2020-09 in haskell

2020/09-Encoding_Error/second.hs

@@ -0,0 +1,47 @@
+module Main (main) where
+import Control.Monad (when)
+import Data.List (foldl')
+import Data.Maybe (fromJust, isNothing)
+import System.Exit (die)
+
+exampleExpectedOutput = 62
+
+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
+
+isContiguousSum :: Int -> [Int] -> Int -> Maybe Int
+isContiguousSum target list counter
+  | sum' == target = Just $ (minimum lst) + (maximum lst)
+  | sum' < target = isContiguousSum target list (counter + 1)
+  | otherwise = Nothing
+  where
+    lst = take counter list
+    sum' = sum lst
+
+compute :: Int -> [Int] -> Int
+compute len list
+  | validate (take len list) (list !! len) = compute len $ tail list
+  | otherwise = list !! len
+
+compute' :: Int -> [Int] -> Int
+compute' target list = case isContiguousSum target list 2 of
+    Nothing -> compute' target (tail list)
+    Just a -> a
+
+main :: IO ()
+main = do
+  example <- parseInput "example"
+  let exampleOutput = compute 5 example
+      exampleOutput' = compute' exampleOutput example
+  when  (exampleOutput' /= exampleExpectedOutput)  (die $ "example failed: got " ++ show exampleOutput' ++ " instead of " ++ show exampleExpectedOutput)
+  input <- parseInput "input"
+  print $ compute' (compute 25 input) input