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-- requires cabal install --lib megaparsec parser-combinators
module Main (main) where
import Control.Applicative.Permutations
import Control.Monad (void, when)
import Data.Char qualified as C
import Data.Either
import Data.List qualified as L
import Data.Map qualified as M
import Data.Maybe
import Data.Set qualified as S
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
import Debug.Trace
exampleExpectedOutput = 71503
data Input = Input [Int] [Int] deriving Show -- time distance
type Parser = Parsec Void String
parseNumber :: Parser Int
parseNumber = read <$> some (digitChar <* many (char ' '))
parseInput' :: Parser Input
parseInput' = Input <$> (string "Time:" *> many (char ' ') *> some parseNumber <* eol)
<*> (string "Distance:" *> many (char ' ') *> some parseNumber <* eol <* eof)
parseInput :: String -> IO Input
parseInput filename = do
input <- readFile filename
case runParser parseInput' filename input of
Left bundle -> error $ errorBundlePretty bundle
Right input' -> return input'
race :: Int -> Int -> Int
race limit record = limit + 1 - 2 * threshold
where
threshold = minTimeToBeat 0
minTimeToBeat :: Int -> Int
minTimeToBeat t | (limit - t) * t > record = t
| otherwise = minTimeToBeat (t+1)
compute :: Input -> Int
compute (Input times distances) = product $ zipWith race times distances
main :: IO ()
main = do
example <- parseInput "example"
let exampleOutput = compute example
when (exampleOutput /= exampleExpectedOutput) (error $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
input <- parseInput "input"
print $ compute input
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