1 files changed, 52 insertions, 0 deletions
diff --git a/2024/11-Plutonian_Pebbles/second.hs b/2024/11-Plutonian_Pebbles/second.hs
new file mode 100644
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+++ b/2024/11-Plutonian_Pebbles/second.hs
@@ -0,0 +1,52 @@
+-- requires cabal install --lib megaparsec parser-combinators heap vector
+module Main (main) where
+
+import           Control.Monad        (void, when)
+import qualified Data.List            as L
+import qualified Data.Map             as M
+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'
+
+type Memo = M.Map (Int, Int) Int
+
+compute :: Input -> Int
+compute input = fst $ L.foldl' (blink (75)) (0, M.empty) input
+  where
+    blink :: Int -> (Int, Memo) -> Int -> (Int, Memo)
+    blink 0 (acc, m) _ = (acc + 1, m)
+    blink i (acc, m) n = case M.lookup (n, i) m of
+      Just n' -> (acc + n', m)
+      Nothing -> let (n', m') = L.foldl' (blink (i-1)) (0, m) $ blinkOne n
+                 in (acc + n', M.insert (n, i) n' m')
+    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
+  input <- parseInput "input"
+  print $ compute input