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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def simulate(sim_type: str, num_qubits: int, num_gates: int, num_prefix_qubits: int = 0, use_processes: bool = False) -> None: """"Runs the simulator."""

    circuit = cirq.Circuit(device=test_device)

    for _ in range(num_gates):
        which = np.random.choice(['expz', 'expw', 'exp11'])
        if which == 'expw':
            q1 = cirq.GridQubit(0, np.random.randint(num_qubits))
            circuit.append(
                cirq.PhasedXPowGate(
                    phase_exponent=np.random.random(),
                    exponent=np.random.random()
                ).on(q1))
        elif which == 'expz':
            q1 = cirq.GridQubit(0, np.random.randint(num_qubits))
            circuit.append(
                cirq.Z(q1)**np.random.random())
        elif which == 'exp11':
            q1 = cirq.GridQubit(0, np.random.randint(num_qubits - 1))
            q2 = cirq.GridQubit(0, q1.col + 1)
            circuit.append(cirq.CZ(q1, q2)**np.random.random())

    if sim_type == _XMON:
        options = cg.XmonOptions(num_shards=2 ** num_prefix_qubits,
                                 use_processes=use_processes)
        cg.XmonSimulator(options).run(circuit)
    elif sim_type == _UNITARY:
        circuit.apply_unitary_effect_to_state(initial_state=0)
    elif sim_type == _DENSITY:
        cirq.DensityMatrixSimulator().run(circuit)