Fixup comment (and indent)

synapse/storage/events.py

@@ -565,6 +565,7 @@ class EventsStore(EventsWorkerStore):
         # map from state_group to ((type, key) -> event_id) state map
         state_groups_map = {}
 
+        # Map from (prev state group, new state group) -> delta state dict
         state_group_deltas = {}
 
         for ev, ctx in events_context:
@@ -631,10 +632,11 @@ class EventsStore(EventsWorkerStore):
         if old_state_groups == new_state_groups:
             defer.returnValue((None, None))
 
-        if len(new_state_groups) == 1:
-            # If there is only one state group, then we know what the current
-            # state is.
-            if len(old_state_groups) == 1:
+        if len(new_state_groups) == 1 and len(old_state_groups) == 1:
+            # If we're going from one state group to another, lets check if
+            # we have a delta for that transition. If we do then we can just
+            # return that.
+
             new_state_group = next(iter(new_state_groups))
             old_state_group = next(iter(old_state_groups))
 
@@ -657,6 +659,8 @@ class EventsStore(EventsWorkerStore):
             state_groups_map.update(group_to_state)
 
         if len(new_state_groups) == 1:
+            # If there is only one state group, then we know what the current
+            # state is.
             defer.returnValue((state_groups_map[new_state_groups.pop()], None))
 
         # Ok, we need to defer to the state handler to resolve our state sets.