1 files changed, 31 insertions, 0 deletions
diff --git a/2020/09-Encoding_Error/first.hs b/2020/09-Encoding_Error/first.hs
new file mode 100644
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+++ b/2020/09-Encoding_Error/first.hs
@@ -0,0 +1,31 @@
+module Main (main) where
+import Control.Monad (when)
+import Data.List (foldl')
+import System.Exit (die)
+
+exampleExpectedOutput = 127
+
+parseInput :: String -> IO [Int]
+parseInput filename = do
+  input <- readFile filename
+  return $ map read $ lines input
+
+validate :: [Int] -> Int -> Bool
+validate list n = processInput list
+  where
+    processInput :: [Int] -> Bool
+    processInput (_:[]) = False
+    processInput (x:xs) = (foldl' (\acc i -> acc || (x + i == n && x /= i)) False xs) || processInput xs
+
+compute :: Int -> [Int] -> Int
+compute len list
+  | validate (take len list) (list !! len) = compute len $ tail list
+  | otherwise = list !! len
+
+main :: IO ()
+main = do
+  example <- parseInput "example"
+  let exampleOutput = compute 5 example
+  when  (exampleOutput /= exampleExpectedOutput)  (die $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
+  input <- parseInput "input"
+  print $ compute 25 input