41 lines
1.1 KiB
Haskell
41 lines
1.1 KiB
Haskell
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import Data.Bits ((.&.), countTrailingZeros, xor)
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import Data.List.Split (splitOn)
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import Data.List (foldl', transpose)
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import qualified Data.Vector.Unboxed as VU
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import Data.Vector.Unboxed (Vector)
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isPowTwo :: Int -> Bool
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isPowTwo n = n .&. (n - 1) == 0
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hashChar :: Char -> Int
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hashChar '#' = 1
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hashChar '.' = 0
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lineToInt :: String -> Int
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lineToInt = foldl' (\x y -> 2 * x + hashChar y) 0
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matchesAt :: Vector Int -> Int -> Bool
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matchesAt v i = VU.length differences == 1 && isPowTwo (VU.head differences)
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where
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nMirrored = min i $ VU.length v - i
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leftSide = VU.slice (i - nMirrored) nMirrored v
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rightSide = VU.slice i nMirrored v
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differences = VU.filter (/= 0)
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$ VU.zipWith xor (VU.reverse rightSide) leftSide
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findMatches :: Vector Int -> [Int]
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findMatches v = filter (matchesAt v) [1 .. VU.length v - 1]
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solveDirection :: [String] -> Int
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solveDirection = last . (0:) . findMatches . VU.fromList . map lineToInt
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solve :: [String] -> Int
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solve l = 100 * solveDirection l + (solveDirection . transpose) l
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main :: IO ()
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main = print . sum . map (solve . lines) . splitOn "\n\n"
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=<< readFile "data.txt"
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