53 lines
1.5 KiB
Haskell
53 lines
1.5 KiB
Haskell
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-- requires cabal install --lib megaparsec parser-combinators heap vector
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module Main (main) where
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import Control.Monad (void, when)
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import qualified Data.List as L
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import qualified Data.Map 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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exampleExpectedOutput = 55312
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type Input = [Int]
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type Parser = Parsec Void String
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parseNumber :: Parser Int
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parseNumber = read <$> some digitChar <* optional hspace
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parseInput' :: Parser Input
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parseInput' = some parseNumber <* eol <* 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 parseInput' filename input of
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Left bundle -> error $ errorBundlePretty bundle
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Right input' -> return input'
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type Memo = M.Map (Int, Int) Int
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compute :: Input -> Int
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compute input = fst $ L.foldl' (blink (75)) (0, M.empty) input
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where
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blink :: Int -> (Int, Memo) -> Int -> (Int, Memo)
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blink 0 (acc, m) _ = (acc + 1, m)
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blink i (acc, m) n = case M.lookup (n, i) m of
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Just n' -> (acc + n', m)
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Nothing -> let (n', m') = L.foldl' (blink (i-1)) (0, m) $ blinkOne n
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in (acc + n', M.insert (n, i) n' m')
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blinkOne :: Int -> [Int]
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blinkOne 0 = [1]
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blinkOne n | m == 0 = [l, r]
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| otherwise = [n * 2024]
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where
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(d, m) = length (show n) `divMod` 2
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(l, r) = n `divMod` (10^d)
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main :: IO ()
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main = do
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input <- parseInput "input"
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print $ compute input
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