55 lines
1.7 KiB
Haskell
55 lines
1.7 KiB
Haskell
-- requires cabal install --lib megaparsec parser-combinators
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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.Char qualified as C
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import Data.Either
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import Data.List qualified as L
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import Data.Map qualified as M
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import Data.Maybe
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import Data.Set qualified as S
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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 = 71503
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data Input = Input [Int] [Int] deriving Show -- time distance
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type Parser = Parsec Void String
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parseNumber :: Parser Int
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parseNumber = read <$> some (digitChar <* many (char ' '))
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parseInput' :: Parser Input
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parseInput' = Input <$> (string "Time:" *> many (char ' ') *> some parseNumber <* eol)
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<*> (string "Distance:" *> many (char ' ') *> some parseNumber <* 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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race :: Int -> Int -> Int
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race limit record = limit + 1 - 2 * threshold
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where
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threshold = minTimeToBeat 0
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minTimeToBeat :: Int -> Int
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minTimeToBeat t | (limit - t) * t > record = t
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| otherwise = minTimeToBeat (t+1)
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compute :: Input -> Int
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compute (Input times distances) = product $ zipWith race times distances
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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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