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-- requires cabal install --lib megaparsec parser-combinators heap vector
module Main (main) where
import Control.Monad (void, when)
import Data.Void (Void)
import Text.Megaparsec
import Text.Megaparsec.Char
exampleExpectedOutput = 55312
type Input = [Int]
type Parser = Parsec Void String
parseNumber :: Parser Int
parseNumber = read <$> some digitChar <* optional hspace
parseInput' :: Parser Input
parseInput' = some parseNumber <* 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 input = sum $ map (blink 25) input
where
blink :: Int -> Int -> Int
blink 0 n = 1
blink i n = sum $ map (blink (i-1)) (blinkOne n)
blinkOne :: Int -> [Int]
blinkOne 0 = [1]
blinkOne n | m == 0 = [l, r]
| otherwise = [n * 2024]
where
(d, m) = length (show n) `divMod` 2
(l, r) = n `divMod` (10^d)
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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