Let the MergingCounter be reset to 0.

Fixes #6

CHANGES.rst

@@ -13,6 +13,10 @@
   the ``increment`` method. See `issue 7
   <https://github.com/NextThought/nti.zodb/issues/7>`_.
 
+- The merging counter does the right thing when reset to zero by two
+  conflicting transactions. See `issue 6
+  <https://github.com/NextThought/nti.zodb/issues/6>`_.
+
 1.1.0 (2020-07-15)
 ==================
 

src/nti/zodb/minmax.py

@@ -152,6 +152,13 @@ class MergingCounter(AbstractNumericValue):
     A :mod:`zope.minmax` item that resolves conflicts by
     merging the numeric value of the difference in magnitude of changes.
     Intented to be used for monotonically increasing counters.
+
+    As a special case, if the counter is reset to zero by both transactions,
+    that becomes the new state.
+
+    .. versionchanged:: 1.2.0
+       Special case setting the counter to zero.
+
     """
 
     def increment(self, amount=1):
@@ -160,6 +167,9 @@ def increment(self, amount=1):
         return self
 
     def _p_resolveConflict(self, oldState, savedState, newState):  # pylint:disable=arguments-differ
+        if savedState == newState == 0:
+            return 0
+
         saveDiff = savedState - oldState
         newDiff = newState - oldState
         savedState = oldState + saveDiff + newDiff

src/nti/zodb/tests/test_minmax.py

@@ -97,10 +97,13 @@ def test_numeric_counter_interface(self):
                     validly_provides(interfaces.INumericCounter))
 
     def test_merge_resolve(self):
-        assert_that(MergingCounter()._p_resolveConflict(0, 0, 1), is_(1))
+        # (original state, currently committed, desired)
+        mc = self._makeOne()
+        assert_that(mc._p_resolveConflict(0, 0, 1), is_(1))
         # simultaneous increment adds
-        assert_that(MergingCounter()._p_resolveConflict(0, 1, 1), is_(2))
-
+        assert_that(mc._p_resolveConflict(0, 1, 1), is_(2))
+        # In the special case that both set it to zero, it becomes 0
+        assert_that(mc._p_resolveConflict(10, 0, 0), is_(0))
 
     def test_str(self):