diff --git a/src/tests/testStatebag.py b/src/tests/testStatebag.py
--- a/src/tests/testStatebag.py
+++ b/src/tests/testStatebag.py
@@ -5,7 +5,7 @@ import config as cfg
 from util import BLACK as B,WHITE as W,EMPTY as _
 from go.engine import SpecGo,Engine
 import go.engine
-from statebag import BoardState,StateBag
+from statebag import BoardState,StateBag,updateDiff
 from .util import simpleLoadSgf,listStates
 
 
@@ -52,6 +52,21 @@ class TestBoardState(TestCase):
 		self.assertIs(s3.getPrev(), s2)
 		self.assertEqual(s3.getWeight(), 1)
 
+	def testUpdateDiff(self):
+		files=["O-Takao-20110106.sgf","Sakai-Iyama-20110110.sgf"]
+
+		for f in files:
+			moves=simpleLoadSgf(os.path.join(cfg.srcDir,"tests/data",f))
+			states=listStates(moves)
+			for (i,j,k) in [(1,2,3),(10,20,30),(90,100,110),(20,70,120)]:
+				s1=states[i]
+				s2=states[j]
+				s3=states[k]
+				diff1=s2-s1
+				diff2=s3-s2
+				with self.subTest(file=f,ijk=(i,j,k)):
+					self.assertEqual(s3-s1,updateDiff(diff1,diff2))
+
 
 class TestStateBag(TestCase):
 	def testReal(self):