2024-12 in haskell

1 files changed, 99 insertions, 0 deletions

diff --git a/2024/12-Garden_Groups/second.hs b/2024/12-Garden_Groups/second.hs
new file mode 100644
index 0000000..98409d6
--- /dev/null
+++ b/2024/12-Garden_Groups/second.hs
@@ -0,0 +1,99 @@
+-- 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 = 80
+example2ExpectedOutput = 436
+example3ExpectedOutput = 1206
+example4ExpectedOutput = 236
+example5ExpectedOutput = 368
+
+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 ()
+type Region = [Coord]
+
+compute :: Input -> Int
+compute input = sum $ map (\r -> corners r * length r) regions
+  where
+    (regions, _) = V.ifoldl' regions' ([], M.empty) input
+    regions' :: ([Region], Visited) -> Int -> Line -> ([Region], Visited)
+    regions' acc y line = VU.ifoldl' (regions'' y) acc line
+    regions'' :: Int -> ([Region], Visited) -> Int -> Char -> ([Region], Visited)
+    regions'' y acc@(rs, visited) x c = case M.lookup (x, y) visited of
+      Just _  -> acc  -- already processed
+      Nothing -> let (r, visited') = flood c ([], visited) (x, y) in (r:rs, visited')
+    flood :: Char -> (Region, Visited) -> (Int, Int) -> (Region, Visited)
+    flood c acc@(rs, visited) (x, y) = case M.lookup (x, y) visited of
+      Just _ -> acc
+      Nothing -> L.foldl' (flood c) ((x, y):rs, M.insert (x, y) () visited) $ neighbors visited c x y
+    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 -> (Int, Int) -> 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
+    corners :: Region -> Int
+    corners r = sum $ map corners' r
+      where
+        inside c = L.elem c r
+        outside = not . inside
+        corners' (x, y) = length $ filter id [ outside (x-1, y) && outside (x, y-1)   -- top left in
+                                             , outside (x+1, y) && outside (x, y-1)   -- top right in
+                                             , outside (x-1, y) && outside (x, y+1)   -- bottom left in
+                                             , outside (x+1, y) && outside (x, y+1)   -- bottom right in
+                                             , inside (x-1, y) && outside (x-1, y-1) && inside (x, y-1)   -- top left out
+                                             , inside (x+1, y) && outside (x+1, y-1) && inside (x, y-1)   -- top right out
+                                             , inside (x-1, y) && outside (x-1, y+1) && inside (x, y+1)   -- bottom left out
+                                             , inside (x+1, y) && outside (x+1, y+1) && inside (x, y+1) ] -- bottom right out
+
+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)
+  example4 <- parseInput "example4"
+  let exampleOutput = compute example4
+  when  (exampleOutput /= example4ExpectedOutput)  (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show example4ExpectedOutput)
+  example5 <- parseInput "example5"
+  let exampleOutput = compute example5
+  when  (exampleOutput /= example5ExpectedOutput)  (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show example5ExpectedOutput)
+  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