fix(#51): use deterministic hashlib.sha256 instead of Python hash()

src/qc_compiler/batching.py

@@ -20,6 +20,7 @@
        same device, maximizing qubit utilization.
 """
 
+import hashlib
 from dataclasses import dataclass, field
 
 from qiskit import QuantumCircuit
@@ -302,7 +303,7 @@ def _compute_core_hash(self, circuit: QuantumCircuit) -> int:
             )
             core_gates.append((gate_name, qubits, params))
 
-        return hash(tuple(core_gates))
+        return int(hashlib.sha256(str(tuple(core_gates)).encode()).hexdigest(), 16)
 
     def _detect_measurement_basis(
         self, circuit: QuantumCircuit

tests/test_batching.py

@@ -352,4 +352,28 @@ def test_circuits_exceeding_device_capacity_split_into_batches(self):
             qc.cx(0, 1)
         plan = batcher.create_batch_plan(circuits, strategy="structural")
         assert plan.total_circuits == 4
-        assert plan.num_batches >= 2
+        assert plan.num_batches >= 2
+
+
+class TestDeterministicCoreHash:
+    """Regression test for deterministic hashing (issue #51)."""
+
+    def test_core_hash_is_deterministic_across_calls(self):
+        batcher = CircuitBatcher(cost_model=CostModel())
+        qc = QuantumCircuit(2)
+        qc.h(0)
+        qc.cx(0, 1)
+        hash1 = batcher._compute_core_hash(qc)
+        hash2 = batcher._compute_core_hash(qc)
+        assert hash1 == hash2
+
+    def test_core_hash_differs_for_different_circuits(self):
+        batcher = CircuitBatcher(cost_model=CostModel())
+        qc1 = QuantumCircuit(2)
+        qc1.h(0)
+        qc1.cx(0, 1)
+        qc2 = QuantumCircuit(2)
+        qc2.x(0)
+        hash1 = batcher._compute_core_hash(qc1)
+        hash2 = batcher._compute_core_hash(qc2)
+        assert hash1 != hash2