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