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diff --git a/2023/18-Lavaduct_Lagoon/first.hs b/2023/18-Lavaduct_Lagoon/first.hs
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+-- requires cabal install --lib megaparsec parser-combinators heap vector
+module Main (main) where
+
+import           Control.Applicative.Permutations
+import           Control.Monad                    (void, when)
+import qualified Data.Char                        as C
+import           Data.Either
+import           Data.Functor
+import qualified Data.Heap                        as H
+import qualified Data.List                        as L
+import qualified Data.Map                         as M
+import           Data.Maybe
+import qualified Data.Set                         as S
+import qualified Data.Vector                      as V
+import qualified Data.Vector.Unboxed              as VU
+import           Data.Void                        (Void)
+import           Text.Megaparsec
+import           Text.Megaparsec.Char
+
+import           Debug.Trace
+
+exampleExpectedOutput = 62
+
+data Heading = U | D | L | R deriving (Eq, Show)
+data Line = Line Heading Int String
+type Input = [Line]
+
+type Parser = Parsec Void String
+
+parseHeading :: Parser Heading
+parseHeading = char 'U' $> U
+           <|> char 'D' $> D
+           <|> char 'L' $> L
+           <|> char 'R' $> R
+
+parseLine :: Parser Line
+parseLine = Line <$> parseHeading <* space
+                 <*> (read <$> some digitChar) <* space
+                 <*> (string "(#" *> some alphaNumChar <* char ')' <* eol)
+
+parseInput' :: Parser Input
+parseInput' = some parseLine <* eof
+
+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 Map = M.Map (Int, Int) Bool
+type Plan = (Map, Int, Int)
+
+compute :: Input -> Int
+compute input = xlen * ylen - M.size (M.filter not floodedPit) -- L.foldl' countLine 0 [ymin..ymax]
+  where
+    -- filling
+    (xs, ys) = unzip $ M.keys trenches
+    xmin = minimum xs - 1
+    xmax = maximum xs + 1
+    xlen = xmax - xmin + 1
+    ymin = minimum ys - 1
+    ymax = maximum ys + 1
+    ylen = ymax - ymin + 1
+    floodedPit = flood (xmin, ymin) trenches
+    flood (x, y) m | x < xmin || x > xmax || y < ymin || y > ymax = m
+                   | M.member (x, y) m = m
+                   | otherwise = flood (x-1, y) $ flood (x+1, y) $ flood (x, y-1) $ flood (x, y+1) $ M.insert (x, y) False m
+    ---- digging
+    (trenches, _, _) = L.foldl' digOne (M.singleton (0, 0) True, 0, 0) input
+    digOne :: Plan-> Line -> Plan
+    digOne (m, x, y) (Line h l _) = (digTrench m h l, x', y')
+      where
+        digTrench :: Map -> Heading -> Int -> Map
+        digTrench m _ (-1) = m
+        digTrench m U i    = digTrench (M.insert (x, y-i) True m) U (i - 1)
+        digTrench m D i    = digTrench (M.insert (x, y+i) True m) D (i - 1)
+        digTrench m L i    = digTrench (M.insert (x-i, y) True m) L (i - 1)
+        digTrench m R i    = digTrench (M.insert (x+i, y) True m) R (i - 1)
+        x' | h == U = x
+           | h == D = x
+           | h == L = x - l
+           | h == R = x + l
+        y' | h == U = y - l
+           | h == D = y + l
+           | h == L = y
+           | h == R = y
+
+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