[codex] fix initial measurement sigma

dpsynth/discrete_mechanisms/aim.py

@@ -190,13 +190,14 @@ def __call__(
     rho_per_round = zcdp_rho / max_rounds
 
     if initial_measurements is None:
-      rho_remaining -= self.one_way_budget_fraction * zcdp_rho
+      one_way_zcdp_rho = self.one_way_budget_fraction * zcdp_rho
+      rho_remaining -= one_way_zcdp_rho
       marginal_queries = [cl for cl in candidates.keys() if len(cl) == 1]
       measurements = common.measure_marginals_with_noise(
           rng,
           data,
           marginal_queries=marginal_queries,
-          gdp_sigma=zcdp_rho * self.one_way_budget_fraction,
+          gdp_sigma=accounting.zcdp_gaussian_sigma(one_way_zcdp_rho),
       )
     else:
       measurements = list(initial_measurements)

dpsynth/discrete_mechanisms/mst.py

@@ -225,12 +225,13 @@ def __call__(
     budget_remaining = self.zcdp_rho
 
     if initial_measurements is None:
-      budget_remaining -= self.one_way_budget_fraction * self.zcdp_rho
+      one_way_zcdp_rho = self.one_way_budget_fraction * self.zcdp_rho
+      budget_remaining -= one_way_zcdp_rho
       one_way_measurements = common.measure_marginals_with_noise(
           rng,
           data,
           marginal_queries=[(a,) for a in data.domain],
-          gdp_sigma=self.zcdp_rho * self.one_way_budget_fraction,
+          gdp_sigma=accounting.zcdp_gaussian_sigma(one_way_zcdp_rho),
       )
     else:
       one_way_measurements = initial_measurements

tests/discrete_mechanisms/aim_test.py

@@ -12,7 +12,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from unittest import mock
+
 from absl.testing import absltest
+from dpsynth.discrete_mechanisms import accounting
 from dpsynth.discrete_mechanisms import aim
 from dpsynth.discrete_mechanisms import aim_gdp
 import mbi
@@ -65,6 +68,23 @@ def test_uncalibrated_aim_gdp_raises(self):
     with self.assertRaisesRegex(ValueError, "calibrate"):
       config(np.random.default_rng(0), data)
 
+  def test_aim_initial_measurement_uses_gaussian_sigma(self):
+    data = mbi.Dataset.synthetic(mbi.Domain(["a"], [3]), N=10)
+    config = aim.AIMMechanism(
+        workload=[("a",)], max_rounds=1, one_way_budget_fraction=0.25
+    ).calibrate(zcdp_rho=2.0)
+    expected_sigma = accounting.zcdp_gaussian_sigma(0.5)
+
+    def assert_sigma(*_, gdp_sigma, **__):
+      self.assertAlmostEqual(gdp_sigma, expected_sigma)
+      raise RuntimeError("stop after checking sigma")
+
+    with mock.patch.object(
+        aim.common, "measure_marginals_with_noise", side_effect=assert_sigma
+    ):
+      with self.assertRaisesRegex(RuntimeError, "checking sigma"):
+        config(np.random.default_rng(0), data)
+
 
 if __name__ == "__main__":
   absltest.main()

tests/discrete_mechanisms/mst_test.py

@@ -12,8 +12,11 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from unittest import mock
+
 from absl.testing import absltest
 import dp_accounting
+from dpsynth.discrete_mechanisms import accounting
 from dpsynth.discrete_mechanisms import mst
 import mbi
 import numpy as np
@@ -97,6 +100,23 @@ def test_dp_event_returns_zcdp(self):
     event = config.dp_event
     self.assertIsInstance(event, dp_accounting.ZCDpEvent)
 
+  def test_initial_measurement_uses_gaussian_sigma(self):
+    data = mbi.Dataset.synthetic(mbi.Domain(['a'], [3]), N=10)
+    config = mst.MSTMechanism(one_way_budget_fraction=0.25).calibrate(
+        zcdp_rho=2.0
+    )
+    expected_sigma = accounting.zcdp_gaussian_sigma(0.5)
+
+    def assert_sigma(*_, gdp_sigma, **__):
+      self.assertAlmostEqual(gdp_sigma, expected_sigma)
+      raise RuntimeError('stop after checking sigma')
+
+    with mock.patch.object(
+        mst.common, 'measure_marginals_with_noise', side_effect=assert_sigma
+    ):
+      with self.assertRaisesRegex(RuntimeError, 'checking sigma'):
+        config(np.random.default_rng(0), data)
+
 
 if __name__ == '__main__':
   absltest.main()