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-- requires cabal install --lib megaparsec parser-combinators heap vector
-- very slow with runghc, use ghc -O3 -o second second.hs and get the result in seconds
module Main (main) where
import Control.Applicative.Permutations
import Control.Monad (void, when)
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
exampleExpectedOutput = 11387
data Equation = Equation Int [Int] deriving Show
type Input = [Equation]
type Parser = Parsec Void String
parseNumber :: Parser Int
parseNumber = read <$> some digitChar <* optional hspace
parseEquation :: Parser Equation
parseEquation = Equation <$> parseNumber <* string ": "
<*> some parseNumber
parseInput' :: Parser Input
parseInput' = some (parseEquation <* 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'
compute :: Input -> Int
compute = sum . map total . filter valid
where
total (Equation t _) = t
valid (Equation total numbers) = elem total $ combinations numbers
combinations [x] = [x]
combinations l = let x = last l
xs = init l
in concatMap (\n -> [x+n, x*n, read (show n ++ show x)]) $ combinations xs
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
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