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diff --git a/2020/20-Jurassic_Jigsaw/first.hs b/2020/20-Jurassic_Jigsaw/first.hs
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+-- requires cabal install --lib megaparsec parser-combinators
+module Main (main) where
+import Control.Monad (void, when)
+import Data.List (intersect, transpose)
+import Data.Void (Void)
+import Text.Megaparsec
+import Text.Megaparsec.Char
+import System.Exit (die)
+
+exampleExpectedOutput = 20899048083289
+
+type TileID = Int
+data Tile = Tile { tileID :: TileID
+                 , image :: [[Bool]]
+                 , edgesPermutations :: [[Bool]]
+                 } deriving Show
+type Input = [Tile]
+
+type Parser = Parsec Void String
+
+parseInt :: Parser Int
+parseInt = do
+  n <- some digitChar
+  return $ read n
+
+parseLine :: Parser [Bool]
+parseLine = do
+  elts <- some (char '#' <|> char '.')
+  void $ char '\n'
+  return $ map (== '#') elts
+
+parseTile :: Parser Tile
+parseTile = do
+  void $ string "Tile "
+  n <- parseInt
+  void $ string ":\n"
+  image <- some parseLine
+  let timage = transpose image
+      edges = [head image, last image, head timage, last timage]
+      inverted = map reverse edges
+  return $ Tile n image (edges ++ inverted)
+
+parseInput' :: Parser Input
+parseInput' = do
+  tiles <- some $ parseTile <* (optional $ char '\n')
+  void eof
+  return tiles
+
+parseInput :: String -> IO Input
+parseInput filename = do
+  input <- readFile filename
+  case runParser parseInput' filename input of
+    Left bundle -> die $ errorBundlePretty bundle
+    Right input' -> return input'
+
+compute :: Input -> Int
+compute tiles = product . map fst . filter ((== 4) . snd) $ map matchingEdges tiles
+  where
+    matchingEdges :: Tile -> (TileID, Int)
+    matchingEdges tile = (tileID tile, sum $ map (matches tile) tiles)
+    matches :: Tile -> Tile -> Int
+    matches Tile{tileID=a, image=_, edgesPermutations=e} Tile{tileID=b, image=_, edgesPermutations=f}
+      | a == b = 0
+      | otherwise = length $ intersect e f
+
+main :: IO ()
+main = do
+  example <- parseInput "example"
+  let exampleOutput = compute example
+  when  (exampleOutput /= exampleExpectedOutput)  (die $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+  input <- parseInput "input"
+  print $ compute input