53 lines
1.6 KiB
Haskell
53 lines
1.6 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.List (foldl')
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import Data.Map qualified as M
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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 = 436
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type Input = [Int]
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type Parser = Parsec Void String
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parseInt :: Parser Int
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parseInt = do
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n <- some digitChar
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void $ optional (char ',')
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return $ read n
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parseOps :: Parser Input
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parseOps = some parseInt <* char '\n' <* eof
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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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compute :: Input -> Int
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compute input = compute' (length input) (foldl' prepare M.empty $ zip (init input) [1..]) (last input)
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where
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compute' :: Int -> M.Map Int (Int, Int) -> Int -> Int
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compute' 2020 _ n = n
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compute' t m n = case M.lookup n m of
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Just (t', i) -> compute' (t+1) (M.insert n (t, i+1) m) (t - t')
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Nothing -> compute' (t+1) (M.insert n (t, 0) m) 0
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prepare :: M.Map Int (Int, Int) -> (Int, Int) -> M.Map Int (Int, Int)
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prepare acc (n, t) = case M.lookup n acc of
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Just (_, i) -> M.insert n (t, i+1) acc
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Nothing -> M.insert n (t, 0) acc
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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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