1 files changed, 49 insertions, 0 deletions
diff --git a/hs/GCStore.hs b/hs/GCStore.hs
new file mode 100644
index 0000000..d1e550f
--- /dev/null
+++ b/hs/GCStore.hs
@@ -0,0 +1,49 @@
+-- Intended to be imported qualified.
+
+module GCStore(Store, Id, empty, store, retrieve, update, refcount, ref, deref) where
+
+import qualified Data.Map.Strict as Map
+import qualified Data.Set as Set
+
+-- import Debug.Trace
+
+
+type Id = Int
+
+data Store a = Store (Map.Map Id (a, Int)) (Set.Set Id) (Set.Set Id)
+  deriving (Show)
+
+empty :: Store a
+empty = Store Map.empty Set.empty Set.empty
+
+store :: Show a => Store a -> a -> (Id, Store a)
+store (Store vm fs us) a =
+    let (i, fs') = if Set.null fs
+                       then (maybe 1 succ (Set.lookupMax us), fs)
+                       else Set.deleteFindMin fs
+    in (i, Store (Map.insert i (a, 1) vm) fs' (Set.insert i us))
+
+retrieve :: Store a -> Id -> Maybe a
+retrieve (Store vm _ _) i = fst <$> Map.lookup i vm
+
+update :: Show a => Store a -> Id -> a -> Store a
+update (Store vm fs us) i a =
+    maybe (error $ "GCStore.update on nonexistent id " ++ show i)
+          (\(_, rc) -> Store (Map.insert i (a, rc) vm) fs us)
+          (Map.lookup i vm)
+
+refcount :: Store a -> Id -> Int
+refcount (Store vm _ _) i = maybe 0 snd $ Map.lookup i vm
+
+ref :: Store a -> Id -> Store a
+ref (Store vm fs us) i = Store (Map.alter updf i vm) fs us
+  where updf Nothing = error $ "GCStore.ref on nonexistent id " ++ show i
+        updf (Just (a, rc)) = Just (a, rc + 1)
+
+deref :: Store a -> Id -> Store a
+deref (Store vm fs us) i =
+    maybe (error $ "GCStore.deref on nonexistent id " ++ show i)
+          (\(a, rc) -> if rc == 1
+              then Store (Map.delete i vm) (Set.insert i fs) (Set.delete i us)
+              else Store (Map.insert i (a, rc - 1) vm) fs us)
+          (Map.lookup i vm)