BUG: sample StochasticFlight inputs once per simulation (#1090) (#1126)

* BUG: sample StochasticFlight inputs once per simulation (#1090)

* MNT: satisfy ruff format on MonteCarlo.__sim_producer (#1090)

---------

Co-authored-by: Gui-FernandesBR <guilherme_fernandes@usp.br>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: 3 people

rocketpy/simulation/monte_carlo.py

@@ -588,12 +588,15 @@ def __run_single_simulation(self):
         Flight
             The flight object of the simulation.
         """
+        rocket = self.rocket.create_object()
+        environment = self.environment.create_object()
+        flight_inputs = self.flight._sample_flight_inputs()
         return Flight(
-            rocket=self.rocket.create_object(),
-            environment=self.environment.create_object(),
-            rail_length=self.flight._randomize_rail_length(),
-            inclination=self.flight._randomize_inclination(),
-            heading=self.flight._randomize_heading(),
+            rocket=rocket,
+            environment=environment,
+            rail_length=flight_inputs["rail_length"],
+            inclination=flight_inputs["inclination"],
+            heading=flight_inputs["heading"],
             initial_solution=self.flight.initial_solution,
             terminate_on_apogee=self.flight.terminate_on_apogee,
             time_overshoot=self.flight.time_overshoot,
@@ -1522,7 +1525,7 @@ def export_ellipses_to_kml(  # pylint: disable=too-many-statements
             except KeyError as e:
                 raise KeyError("No impact data found. Skipping impact ellipses.") from e
 
-        (apogee_ellipses, impact_ellipses) = generate_monte_carlo_ellipses(
+        apogee_ellipses, impact_ellipses = generate_monte_carlo_ellipses(
             impact_x,
             impact_y,
             apogee_x,

rocketpy/stochastic/stochastic_flight.py

@@ -123,21 +123,28 @@ def _validate_initial_solution(self, initial_solution):
             else:
                 raise TypeError("`initial_solution` must be a tuple of numbers")
 
-    # TODO: these methods call dict_generator a lot of times unnecessarily
+    def _sample_flight_inputs(self):
+        """Sample rail_length, inclination, and heading in a single draw.
+
+        Returns
+        -------
+        dict
+            Mapping with keys ``rail_length``, ``inclination``, and ``heading``.
+            Also updates ``last_rnd_dict``.
+        """
+        return next(self.dict_generator())
+
     def _randomize_rail_length(self):
         """Randomizes the rail length of the flight."""
-        generated_dict = next(self.dict_generator())
-        return generated_dict["rail_length"]
+        return self._sample_flight_inputs()["rail_length"]
 
     def _randomize_inclination(self):
         """Randomizes the inclination of the flight."""
-        generated_dict = next(self.dict_generator())
-        return generated_dict["inclination"]
+        return self._sample_flight_inputs()["inclination"]
 
     def _randomize_heading(self):
         """Randomizes the heading of the flight."""
-        generated_dict = next(self.dict_generator())
-        return generated_dict["heading"]
+        return self._sample_flight_inputs()["heading"]
 
     def create_object(self):
         """Creates and returns a Flight object from the randomly generated input
@@ -148,12 +155,11 @@ def create_object(self):
         flight : Flight
             Flight object with the randomly generated input arguments.
         """
-        generated_dict = next(self.dict_generator())
-        # TODO: maybe we should use generated_dict["rail_length"] instead
+        generated_dict = self._sample_flight_inputs()
         return Flight(
             rocket=self.obj.rocket,
             environment=self.obj.env,
-            rail_length=self._randomize_rail_length(),
+            rail_length=generated_dict["rail_length"],
             inclination=generated_dict["inclination"],
             heading=generated_dict["heading"],
             initial_solution=self.initial_solution,

tests/unit/simulation/test_monte_carlo.py

@@ -614,9 +614,13 @@ def test_a_monte_carlo_flight_keeps_the_configuration_it_was_given(monkeypatch):
         initial_solution=None,
         terminate_on_apogee=True,
         time_overshoot=False,
-        _randomize_rail_length=lambda: 5.0,
-        _randomize_inclination=lambda: 84.0,
-        _randomize_heading=lambda: 133.0,
+        # One draw for all three, as #1090 requires: three separate calls
+        # meant the flight flew one sample and the exported row logged another.
+        _sample_flight_inputs=lambda: {
+            "rail_length": 5.0,
+            "inclination": 84.0,
+            "heading": 133.0,
+        },
     )
     analysis = object.__new__(MonteCarlo)
     analysis.flight = stochastic_flight
@@ -633,6 +637,12 @@ def test_a_monte_carlo_flight_keeps_the_configuration_it_was_given(monkeypatch):
     assert flight.equations_of_motion == "solid_propulsion"
     assert flight.simulation_mode == "native"
     assert flight.name == "named"
+    # and the single draw is what the flight is actually built from
+    assert (flight.rail_length, flight.inclination, flight.heading) == (
+        5.0,
+        84.0,
+        133.0,
+    )
 
 
 @pytest.mark.parametrize(

tests/unit/stochastic/test_stochastic_flight.py

@@ -71,3 +71,39 @@ def test_dict_generator_skips_initial_solution_list(flight_calisto_robust):
     generated = next(stochastic_flight.dict_generator())
     assert "initial_solution" not in generated
     assert stochastic_flight.initial_solution == initial_solution
+
+
+def test_create_object_matches_last_rnd_dict(flight_calisto_robust):
+    """Regression for #1090: create_object must use one dict_generator draw.
+
+    Spreads are set on all three flight inputs so a second draw would diverge
+    from ``last_rnd_dict``.
+    """
+    stochastic_flight = StochasticFlight(
+        flight=flight_calisto_robust,
+        rail_length=(5.2, 0.5),
+        inclination=(84.7, 1),
+        heading=(53, 2),
+    )
+    stochastic_flight._set_stochastic(4242)
+
+    flight = stochastic_flight.create_object()
+    sampled = stochastic_flight.last_rnd_dict
+
+    assert flight.rail_length == sampled["rail_length"]
+    assert flight.inclination == sampled["inclination"]
+    assert flight.heading == sampled["heading"]
+
+
+def test_monte_carlo_single_simulation_matches_flight_last_rnd_dict(
+    monte_carlo_calisto,
+):
+    """Regression for #1090: MonteCarlo must fly the same sample it logs."""
+    monte_carlo_calisto.flight._set_stochastic(4242)
+
+    flight = monte_carlo_calisto._MonteCarlo__run_single_simulation()
+    sampled = monte_carlo_calisto.flight.last_rnd_dict
+
+    assert flight.rail_length == sampled["rail_length"]
+    assert flight.inclination == sampled["inclination"]
+    assert flight.heading == sampled["heading"]