Day 24

2 files changed, 91 insertions, 0 deletions

diff --git a/2017/24.hs b/2017/24.hs
new file mode 100644
index 0000000..fe71f27
--- /dev/null
+++ b/2017/24.hs
@@ -0,0 +1,34 @@
+import Control.Monad
+import Data.List
+import qualified Data.Map.Strict as Map
+
+
+type Inventory = Map.Map Int [Int]
+
+findbridges :: Int -> Inventory -> [[(Int, Int)]]
+findbridges start inven = case Map.lookup start inven of
+    Nothing -> [[]]
+    Just nexts ->
+        concatMap (\n -> map ((start, n) :) $ findbridges n (delpair start n $ delpair n start inven)) $
+        nub nexts
+  where
+    delpair :: Int -> Int -> Inventory -> Inventory
+    delpair a b = Map.update (\l -> let l' = l \\ [b] in if null l' then Nothing else Just l') a .
+                  Map.update (\l -> let l' = l \\ [a] in if null l' then Nothing else Just l') b
+
+strength :: [(Int, Int)] -> Int
+strength = sum . map (uncurry (+))
+
+parse :: String -> (Int, Int)
+parse str = let Just idx = findIndex (== '/') str in (read $ take idx str, read $ drop (succ idx) str)
+
+main :: IO ()
+main = do
+    input <- liftM (map parse . lines) (readFile "24.in")
+    let mp = foldl (\m (a,b) -> Map.insertWith (++) b [a] $ Map.insertWith (++) a [b] m) Map.empty input
+        br = findbridges 0 mp
+
+    print $ maximum (map strength br)
+
+    let maxlen = maximum (map length br)
+    print $ maximum $ map strength $ filter ((== maxlen) . length) br
diff --git a/2017/24.in b/2017/24.in
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index 0000000..1fbfe25
--- /dev/null
+++ b/2017/24.in
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