1 files changed, 28 insertions, 0 deletions
diff --git a/src/Much/MappedSets.hs b/src/Much/MappedSets.hs
new file mode 100644
index 0000000..ec0ae73
--- /dev/null
+++ b/src/Much/MappedSets.hs
@@ -0,0 +1,28 @@
+module Much.MappedSets (invert, mk) where
+
+import           Control.Arrow
+import           Data.Map.Strict   (Map)
+import qualified Data.Map.Strict as Map
+import           Data.Maybe
+import           Data.Set   (Set)
+import qualified Data.Set as Set
+
+
+mk :: (Ord a, Ord b) => [(a, [b])] -> Map a (Set b)
+mk =
+    Map.fromList . map (second Set.fromList)
+
+
+invert :: (Ord a, Ord b) => Map a (Set b) -> Map b (Set a)
+invert =
+    Map.foldrWithKey invert1 Map.empty
+
+
+invert1 :: (Ord a, Ord b) => a -> Set b -> Map b (Set a) -> Map b (Set a)
+invert1 k v a =
+    Set.foldr (upsert k) a v
+
+
+upsert :: (Ord a, Ord b) => a -> b -> Map b (Set a) -> Map b (Set a)
+upsert k =
+    Map.alter (Just . Set.insert k . fromMaybe Set.empty)