def _transform_registers(
circuit: qiskit.QuantumCircuit,
new_qregs: Optional[List[qiskit.QuantumRegister]] = None,
) -> None:
"""Transforms the registers in the circuit to the new registers.
Args:
circuit: Qiskit circuit with at most one quantum register.
new_qregs: The new quantum registers for the circuit.
Raises:
ValueError:
* If the input circuit has more than one quantum register.
* If the number of qubits in the new quantum registers is
greater than the number of qubits in the circuit.
"""
if new_qregs is None:
return
if len(circuit.qregs) > 1:
raise ValueError(
"Input circuit is required to have <= 1 quantum register but has "
f"{len(circuit.qregs)} quantum registers."
)
qreg_sizes = [qreg.size for qreg in new_qregs]
nqubits_in_circuit = circuit.num_qubits
if len(qreg_sizes) and sum(qreg_sizes) < nqubits_in_circuit:
raise ValueError(
f"The circuit has {nqubits_in_circuit} qubit(s), but the provided "
f"quantum registers have {sum(qreg_sizes)} qubit(s)."
)
# Copy the circuit data.
data = copy.deepcopy(circuit._data)
# Remove the old qubits and add the new ones.
circuit._qubits = []
circuit._qubit_set = set()
circuit.qregs = []
circuit._data = []
circuit._qubit_indices = {}
circuit.add_register(*new_qregs)
# Map the qubits in operations to the new qubits.
new_ops = []
for op in data:
gate, qubits, cbits = op
new_qubits = _map_qubits(qubits, qreg_sizes, new_qregs)
new_ops.append((gate, new_qubits, cbits))
circuit._data = new_ops
