64 lines
2.2 KiB
Haskell
64 lines
2.2 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 Data.Either
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import qualified Data.Matrix as MTX
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import Data.Ratio
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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 = 480
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type Pair = (Rational, Rational) -- X Y
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type Entry = (Pair, Pair, Pair) -- ButtonA ButtonB Prize
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type Input = [Entry]
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type Parser = Parsec Void String
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parseNumber :: Parser Rational
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parseNumber = fromInteger . read <$> some digitChar
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parseButton :: Parser Pair
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parseButton = (,) <$> (string "Button " *> letterChar *> string ": X+" *> parseNumber)
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<*> (string ", Y+" *> parseNumber <* eol)
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parsePrize :: Parser Pair
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parsePrize = (,) <$> (string "Prize: X=" *> parseNumber)
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<*> (string ", Y=" *> parseNumber <* eol)
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parseEntry :: Parser Entry
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parseEntry = (,,) <$> parseButton
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<*> parseButton
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<*> parsePrize
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parseInput' :: Parser Input
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parseInput' = some (parseEntry <* optional 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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-- ax + bx = c
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-- ay + by = d
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compute :: Input -> Rational
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compute = sum . map compute'
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where
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compute' ((ax, ay), (bx, by), (px, py)) = let (Right sol) = MTX.rref $ MTX.fromList 2 3 [ ax, bx, px
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, ay, by, py ]
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x = sol MTX.! (1, 3)
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y = sol MTX.! (2, 3)
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in if (denominator x == 1 && denominator y == 1) then 3 * x + y else 0
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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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