52 lines
1.7 KiB
Haskell
52 lines
1.7 KiB
Haskell
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-- requires cabal install --lib megaparsec parser-combinators heap vector
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-- very slow with runghc, use ghc -O3 -o second second.hs and get the result in seconds
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module Main (main) where
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import Control.Applicative.Permutations
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import Control.Monad (void, when)
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import Data.Void (Void)
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import Text.Megaparsec
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import Text.Megaparsec.Char
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exampleExpectedOutput = 11387
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data Equation = Equation Int [Int] deriving Show
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type Input = [Equation]
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type Parser = Parsec Void String
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parseNumber :: Parser Int
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parseNumber = read <$> some digitChar <* optional hspace
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parseEquation :: Parser Equation
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parseEquation = Equation <$> parseNumber <* string ": "
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<*> some parseNumber
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parseInput' :: Parser Input
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parseInput' = some (parseEquation <* eol) <* eof
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parseInput :: String -> IO Input
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parseInput filename = do
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input <- readFile filename
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case runParser parseInput' filename input of
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Left bundle -> error $ errorBundlePretty bundle
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Right input' -> return input'
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compute :: Input -> Int
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compute = sum . map total . filter valid
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where
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total (Equation t _) = t
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valid (Equation total numbers) = elem total $ combinations numbers
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combinations [x] = [x]
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combinations l = let x = last l
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xs = init l
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in concatMap (\n -> [x+n, x*n, read (show n ++ show x)]) $ combinations xs
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main :: IO ()
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main = do
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example <- parseInput "example"
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let exampleOutput = compute example
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when (exampleOutput /= exampleExpectedOutput) (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
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input <- parseInput "input"
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print $ compute input
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