58 lines
1.6 KiB
Haskell
58 lines
1.6 KiB
Haskell
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-- requires cabal install --lib megaparsec parser-combinators
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module Main (main) where
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import Control.Monad (void, when)
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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 (fromJust)
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import Data.Set qualified as S
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import Data.Void (Void)
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import Math.NumberTheory.Powers.Modular
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import Text.Megaparsec
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import Text.Megaparsec.Char
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import System.Exit (die)
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exampleExpectedOutput = 14897079
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type Input = (Integer, Integer)
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type Parser = Parsec Void String
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parseInteger :: Parser Integer
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parseInteger = do
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n <- some digitChar
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void $ optional (char '\n')
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return $ read n
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parseInput' :: Parser Input
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parseInput' = do
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a <- parseInteger
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b <- parseInteger
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void eof
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return (a, b)
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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 -> die $ errorBundlePretty bundle
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Right input' -> return input'
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transform :: Integer -> Int -> Integer
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transform subjectNum loopSize = powMod subjectNum loopSize 20201227
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compute :: Input -> Integer
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compute (cardPubKey, doorPubKey) = encryptionKey
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where
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cardLoopSize = fromJust . L.elemIndex cardPubKey $ map (transform 7) [0..]
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doorLoopSize = fromJust . L.elemIndex doorPubKey $ map (transform 7) [0..]
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encryptionKey = transform doorPubKey cardLoopSize
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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) (die $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
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print "OK"
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input <- parseInput "input"
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print $ compute input
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