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diff --git a/2023/18-Lavaduct_Lagoon/second.hs b/2023/18-Lavaduct_Lagoon/second.hs
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+-- requires cabal install --lib megaparsec parser-combinators heap vector text
+module Main (main) where
+
+import           Control.Applicative.Permutations
+import           Control.Monad                    (void, when)
+import           Data.Bifunctor                   (bimap)
+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.Text                        as T
+import qualified Data.Text.Read                   as T
+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 = 952408144115
+
+data Heading = U | D | L | R deriving (Eq, Show)
+data Line = Line Int Heading
+type Input = [Line]
+
+type Parser = Parsec Void String
+
+parseHeading :: Parser Heading
+parseHeading = char '3' $> U
+           <|> char '1' $> D
+           <|> char '2' $> L
+           <|> char '0' $> R
+
+parseLine :: Parser Line
+parseLine = Line <$> (letterChar *> space *> some digitChar *> string " (#" *> (fst . fromRight (0, T.pack "") . T.hexadecimal . T.pack <$> count 5 alphaNumChar))
+                 <*> (parseHeading <* 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'
+
+pick_theorem :: [(Int, Int)] -> Int
+pick_theorem vertices = 1 + perimeter `div` 2 + (abs . (`div` 2) . sum . zipWith crossProduct vertices $ tail vertices)
+    where crossProduct (x1, y1) (x2, y2) = y1 * x2 - x1 * y2
+          dist         (x1, y1) (x2, y2) = abs (x1 - x2) + abs (y1 - y2)
+          perimeter = sum . zipWith dist vertices $ tail vertices
+
+compute :: Input -> Int
+compute input = pick_theorem $ (0, 0) : L.foldl' coords [(0, 0)] input
+  where
+    coords :: [(Int, Int)] -> Line -> [(Int, Int)]
+    coords acc@((x, y):_) (Line l h) | h == U = (x, y - l):acc
+                                     | h == D = (x, y + l):acc
+                                     | h == L = (x - l, y):acc
+                                     | h == R = (x + l, y):acc
+
+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