fix(#45): structural batching uses device qubit count for non-overlapping placement

src/qc_compiler/batching.py

@@ -218,7 +218,9 @@ def _structural_batch(
         """Batch circuits with same depth on non-overlapping qubit subsets.
 
         Circuits using different qubits of the same device can run
-        in parallel, maximizing qubit utilization.
+        in parallel, maximizing qubit utilization. Uses the device's
+        total qubit count (max_qubits) to determine how many circuits
+        can be placed without overlapping.
 
         Args:
             circuits: List of quantum circuits.
@@ -240,22 +242,22 @@ def _structural_batch(
         batch_sizes = []
 
         for depth, indices in depth_groups.items():
-            current_batch_qubits = set()
             current_batch = []
+            total_qubits_used = 0
 
             for idx in indices:
                 circuit = circuits[idx]
-                circuit_qubits = set(range(circuit.num_qubits))
+                n_qubits = circuit.num_qubits
 
-                if not current_batch_qubits & circuit_qubits:
+                if total_qubits_used + n_qubits <= self.max_qubits:
                     current_batch.append(circuit)
-                    current_batch_qubits |= circuit_qubits
+                    total_qubits_used += n_qubits
                 else:
                     if current_batch:
                         batches.append(current_batch)
                         batch_sizes.append(len(current_batch))
                     current_batch = [circuit]
-                    current_batch_qubits = circuit_qubits.copy()
+                    total_qubits_used = n_qubits
 
             if current_batch:
                 batches.append(current_batch)

tests/test_batching.py

@@ -318,4 +318,38 @@ def test_estimate_speedup_empty_groups(self, batcher):
 
     def test_estimate_structural_speedup_empty(self, batcher):
         speedup = batcher._estimate_structural_speedup([], [])
-        assert speedup == 1.0
+        assert speedup == 1.0
+
+
+class TestStructuralBatchingDeviceQubits:
+    """Regression tests for structural batching with device qubit count (issue #45)."""
+
+    def test_same_size_circuits_can_be_batched_on_large_device(self):
+        batcher = CircuitBatcher(cost_model=CostModel(), max_qubits=127)
+        circuits = [QuantumCircuit(3) for _ in range(4)]
+        for qc in circuits:
+            qc.h(0)
+            qc.cx(0, 1)
+            qc.cx(1, 2)
+        plan = batcher.create_batch_plan(circuits, strategy="structural")
+        assert plan.num_batches >= 1
+        assert plan.total_circuits == 4
+
+    def test_small_circuits_batch_together_on_device(self):
+        batcher = CircuitBatcher(cost_model=CostModel(), max_qubits=10)
+        circuits = [QuantumCircuit(3) for _ in range(3)]
+        for qc in circuits:
+            qc.h(0)
+            qc.cx(0, 1)
+        plan = batcher.create_batch_plan(circuits, strategy="structural")
+        assert plan.total_circuits == 3
+
+    def test_circuits_exceeding_device_capacity_split_into_batches(self):
+        batcher = CircuitBatcher(cost_model=CostModel(), max_qubits=5)
+        circuits = [QuantumCircuit(3) for _ in range(4)]
+        for qc in circuits:
+            qc.h(0)
+            qc.cx(0, 1)
+        plan = batcher.create_batch_plan(circuits, strategy="structural")
+        assert plan.total_circuits == 4
+        assert plan.num_batches >= 2