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module Day25A where
import System.Environment (getArgs)
import Text.Printf (printf)
import Data.Bits
modExp :: Integer -> Integer -> Integer -> Integer
modExp b 0 m = 1
modExp b e m = t * modExp ((b * b) `mod` m) (shiftR e 1) m `mod` m
where t = if testBit e 0 then b `mod` m else 1
-- subject, loop size
encrypt :: Integer -> Integer -> Integer
encrypt s n = modExp s n 20201227
-- Brute force the loop size used for the given number with the given subject
bruteLoopSize :: Integer -> Integer -> Integer
bruteLoopSize k s = getNEqual 1 1
where getNEqual n x | new == k = n
| otherwise = getNEqual (n + 1) new
where new = (x * s) `mod` 20201227
parseFile :: String -> IO (Integer, Integer)
parseFile f = do
c <- readFile f;
let [a, b] = map read $ lines c;
return (a, b);
-- runghc 25a.hs inputs/day25
main :: IO ()
main = do
args <- getArgs;
(a, b) <- parseFile $ head args;
let aLoop = bruteLoopSize a 7;
let bLoop = bruteLoopSize b 7;
printf "a loop size = %d\nb loop size = %d\n" aLoop bLoop;
let enc = encrypt a bLoop;
printf "key = %d\n" enc;
return ();
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