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-- requires cabal install --lib megaparsec parser-combinators
module Main (main) where
import Control.Monad (void, when)
import Data.Maybe (fromJust)
import qualified Data.Set as S
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
import System.Exit (die)
exampleExpectedOutput = 8
data Op = Nop Int | Acc Int | Jmp Int
type Parser = Parsec Void String
parseOp :: Parser Op
parseOp = do
op <- string "nop " <|> string "acc " <|> string "jmp "
sign <- (char '+' *> return 1) <|> (char '-' *> return (-1))
num <- some digitChar
void $ char '\n'
let n = sign * (read num)
return $ case op of
"nop " -> Nop n
"acc " -> Acc n
"jmp " -> Jmp n
parseOps :: Parser [Op]
parseOps = some parseOp <* eof
parseInput :: String -> IO [Op]
parseInput filename = do
input <- readFile filename
case runParser parseOps filename input of
Left bundle -> die $ errorBundlePretty bundle
Right ops -> return ops
run :: [Op] -> Int -> Int -> S.Set Int -> Bool -> Maybe Int
run ops pointer acc visited mut
| pointer == last = Just acc
| S.member pointer visited = Nothing
| otherwise = case ops !! pointer of
Nop i -> case runNop mut of
Nothing -> if mut then Nothing else runJmp i True
Just n -> Just n
Acc i -> runAcc i mut
Jmp i -> case runJmp i mut of
Nothing -> if mut then Nothing else runNop True
Just n -> Just n
where
last = length ops
runAcc i = run ops (pointer + 1) (acc + i) visited'
runJmp i = run ops (pointer + i) acc visited'
runNop = run ops (pointer + 1) acc visited'
visited' = S.insert pointer visited
compute :: [Op] -> Int
compute ops = fromJust $ run ops 0 0 S.empty False
main :: IO ()
main = do
example <- parseInput "example"
let exampleOutput = compute example
when (exampleOutput /= exampleExpectedOutput) (die $ "example failed: got " ++ show exampleOutput ++ " instead of " ++ show exampleExpectedOutput)
input <- parseInput "input"
print $ compute input
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