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Solution for day 9

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Jan 15, 2021, 12:17 AM
CUOTDYHN6R6ZZAH4S2JF65QP5MQ4OZWLXB5JCL4P6RHMP2QRESXAC

Dependencies

Change contents

  • file addition: day9.hs (----------)
    [2.17]
    import Control.Monad
    import Data.List
    import qualified Data.Set as S
    import System.Environment
    stretches :: Int -> [a] -> [([a],a)]
    stretches n l = let
    (i,z) = splitAt n l
    go _ [] = []
    go p (a:r) = (p,a) : go (drop 1 p ++ [a]) r
    in go i z
    valid p n = not $ null $ do
    (a:r) <- tails $ sort $ filter (< n) p
    let t = n - a
    (_,ge) <- [break (>= t) r]
    (_:_) <- [takeWhile (== t) ge]
    return ()
    part1 :: (Num n, Ord n) => [n] -> n
    part1 = snd . head . filter (not . uncurry valid) . stretches 25
    stretchesWithSum :: (Num a, Ord a) => a -> [a] -> [[a]]
    stretchesWithSum t = go 0 [] where
    go es e r = case es `compare` t of
    LT -> case r of
    (h:r') -> go (es + h) (e ++ [h]) r'
    [] -> []
    EQ -> e : let
    (d:r') = e
    in go (es - d) r' r
    GT -> let
    (d:r') = e
    in go (es - d) r' r
    part2 :: (Num n, Ord n) => [n] -> n -> n
    part2 l e = head $ do
    l'@(_:_:_) <- stretchesWithSum e l
    Just (MinMax a h) <- pure $ foldMap (Just . minMax) l'
    return (a + h)
    data MinMax a = MinMax a a
    instance Ord a => Semigroup (MinMax a) where
    MinMax l1 h1 <> MinMax l2 h2 = MinMax (min l1 l2) (max h1 h2)
    minMax :: a -> MinMax a
    minMax = join MinMax
    main :: IO ()
    main = do
    [fn] <- getArgs
    d <- (map (read :: String -> Integer) . words) <$> readFile fn
    let p1 = part1 d
    print p1
    print $ part2 d p1