aoc2020/day9.hs

main :: IO ()
main = do
  raw <- readFile "day9.txt"
  let nums = (map read $ lines raw) :: [Int]
      ansA = solveA nums
      ansB = solveB ansA nums
   in do
     putStrLn $ "day9a: " ++ (show ansA)
     putStrLn $ "day9b: " ++ (show ansB)

solveA :: [Int] -> Int
solveA nums = runWindow' (take 25 nums) (drop 25 nums)

solveB :: Int -> [Int] -> Int
solveB t nums = (maximum ans) + (minimum ans)
  where
    ans = testRuns t nums

runWindow' :: [Int] -> [Int] -> Int
runWindow' _ [] = error "reached end of stream"
runWindow' win (n:ns) =
  if n `elem` ws
  then runWindow' ((tail win) ++ [n]) ns
  else n
  where
    ws = [ x+y | x <- ls, y <- ls, x /= y ]
    ls = filter (<= (n - (minimum win))) win

findRun :: Int -> [Int] -> Int -> [Int] -> Maybe [Int]
findRun _ _ _ [] = Nothing
findRun t a r (l:ls)
  | r + l == t = Just (l:a)
  | r + l > t = Nothing
  | otherwise = findRun t (l:a) (r + l) ls

testRuns :: Int -> [Int] -> [Int]
testRuns _ [] = error "no answer"
testRuns t ls = case (findRun t [] 0 ls) of
                  Nothing -> testRuns t (tail ls)
                  Just x -> x