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-- requires cabal install --lib megaparsec parser-combinators
module Main (main) where
import Control.Monad (void, when)
import Data.Either
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
import System.Exit (die)
exampleExpectedOutput = 231 + 51
data Operation = Add | Mul deriving (Eq, Show)
data Chain = Chain [Either Int Chain] [Operation] deriving Show
type Input = [Chain]
type Parser = Parsec Void String
parseInt :: Parser Int
parseInt = do
n <- some digitChar
void $ optional (char ' ')
return $ read n
parseOp :: Parser Operation
parseOp = do
op <- (char '+' *> return Add) <|> (char '*' *> return Mul)
void $ char ' '
return op
parseParenthesis :: Parser Chain
parseParenthesis = do
void $ char '('
a <- parseChain
void $ char ')'
void $ optional (char ' ')
return $ a
parseChain :: Parser Chain
parseChain = do
a <- ((Left <$> parseInt) <|> (Right <$> parseParenthesis))
next <- (Just <$> parseOp) <|> (return Nothing)
case next of
Nothing -> return $ Chain [a] []
Just op -> do
Chain elems ops <- parseChain
return $ Chain (a:elems) (op:ops)
parseInput' :: Parser Input
parseInput' = do
input <- some (parseChain <* void (optional (char '\n')))
void eof
return $ input
parseInput :: String -> IO Input
parseInput filename = do
input <- readFile filename
case runParser parseInput' filename input of
Left bundle -> die $ errorBundlePretty bundle
Right input' -> return input'
compute' :: Chain -> Int
compute' (Chain [Left x] []) = x
compute' (Chain [Right x] []) = compute' x
compute' (Chain (x:y:xs) (op:ops))
| op == Add = compute' $ Chain (Left (cx+cy):xs) ops
| op == Mul = cx * (compute' $ Chain (Left cy:xs) ops)
where
cx = if isLeft x then fromLeft 0 x else compute' (fromRight (Chain [] []) x)
cy = if isLeft y then fromLeft 0 y else compute' (fromRight (Chain [] []) y)
compute :: Input -> Int
compute input = sum $ map compute' input
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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