diff --git a/common/Conway.hs b/common/Conway.hs
new file mode 100644
index 0000000..ad62ef3
--- /dev/null
+++ b/common/Conway.hs
@@ -0,0 +1,28 @@
+module Conway
+    ( Index(..)
+    , update
+    )
+where
+
+import Data.List
+import qualified Data.Vector.Sized as V
+import Data.Map (Map)
+import qualified Data.Map as M
+import Data.Set (Set)
+import qualified Data.Set as S
+
+class Index i where
+    neighbours :: i -> [i]
+    addIndex :: i -> i -> i
+
+counts :: Ord a => [a] -> Map a Int
+counts = M.fromList . map (\xs -> (head xs, length xs)) . group . sort
+
+neighbourCounts :: (Index i, Ord i) => Set i -> Map i Int
+neighbourCounts = counts . concatMap neighbours . S.elems
+
+update :: (Index i, Ord i) => (Int -> Bool) -> (Int -> Bool) -> Set i -> Set i
+update create destroy old = M.keysSet $ M.filterWithKey shouldLive $ M.fromSet (flip (M.findWithDefault 0) nCounts) indices
+    where nCounts = neighbourCounts old
+          indices = S.union old $ M.keysSet nCounts
+          shouldLive ix n = if S.notMember ix old then create n else not $ destroy n
diff --git a/day17/day17.hs b/day17/day17.hs
index 59c35a1..3134d1a 100644
--- a/day17/day17.hs
+++ b/day17/day17.hs
@@ -3,35 +3,18 @@
 module Day17 where
 
 import AoC
+import Conway
 
 import Data.List
 import Data.Vector.Sized (Vector)
 import qualified Data.Vector.Sized as V
-import Data.Map (Map)
-import qualified Data.Map as M
 import Data.Set (Set)
 import qualified Data.Set as S
 
-class Index i where
-    neighbours :: i -> [i]
-    addIndex :: i -> i -> i
-
 instance (Num a, Eq a) => Index (Vector n a) where
     neighbours i = filter (/= i) . V.mapM (flip map [-1, 0, 1] . (+)) $ i
     addIndex = V.zipWith (+)
 
-counts :: Ord a => [a] -> Map a Int
-counts = M.fromList . map (\xs -> (head xs, length xs)) . group . sort
-
-neighbourCounts :: (Index i, Ord i) => Set i -> Map i Int
-neighbourCounts = counts . concatMap neighbours . S.elems
-
-update :: (Index i, Ord i) => (Int -> Bool) -> (Int -> Bool) -> Set i -> Set i
-update create destroy old = M.keysSet $ M.filterWithKey shouldLive $ M.fromSet (flip (M.findWithDefault 0) nCounts) indices
-    where nCounts = neighbourCounts old
-          indices = S.union old $ M.keysSet nCounts
-          shouldLive ix n = if S.notMember ix old then create n else not $ destroy n
-
 run6 :: (Index i, Ord i) => Set i -> Int
 run6 = S.size . (!! 6) . iterate (update (==3) (`notElem` [2, 3]))