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-- requires cabal install --lib megaparsec parser-combinators heap vector
module Main (main) where
import Control.Monad (void, when)
import Data.Bits
import Data.Functor
import qualified Data.List as L
import Data.Maybe
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
type Register = Int
type Op = Int
data Input = Input [Register] [Op] deriving Show
type Parser = Parsec Void String
parseNumber :: Parser Int
parseNumber = read <$> some digitChar
parseRegister :: Parser Register
parseRegister = string "Register " *> (char 'A' <|> char 'B' <|> char 'C') *> string ": " *> parseNumber
parseInput' :: Parser Input
parseInput' = Input <$> some (parseRegister <* eol) <* eol
<*> (string "Program: " *> some (parseNumber <* optional (char ',')) <* 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'
type Pointer = Int
type PState = (Pointer, [Register], [Int])
compute :: Input -> Int
compute (Input start ops) = minimum $ findA 0 (tail $ L.reverse $ L.tails ops)
where
findA :: Int -> [[Int]] -> [Int]
findA a [] = [a]
findA a (l:ls) = concatMap (\next -> findA next ls) nexts
where
candidates :: [Int]
candidates = [a*8..a*8+7]
steps :: [PState]
steps = map (\x -> step (0, [x, 0, 0], [])) candidates
outputs :: [[Int]]
outputs = map (\(_, [_, _, _], x') -> reverse x') steps
valids :: [Int]
valids = L.elemIndices l outputs
nexts :: [Int]
nexts = map (candidates L.!!) valids
end = length ops
step :: PState -> PState
step ps@(p, [a, b, c], _) | p == end = ps
| otherwise = step $ i ps co
where
i = [adv, bxl, bst, jnz, bxc, out, bdv, cdv] L.!! (ops L.!! p)
co = [0, 1, 2, 3, a, b, c] L.!! (ops L.!! (p+1))
adv (p, [a, b, c], acc) n = (p+2, [a `div` (2 ^ n), b, c], acc)
bxl (p, [a, b, c], acc) n = (p+2, [a, xor b n, c], acc)
bst (p, [a, b, c], acc) n = (p+2, [a, n `mod` 8, c], acc)
jnz (p, [0, b, c], acc) n = (p+2, [0, b, c], acc)
jnz (p, [a, b, c], acc) n = (n, [a, b, c], acc)
bxc (p, [a, b, c], acc) _ = (p+2, [a, xor b c, c], acc)
out (p, [a, b, c], acc) n = (p+2, [a, b, c], (n `mod` 8) : acc)
bdv (p, [a, b, c], acc) n = (p+2, [a, a `div` (2 ^ n), c], acc)
cdv (p, [a, b, c], acc) n = (p+2, [a, b, a `div` (2 ^ n)], acc)
main :: IO ()
main = do
input <- parseInput "input"
print $ compute input
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