2020 day9/haskell: add solution

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Xiretza 2020-12-10 19:23:22 +01:00
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@ -14,5 +14,6 @@ https://adventofcode.com/2020/
| 6 | | `**` | | | 6 | | `**` | |
| 7 | `**` | | | | 7 | `**` | | |
| 8 | | `**` | | | 8 | | `**` | |
| 9 | | `**` | |
`test.sh` can be used to run all solutions and automatically compares them to (my) puzzle inputs and the expected outputs. `test.sh` can be used to run all solutions and automatically compares them to (my) puzzle inputs and the expected outputs.

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26134589
3535124

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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