71 lines
2.3 KiB
Haskell
71 lines
2.3 KiB
Haskell
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import Data.List
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import Data.List.Split
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import Data.Maybe
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main :: IO ()
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main = do
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input <- readFile "9/input.txt"
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let x = parse $ lines input
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-- Problem 1
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print $ f x
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-- Problem 2
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print $ g x
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where
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f = risk
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g = solve
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parse :: [String] -> [[Int]]
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parse = map (map read . chunksOf 1)
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risk :: [[Int]] -> Int
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risk a = sum $ map succ $ mapMaybe index (areas a)
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where
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index = ($ a) . uncurry idx
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solve :: [[Int]] -> Int
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solve x = product $ take 3 $ reverse $ sort $ map (basin x) $ areas x
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-- | Find risk areas (low points).
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areas :: [[Int]] -> [(Int,Int)]
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areas a = [ (x,y) | y <- [ 0 .. (pred $ length a) ],
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x <- [ 0 .. (length (a !! y)) ],
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Just n <- [idx x y a],
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all ((> n) . snd) (adj x y a) ]
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-- | Get the size of a basin centered on the given point.
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basin :: [[Int]] -> (Int, Int) -> Int
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basin m (x,y) = length a
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where
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a = case idx x y m of
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Just v -> let t = ((x,y),v) in t : search t m
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Nothing -> []
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-- | Search for neighbors of the given node where the value of the neighbor
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-- | is not 9 and is greater than the value of the given node. This search
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-- | is then repeated for each result it finds until no nodes satisfying the
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-- | constraint can be found in the immediate neighborhood of the cluster.
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search :: ((Int, Int), Int) -> [[Int]] -> [((Int, Int), Int)]
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search t @ ((x,y),v) b = nub (next ++ (next >>= (`search` b)))
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where
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neighbors = adj x y b
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next = nub $ filter (\(_,n) -> n /= 9 && n > v) neighbors
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-- | Find elements that are directly adjacent to a point in a matrix.
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adj :: Int -> Int -> [[a]] -> [((Int, Int), a)]
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adj x y m = [ ((a, b), v) | a <- [pred x .. succ x],
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b <- [pred y .. succ y],
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x /= a || y /= b,
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y == b || x == a,
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Just v <- [idx a b m] ]
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-- | Index into a matrix safely. If out of bounds on either axis,
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-- | returns `Nothing`.
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idx :: Int -> Int -> [[a]] -> Maybe a
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idx x y m = get y m >>= get x
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-- | Index into a list safely. Returns `Nothing` if out of bounds.
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get :: Int -> [a] -> Maybe a
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get i l | i < 0 = Nothing
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| i >= length l = Nothing
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| otherwise = Just (l !! i)
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