2020 day9/haskell: add solution

2020/README.md

@@ -14,5 +14,6 @@ https://adventofcode.com/2020/
 | 6 |        |  `**`   |      |
 | 7 |  `**`  |         |      |
 | 8 |        |  `**`   |      |
+| 9 |        |  `**`   |      |
 
 `test.sh` can be used to run all solutions and automatically compares them to (my) puzzle inputs and the expected outputs.

2020/data/day9.expected

@@ -0,0 +1,2 @@
+26134589
+3535124

2020/day9/day9.hs

@@ -0,0 +1,30 @@
+import AoC
+
+import Control.Monad
+import Data.List
+import Data.Maybe
+
+slidingWindow :: Int -> [a] -> [[a]]
+slidingWindow n xs
+  | length xs < n = []
+  | otherwise = take n xs : slidingWindow n (tail xs)
+
+summingTo :: (Eq a, Num a) => Int -> a -> [a] -> Maybe [a]
+n `summingTo` x = find ((==x) . sum) . replicateM n
+
+sublistSummingTo :: (Num a, Ord a) => a -> [a] -> Maybe [a]
+sublistSummingTo n = fmap snd . find ((== n) . fst) . scanl go (0, [])
+    where go = dropNeeded .: addToAcc
+          addToAcc (sum, parts) x = (sum+x, parts++[x])
+          dropNeeded = fromJust . find ((<= n) . fst) . iterate dropPart
+          dropPart (sum, (x:xs)) = (sum-x, xs)
+
+part1 :: [Int] -> Int
+part1 = snd . fromJust . find (not . uncurry doesSum) . map (\xs -> (init xs, last xs)) . slidingWindow 26
+    where doesSum xs y = isJust $ 2 `summingTo` y $ xs
+
+part2 :: [Int] -> Int
+part2 xs = minimum range + maximum range
+    where range = fromJust $ sublistSummingTo (part1 xs) xs
+
+main = runAoC (map read . lines) part1 part2