2024-11 in haskell

1 files changed, 48 insertions, 0 deletions

diff --git a/2024/11-Plutonian_Pebbles/first.hs b/2024/11-Plutonian_Pebbles/first.hs
new file mode 100644
index 0000000..b80f5eb
--- /dev/null
+++ b/2024/11-Plutonian_Pebbles/first.hs
@@ -0,0 +1,48 @@
+-- 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