-- 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.Void            (Void)
import           Text.Megaparsec
import           Text.Megaparsec.Char

exampleExpectedOutput = "4,6,3,5,6,3,5,2,1,0"

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 -> String
compute (Input start ops) = L.intercalate "," . L.reverse . map show . (\(_, _, s) -> s) $ step (0, start, [])
  where
    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
  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