2024-23 part 2 in haskell

2024/23-LAN_Party/second.hs

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+-- requires cabal install --lib megaparsec parser-combinators heap vector
+module Main (main) where
+
+import           Control.Monad        (void, when)
+import           Data.Functor
+import qualified Data.List            as L
+import qualified Data.Map             as M
+import           Data.Maybe
+import           Data.Ord             (comparing)
+import qualified Data.Set             as S
+import           Data.Void            (Void)
+import           Text.Megaparsec
+import           Text.Megaparsec.Char
+
+import           Debug.Trace
+
+exampleExpectedOutput = "co,de,ka,ta"
+
+type Computer = String
+type Link = (Computer, Computer)
+type Input = [Link]
+
+type Parser = Parsec Void String
+
+parseLink :: Parser Link
+parseLink = (,) <$> some letterChar <* char '-'
+                <*> some letterChar
+
+parseInput' :: Parser Input
+parseInput' = some (parseLink <* 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 Network = M.Map Computer [Computer]
+type Negroup = S.Set [Computer]
+
+-- All nodes have 4 neighbours in the example and 13 in the input. Since all
+-- nodes are connected and the example having a biggest group size of 4, I tried
+-- to solve for this and it worked. This means looking for nodes where all
+-- neighbors except one are the same.
+--
+-- Luckily, there is a single group of this size that matches.
+compute :: Input -> String
+compute links = L.intercalate "," . L.sort . fromJust . snd $ L.foldl' biggestGroup (network, Nothing) hosts
+  where
+    biggestGroup :: (Network, Maybe [Computer]) -> Computer -> (Network, Maybe [Computer])
+    biggestGroup a@(_, Just _) _ = a
+    biggestGroup (net, Nothing) n | M.size sharedNs == desiredSize = (net', Just $ n : M.keys sharedNs)
+                                  | otherwise = (net', Nothing)
+      where
+        net' = M.delete n net
+        ns = n : net M.! n
+        sharedNs = M.filter (\ns' -> length (L.filter (\n' -> L.elem n' ns) ns') == desiredSize) net
+    hosts :: [Computer]
+    hosts = M.keys network
+    desiredSize = (length $ network M.! "ta") - 1
+    network = L.foldl' connect M.empty links
+    connect :: Network -> Link -> Network
+    connect net (c1, c2) = connectOne c1 c2 $ connectOne c2 c1 net
+    connectOne :: Computer -> Computer -> Network -> Network
+    connectOne c1 c2 net = case M.lookup c1 net of
+      Just l  -> M.insert c1 (c2:l) net
+      Nothing -> M.insert c1 [c2] net
+
+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