DpEventBuilder tracks the order of events, instead of just maintaining a multiset.

Existing approaches to accounting are generally agnostic to the order of composition, even when the composition is adaptive. But in principle it is possible for an accountant to require such information, so we had better not throw it away.

Note that `ComposedDpEvent` is now treated like any other `DpEvent`, not taken apart and the components added separately as it was. The reason for this is that a common pattern may be to compose a series of `ComposedDpEvent`s that have identical substructure. We want the `DpEventBuilder` to represent this as a single `SelfComposedDpEvent`, not a linearly-growing `ComposedDpEvent`.

PiperOrigin-RevId: 398359519

tensorflow_privacy/privacy/analysis/dp_event_builder.py

@@ -13,8 +13,6 @@
 # limitations under the License.
 """Builder class for ComposedDpEvent."""
 
-import collections
-
 from tensorflow_privacy.privacy.analysis import dp_event
 
 
@@ -28,7 +26,8 @@ class DpEventBuilder(object):
   """
 
   def __init__(self):
-    self._events = collections.OrderedDict()
+    # A list of (event, count) pairs.
+    self._event_counts = []
     self._composed_event = None
 
   def compose(self, event: dp_event.DpEvent, count: int = 1):
@@ -46,33 +45,32 @@ class DpEventBuilder(object):
     if count < 1:
       raise ValueError(f'`count` must be positive. Found {count}.')
 
-    if isinstance(event, dp_event.ComposedDpEvent):
-      for composed_event in event.events:
-        self.compose(composed_event, count)
+    if isinstance(event, dp_event.NoOpDpEvent):
+      return
     elif isinstance(event, dp_event.SelfComposedDpEvent):
       self.compose(event.event, count * event.count)
-    elif isinstance(event, dp_event.NoOpDpEvent):
-      return
     else:
-      current_count = self._events.get(event, 0)
-      self._events[event] = current_count + count
+      if self._event_counts and self._event_counts[-1][0] == event:
+        new_event_count = (event, self._event_counts[-1][1] + count)
+        self._event_counts[-1] = new_event_count
+      else:
+        self._event_counts.append((event, count))
       self._composed_event = None
 
   def build(self) -> dp_event.DpEvent:
     """Builds and returns the composed DpEvent represented by the builder."""
     if not self._composed_event:
-      self_composed_events = []
-      for event, count in self._events.items():
+      events = []
+      for event, count in self._event_counts:
         if count == 1:
-          self_composed_events.append(event)
+          events.append(event)
         else:
-          self_composed_events.append(
-              dp_event.SelfComposedDpEvent(event, count))
-      if not self_composed_events:
+          events.append(dp_event.SelfComposedDpEvent(event, count))
+      if not events:
         self._composed_event = dp_event.NoOpDpEvent()
-      elif len(self_composed_events) == 1:
-        self._composed_event = self_composed_events[0]
+      elif len(events) == 1:
+        self._composed_event = events[0]
       else:
-        self._composed_event = dp_event.ComposedDpEvent(self_composed_events)
+        self._composed_event = dp_event.ComposedDpEvent(events)
 
     return self._composed_event

tensorflow_privacy/privacy/analysis/dp_event_builder_test.py

@@ -20,8 +20,6 @@ from tensorflow_privacy.privacy.analysis import dp_event_builder
 _gaussian_event = dp_event.GaussianDpEvent(1.0)
 _poisson_event = dp_event.PoissonSampledDpEvent(_gaussian_event, 0.1)
 _self_composed_event = dp_event.SelfComposedDpEvent(_gaussian_event, 3)
-_composed_event = dp_event.ComposedDpEvent(
-    [_self_composed_event, _poisson_event])
 
 
 class DpEventBuilderTest(absltest.TestCase):
@@ -50,22 +48,27 @@ class DpEventBuilderTest(absltest.TestCase):
 
   def test_compose_heterogenous(self):
     builder = dp_event_builder.DpEventBuilder()
+    builder.compose(_poisson_event)
     builder.compose(_gaussian_event)
-    builder.compose(_poisson_event)
     builder.compose(_gaussian_event, 2)
-    self.assertEqual(_composed_event, builder.build())
+    builder.compose(_poisson_event)
+    expected_event = dp_event.ComposedDpEvent(
+        [_poisson_event, _self_composed_event, _poisson_event])
+    self.assertEqual(expected_event, builder.build())
 
-  def test_compose_complex(self):
+  def test_compose_composed(self):
     builder = dp_event_builder.DpEventBuilder()
-    builder.compose(_gaussian_event, 2)
-    builder.compose(_composed_event)
+    composed_event = dp_event.ComposedDpEvent(
+        [_gaussian_event, _poisson_event, _self_composed_event])
+    builder.compose(_gaussian_event)
+    builder.compose(composed_event)
+    builder.compose(composed_event, 2)
+    builder.compose(_poisson_event)
     builder.compose(_poisson_event)
-    builder.compose(_composed_event, 2)
-
     expected_event = dp_event.ComposedDpEvent([
-        dp_event.SelfComposedDpEvent(_gaussian_event, 11),
-        dp_event.SelfComposedDpEvent(_poisson_event, 4)
-    ])
+        _gaussian_event,
+        dp_event.SelfComposedDpEvent(composed_event, 3),
+        dp_event.SelfComposedDpEvent(_poisson_event, 2)])
     self.assertEqual(expected_event, builder.build())