# Source code for qiskit.circuit.library.generalized_gates.permutation

```
# This code is part of Qiskit.
#
# (C) Copyright IBM 2017, 2020.
#
# This code is licensed under the Apache License, Version 2.0. You may
# obtain a copy of this license in the LICENSE.txt file in the root directory
# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
#
# Any modifications or derivative works of this code must retain this
# copyright notice, and modified files need to carry a notice indicating
# that they have been altered from the originals.
"""Permutation circuit."""
from typing import List, Optional
import numpy as np
from qiskit.circuit.quantumcircuit import QuantumCircuit
from qiskit.circuit.exceptions import CircuitError
[docs]class Permutation(QuantumCircuit):
"""An n_qubit circuit that permutes qubits."""
def __init__(
self,
num_qubits: int,
pattern: Optional[List[int]] = None,
seed: Optional[int] = None,
) -> None:
"""Return an n_qubit permutation circuit implemented using SWAPs.
Args:
num_qubits: circuit width.
pattern: permutation pattern. If None, permute randomly.
seed: random seed in case a random permutation is requested.
Raises:
CircuitError: if permutation pattern is malformed.
Reference Circuit:
.. jupyter-execute::
:hide-code:
from qiskit.circuit.library import Permutation
import qiskit.tools.jupyter
A = [2,4,3,0,1]
circuit = Permutation(5, A)
circuit.draw('mpl')
Expanded Circuit:
.. jupyter-execute::
:hide-code:
from qiskit.circuit.library import Permutation
import qiskit.tools.jupyter
A = [2,4,3,0,1]
circuit = Permutation(5, A)
%circuit_library_info circuit.decompose()
"""
if pattern is not None:
if sorted(pattern) != list(range(num_qubits)):
raise CircuitError(
"Permutation pattern must be some " "ordering of 0..num_qubits-1 in a list."
)
pattern = np.array(pattern)
else:
rng = np.random.default_rng(seed)
pattern = np.arange(num_qubits)
rng.shuffle(pattern)
name = "permutation_" + np.array_str(pattern).replace(" ", ",")
circuit = QuantumCircuit(num_qubits, name=name)
super().__init__(num_qubits, name=name)
for i, j in _get_ordered_swap(pattern):
circuit.swap(i, j)
all_qubits = self.qubits
self.append(circuit.to_gate(), all_qubits)
def _get_ordered_swap(permutation_in):
"""This attempts to sort the input permutation by iterating through the
permutation list and swapping the element with where the actual index occurs and
and tracking the swaps.
"""
permutation = list(permutation_in[:])
swap_list = []
for i, val in enumerate(permutation):
if val != i:
j = permutation.index(i)
swap_list.append((i, j))
permutation[i], permutation[j] = permutation[j], permutation[i]
swap_list.reverse()
return swap_list
```