day9

day9.hs

@@ -0,0 +1,40 @@
+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