Merge pull request #23 from rickowens/pr

feat: Add `diffSize`.

.gitignore

@@ -3,3 +3,4 @@
 /cabal.project.local
 /.tags.lock
 /tags
+*.full-imports

crdt-event-fold.cabal

@@ -1,6 +1,6 @@
 cabal-version:       3.0
 name:                crdt-event-fold
-version:             1.8.0.2
+version:             1.8.1.0
 synopsis:            Garbage collected event folding CRDT.
 description:         Garbage collected event folding CRDT. Consistently
                      apply arbitrary operations to replicated data.

src/Data/CRDT/EventFold.hs

@@ -139,6 +139,7 @@ module Data.CRDT.EventFold (
   fullMerge_,
   UpdateResult(..),
   events,
+  diffSize,
   diffMerge,
   diffMerge_,
   MergeError(..),
@@ -184,8 +185,8 @@ import GHC.Generics (Generic)
 import Prelude (Applicative(pure), Bool(False, True), Either(Left, Right),
   Enum(succ), Eq((/=), (==)), Foldable(foldr, maximum), Functor(fmap),
   Maybe(Just, Nothing), Monoid(mempty), Ord((<), (<=), compare, max),
-  Semigroup((<>)), ($), (.), (<$>), (||), Num, Show, const, fst, id,
-  not, otherwise, snd)
+  Semigroup((<>)), ($), (.), (<$>), (||), Int, Num, Show, const, fst,
+  id, not, otherwise, snd)
 import Type.Reflection (Typeable)
 import qualified Data.DoubleWord as DW
 import qualified Data.Map as Map
@@ -590,11 +591,11 @@ events peer ef =
 
 
 {- | A package containing events that can be merged into an event fold. -}
-data Diff o p e = Diff {
-     diffEvents :: Map (EventId p) (Maybe (Delta p e), Set p),
-     diffOrigin :: o,
-    diffInfimum :: EventId p,
-    diffUnjoins :: Set (EventId p)
+data Diff o p e = Diff
+  {  diffEvents :: Map (EventId p) (Maybe (Delta p e), Set p)
+  ,  diffOrigin :: o
+  , diffInfimum :: EventId p
+  , diffUnjoins :: Set (EventId p)
   }
   deriving stock (Generic)
 deriving stock instance (Eq o, Eq p, Eq e, Eq (Output e)) => Eq (Diff o p e)
@@ -610,6 +611,16 @@ instance (
     Binary (Diff o p e)
 
 
+{-|
+  Return the number of events contained in the diff. This information
+  might be useful for optimizing performance by, for instance, choosing to
+  use `diffMerge` instead of `fullMerge` when the diff is small or zero.
+-}
+diffSize :: Diff o p e -> Int
+diffSize Diff { diffEvents } =
+  Map.size diffEvents
+
+
 {- |
   Like 'fullMerge', but merge a remote 'Diff' instead of a full remote
   'EventFold'.