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diff --git a/2020/25-Combo_Breaker/example b/2020/25-Combo_Breaker/example
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+5764801
+17807724
diff --git a/2020/25-Combo_Breaker/first.hs b/2020/25-Combo_Breaker/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 qualified as L
+import Data.Map qualified as M
+import Data.Maybe (fromJust)
+import Data.Set qualified as S
+import Data.Void (Void)
+import Math.NumberTheory.Powers.Modular
+import Text.Megaparsec
+import Text.Megaparsec.Char
+import System.Exit (die)
+
+exampleExpectedOutput = 14897079
+
+type Input = (Integer, Integer)
+
+type Parser = Parsec Void String
+
+parseInteger :: Parser Integer
+parseInteger = do
+  n <- some digitChar
+  void $ optional (char '\n')
+  return $ read n
+
+parseInput' :: Parser Input
+parseInput' = do
+  a <- parseInteger
+  b <- parseInteger
+  void eof
+  return (a, b)
+
+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'
+
+transform :: Integer -> Int -> Integer
+transform subjectNum loopSize = powMod subjectNum loopSize 20201227
+
+compute :: Input -> Integer
+compute (cardPubKey, doorPubKey) = encryptionKey
+  where
+    cardLoopSize = fromJust . L.elemIndex cardPubKey $ map (transform 7) [0..]
+    doorLoopSize = fromJust . L.elemIndex doorPubKey $ map (transform 7) [0..]
+    encryptionKey = transform doorPubKey cardLoopSize
+
+main :: IO ()
+main = do
+  example <- parseInput "example"
+  let exampleOutput = compute example
+  when  (exampleOutput /= exampleExpectedOutput)  (die $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+  print "OK"
+  input <- parseInput "input"
+  print $ compute input
diff --git a/2020/25-Combo_Breaker/input b/2020/25-Combo_Breaker/input
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+17773298
+15530095