78 lines
2.2 KiB
Haskell
78 lines
2.2 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, zipWithM)
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import Data.Either (fromRight)
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import Data.List (foldl')
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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 = 1068781
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type Input = [Maybe Int]
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type Parser = Parsec Void String
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parseShuttle :: Parser (Maybe Int)
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parseShuttle = do
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num <- (Just . read <$> some digitChar) <|> (char 'x' *> return Nothing)
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void . optional $ char ','
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return $ num
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parseOps :: Parser Input
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parseOps = do
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void $ some digitChar
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void $ char '\n'
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shuttles <- some parseShuttle
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void $ char '\n'
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void $ eof
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return $ shuttles
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parseInput :: String -> IO Input
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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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-- From rosetta code https://rosettacode.org/wiki/Chinese_remainder_theorem#Haskell
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egcd :: Int -> Int -> (Int, Int)
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egcd _ 0 = (1, 0)
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egcd a b = (t, s - q * t)
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where
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(s, t) = egcd b r
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(q, r) = a `quotRem` b
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modInv :: Int -> Int -> Either String Int
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modInv a b =
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case egcd a b of
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(x, y)
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| a * x + b * y == 1 -> Right x
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| otherwise ->
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Left $ "No modular inverse for " ++ show a ++ " and " ++ show b
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chineseRemainder :: [Int] -> [Int] -> Either String Int
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chineseRemainder residues modulii =
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zipWithM modInv crtModulii modulii >>=
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(Right . (`mod` modPI) . sum . zipWith (*) crtModulii . zipWith (*) residues)
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where
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modPI = product modulii
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crtModulii = (modPI `div`) <$> modulii
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-- end of snippet from rosetta code
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compute :: Input -> Int
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compute input = fromRight 0 . uncurry chineseRemainder $ unzip $ fst $ foldl' accumul ([], 0) input
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where
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accumul :: ([(Int, Int)], Int) -> Maybe Int -> ([(Int, Int)], Int)
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accumul (acc, i) Nothing = (acc, i+1)
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accumul (acc, i) (Just v) = (acc ++ [(-i, v)], i+1)
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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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