Add CSWAP support to ClassicalStateSimulator closes #7841

Author: rosspeili

cirq-core/cirq/sim/classical_simulator.py

@@ -154,6 +154,10 @@ def _act_on_fallback_(self, action, qubits: Sequence[cirq.Qid], allow_decompose:
         elif gate == ops.SWAP:
             a, b = mapped_qubits
             self._state.basis[a], self._state.basis[b] = self._state.basis[b], self._state.basis[a]
+        elif gate == ops.CSWAP:
+            c, a, b = mapped_qubits
+            if self._state.basis[c]:
+                self._state.basis[a], self._state.basis[b] = self._state.basis[b], self._state.basis[a]
         elif gate == ops.TOFFOLI:
             c1, c2, q = mapped_qubits
             self._state.basis[q] ^= self._state.basis[c1] & self._state.basis[c2]

cirq-core/cirq/sim/classical_simulator_test.py

@@ -146,6 +146,23 @@ def test_CSWAP(initial_state) -> None:
     np.testing.assert_equal(results, final_state)
 
 
+def test_named_CSWAP() -> None:
+    # Specifically test named CSWAP gate, not just controlled(SWAP)
+    q0, q1, q2 = cirq.LineQubit.range(3)
+    circuit = cirq.Circuit()
+    # Control q0=1, so swap q1, q2
+    circuit.append(cirq.X(q0))
+    circuit.append(cirq.X(q1))
+    circuit.append(cirq.CSWAP(q0, q1, q2))  # q0=1 -> swap q1=1, q2=0 -> q1=0, q2=1
+    circuit.append(cirq.measure((q0, q1, q2), key='key'))
+    
+    sim = cirq.ClassicalStateSimulator()
+    result = sim.run(circuit, repetitions=1)
+    # Expected: 1, 0, 1
+    expected = np.array([[[1, 0, 1]]], dtype=np.uint8)
+    np.testing.assert_equal(result.records['key'], expected)
+
+
 def test_measurement_gate() -> None:
     q0, q1 = cirq.LineQubit.range(2)
     circuit = cirq.Circuit()