56 lines
1.7 KiB
Haskell
56 lines
1.7 KiB
Haskell
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-- requires cabal install --lib megaparsec parser-combinators heap vector
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module Main (main) where
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import Control.Monad (void, when)
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import qualified Data.List as L
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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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import Debug.Trace
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exampleExpectedOutput = 4
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type Report = [Int]
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type Input = [Report]
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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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parseReport :: Parser Report
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parseReport = some parseNumber
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parseInput' :: Parser Input
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parseInput' = some (parseReport <* 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 input = length . filter id $ map (any (valid . differences) . candidates) input
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where
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candidates :: Report -> [Report]
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candidates report = report : [splitThis i|i<-[1..length report]]
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where
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splitThis i = let (l, r) = L.splitAt i report
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in init l ++ r
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differences report = zipWith (-) (init report) (tail report)
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valid :: Report -> Bool
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valid rs@(r:_) | r > 0 = all (\v -> v > 0 && v <= 3) rs
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| r < 0 = all (\v -> v < 0 && v >= (-3)) rs
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| otherwise = False
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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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