2 files changed, 76 insertions, 0 deletions
diff --git a/2021/12.hs b/2021/12.hs
new file mode 100644
index 0000000..03b2d77
--- /dev/null
+++ b/2021/12.hs
@@ -0,0 +1,52 @@
+module Main where
+
+import qualified Data.Array as A
+import Data.Bifunctor (bimap)
+import Data.Char
+import Data.List
+import qualified Data.Map.Strict as Map
+import qualified Data.Set as Set
+import Data.Tuple (swap)
+
+import Input
+import Util
+
+
+uniq :: Eq a => [a] -> [a]
+uniq (x : y : xs) | x == y = uniq (y : xs)
+                  | otherwise = x : uniq (y : xs)
+uniq l = l
+
+main :: IO ()
+main = do
+    inp <- getInput 12
+    let listpairs = map (toList . splitOn (== '-')) inp
+        pairs = map (\[a,b] -> (a, b)) listpairs
+        nodes = uniq (sort (concat listpairs))
+        nodeids = Map.fromList (zip nodes [0::Int ..])
+        idpairs = map (bimap (nodeids Map.!) (nodeids Map.!)) pairs
+        startid = nodeids Map.! "start"
+        endid = nodeids Map.! "end"
+        collect l = A.listArray (0, length l - 1) l
+        graph = collect
+            [collect [b | (a, b) <- idpairs ++ map swap idpairs
+                     , a == node]
+            | node <- [0 .. length nodes - 1]]
+        issmall = collect (map (all isLower) nodes)
+        npathsfrom :: Int -> Set.Set Int -> Bool -> [[Int]]
+        npathsfrom node seen permissive
+          | node == endid = [[endid]]
+          | otherwise =
+              let nexts = A.elems (graph A.! node)
+                  allowed n = not (issmall A.! n) || n `Set.notMember` seen
+                  doubling n = n /= startid && n /= endid && not (allowed n)
+                  nexts_allowed = filter allowed nexts
+                  nexts_doubling = filter doubling nexts
+                  seen' | issmall A.! node = Set.insert node seen
+                        | otherwise = seen
+              in concatMap (\n -> map (node :) (npathsfrom n seen' permissive)) nexts_allowed
+                 ++ (if permissive
+                         then concatMap (\n -> map (node :) (npathsfrom n seen' False)) nexts_doubling
+                         else [])
+    print (length $ npathsfrom startid mempty False)
+    print (length $ npathsfrom startid mempty True)
diff --git a/2021/12.in b/2021/12.in
new file mode 100644
index 0000000..e1a2275
--- /dev/null
+++ b/2021/12.in
@@ -0,0 +1,24 @@
+pf-pk
+ZQ-iz
+iz-NY
+ZQ-end
+pf-gx
+pk-ZQ
+ZQ-dc
+NY-start
+NY-pf
+NY-gx
+ag-ZQ
+pf-start
+start-gx
+BN-ag
+iz-pf
+ag-FD
+pk-NY
+gx-pk
+end-BN
+ag-pf
+iz-pk
+pk-ag
+iz-end
+iz-BN