+    Left bundle  -> error $ errorBundlePretty bundle
+    Right input' -> return input'
+
+type Cost = Int
+data Heading = N | S | E | W deriving (Eq, Ord, Show)
+type Coord = (Int, Int, Heading)
+type Path = S.Set Coord
+data Position = Position Coord Cost Path deriving (Eq, Show)
+instance Ord Position where
+  compare (Position _ c1 _) (Position _ c2 _) = c1 `compare` c2
+type Visited = M.Map Coord Int
+
+type Candidates = H.MinHeap Position
+
+compute :: Input -> Int
+compute input = S.size $ S.map (\(x, y, _) -> (x, y)) $ walk infinity M.empty $ H.singleton (Position start 0 S.empty)
+  where
+    walk :: Int -> Visited -> Candidates -> S.Set Coord
+    walk l v h | H.size h == 0 = S.empty
+               | c > l = walk l v' h'
+               | t == End = S.union s' $ walk c v' h'
+               | otherwise = walk l v' $ H.union h' $ H.fromList $ nexts v' $ Position p c s'
+      where
+        ([(Position p@(x, y, d) c s)], h') = H.splitAt 1 h
+        t = input V.! y V.! x
+        v' = case M.lookup p v of
+          Just c' -> if c < c' then M.insert p c v else v
+          Nothing -> M.insert p c v
+        s' = S.insert p s
+    nexts :: Visited -> Position -> [Position]
+    nexts v p = L.filter (valid v) $ candidates p
+    valid :: Visited -> Position -> Bool
+    valid v (Position p@(x, y, _) c _) = input V.! y V.! x /= Wall && case M.lookup p v of
+      Just c' -> c <= c'
+      Nothing -> True
+    candidates :: Position -> [Position]
+    candidates (Position (x, y, N) c s) = [ Position (x-1, y, W) (c+1001) s, Position (x+1, y, E) (c+1001) s, Position (x, y-1, N) (c+1) s ]
+    candidates (Position (x, y, S) c s) = [ Position (x-1, y, W) (c+1001) s, Position (x+1, y, E) (c+1001) s, Position (x, y+1, S) (c+1) s ]
+    candidates (Position (x, y, E) c s) = [ Position (x, y-1, N) (c+1001) s, Position (x, y+1, S) (c+1001) s, Position (x+1, y, E) (c+1) s ]
+    candidates (Position (x, y, W) c s) = [ Position (x, y-1, N) (c+1001) s, Position (x, y+1, S) (c+1001) s, Position (x-1, y, W) (c+1) s ]
+    height = V.length input
+    width = V.length (input V.! 0)
+    start = (1, height - 2, E)
+    infinity = maxBound :: Int
+
+main :: IO ()
+main = do
+  example <- parseInput "example"
+  let exampleOutput = compute example
+  when  (exampleOutput /= exampleExpectedOutput)  (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+  input <- parseInput "input"
+  print $ compute input