fix: preserve system IDs during inflight refill

nvalchemi/dynamics/base.py

@@ -2047,6 +2047,22 @@ def refill_check(self, batch: Batch, exit_status: int) -> Batch | None:
             device = result.device
             for key, default_fn in self._bookkeeping_keys.items():
                 new_tensor = default_fn(n_total, device)
+                # Preserve values already carried by appended replacements before
+                # restoring the prefix for systems that stayed active.
+                result_vals = getattr(result, key, None)
+                if result_vals is not None:
+                    result_vals = (
+                        result_vals.unsqueeze(-1)
+                        if result_vals.dim() == 1
+                        else result_vals
+                    )
+                    if result_vals.shape == new_tensor.shape:
+                        new_tensor.copy_(result_vals)
+                    else:
+                        raise RuntimeError(
+                            f"Bookkeeping key '{key}' has shape {result_vals.shape} "
+                            f"after refill, expected {new_tensor.shape}."
+                        )
                 remaining_vals = getattr(batch, key, None)
                 if remaining_vals is not None and n_remaining > 0:
                     src = remaining_vals[remaining_indices]

test/dynamics/test_inflight.py

@@ -635,6 +635,38 @@ def test_refill_writes_bookkeeping_to_storage(self) -> None:
 
         assert "status" in result
 
+    def test_refill_preserves_replacement_system_id(self) -> None:
+        """Replacement systems keep sampler-assigned system IDs after refill."""
+        dataset = MockDataset([(1, 0)] * 3)
+        sampler = SizeAwareSampler(dataset, max_atoms=2)
+        dynamics = BaseDynamics(model=self.model, sampler=sampler, device_type="cpu")
+
+        batch = sampler.build_initial_batch()
+        assert batch.system_id.view(-1).tolist() == [0, 1]
+
+        batch["status"] = torch.tensor([[1], [1]])
+        result = dynamics.refill_check(batch, exit_status=1)
+
+        assert result is not None
+        assert result.system_id.view(-1).tolist() == [2]
+        assert result.status.view(-1).tolist() == [0]
+
+    def test_refill_preserves_mixed_system_ids(self) -> None:
+        """Remaining and replacement systems both keep their system IDs."""
+        dataset = MockDataset([(1, 0)] * 3)
+        sampler = SizeAwareSampler(dataset, max_atoms=2)
+        dynamics = BaseDynamics(model=self.model, sampler=sampler, device_type="cpu")
+
+        batch = sampler.build_initial_batch()
+        assert batch.system_id.view(-1).tolist() == [0, 1]
+
+        batch["status"] = torch.tensor([[1], [0]])
+        result = dynamics.refill_check(batch, exit_status=1)
+
+        assert result is not None
+        assert result.system_id.view(-1).tolist() == [1, 2]
+        assert result.status.view(-1).tolist() == [0, 0]
+
     def test_refill_partial_replacement(self) -> None:
         """When sampler has fewer replacements than graduated, batch shrinks.