1 files changed, 80 insertions, 0 deletions
diff --git a/2024/12-Garden_Groups/first.hs b/2024/12-Garden_Groups/first.hs
new file mode 100644
index 0000000..d163999
--- /dev/null
+++ b/2024/12-Garden_Groups/first.hs
@@ -0,0 +1,80 @@
+-- requires cabal install --lib megaparsec parser-combinators heap vector
+module Main (main) where
+
+import           Control.Monad        (void, when)
+import qualified Data.List            as L
+import qualified Data.Map             as M
+import qualified Data.Vector          as V
+import qualified Data.Vector.Unboxed  as VU
+import           Data.Void            (Void)
+import           Text.Megaparsec
+import           Text.Megaparsec.Char
+
+example1ExpectedOutput = 140
+example2ExpectedOutput = 772
+example3ExpectedOutput = 1930
+
+type Line = VU.Vector Char
+type Input = V.Vector Line
+
+type Parser = Parsec Void String
+
+parseLine :: Parser Line
+parseLine = do
+  line <- some letterChar <* eol
+  return $ VU.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 Coord = (Int, Int)
+type Visited = M.Map Coord ()
+
+compute :: Input -> Int
+compute input = fst $ V.ifoldl' compute' (0, M.empty) input
+  where
+    compute' :: (Int, Visited) -> Int -> Line -> (Int, Visited)
+    compute' (acc, visited) y line = VU.ifoldl' (compute'' y) (acc, visited) line
+    compute'' :: Int -> (Int, Visited) -> Int -> Char -> (Int, Visited)
+    compute'' y acc@(cost, visited) x c = case M.lookup (x, y) visited of
+      Just _  -> acc  -- already processed
+      Nothing -> let (area, perimeter, visited') = enclose c (0, 0, visited) (x, y) in (cost + area * perimeter, M.union visited visited')
+    enclose :: Char -> (Int, Int, Visited) -> Coord -> (Int, Int, Visited)
+    enclose c acc@(area, perimeter, visited) (x, y) = case M.lookup (x, y) visited of
+      Just _ -> acc
+      Nothing -> case input V.!? y of
+        Just line -> case line VU.!? x of
+          Just c' -> let n = neighbors visited c x y in L.foldl' (enclose c) (1 + area, 4 - length n + perimeter, M.insert (x, y) () visited) n
+          Nothing -> (0, 0, visited)
+        Nothing -> (0, 0, visited)
+    neighbors :: Visited -> Char -> Int -> Int -> [Coord]
+    neighbors visited c x y = filter (valid visited c) [ (x-1, y), (x+1, y), (x, y-1), (x, y+1) ]
+    valid :: Visited -> Char -> Coord -> Bool
+    valid visited c (x, y) = case input V.!? y of
+      Just line -> case line VU.!? x of
+        Just c' -> c == c'
+        Nothing -> False
+      Nothing -> False
+
+main :: IO ()
+main = do
+  example1 <- parseInput "example1"
+  let exampleOutput = compute example1
+  when  (exampleOutput /= example1ExpectedOutput)  (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show example1ExpectedOutput)
+  example2 <- parseInput "example2"
+  let exampleOutput = compute example2
+  when  (exampleOutput /= example2ExpectedOutput)  (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show example2ExpectedOutput)
+  example3 <- parseInput "example3"
+  let exampleOutput = compute example3
+  when  (exampleOutput /= example3ExpectedOutput)  (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show example3ExpectedOutput)
+  input <- parseInput "input"
+  print $ compute input