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