# -*- coding: utf-8 -*-
# This code is part of Qiskit.
#
# (C) Copyright IBM 2017, 2018.
#
# 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.
"""Cancel back-to-back `cx` gates in dag."""
from qiskit.transpiler.basepasses import TransformationPass
[docs]class CXCancellation(TransformationPass):
"""Cancel back-to-back `cx` gates in dag."""
[docs] def run(self, dag):
"""Run the CXCancellation pass on `dag`.
Args:
dag (DAGCircuit): the directed acyclic graph to run on.
Returns:
DAGCircuit: Transformed DAG.
"""
cx_runs = dag.collect_runs(["cx"])
for cx_run in cx_runs:
# Partition the cx_run into chunks with equal gate arguments
partition = []
chunk = []
for i in range(len(cx_run) - 1):
chunk.append(cx_run[i])
qargs0 = cx_run[i].qargs
qargs1 = cx_run[i + 1].qargs
if qargs0 != qargs1:
partition.append(chunk)
chunk = []
chunk.append(cx_run[-1])
partition.append(chunk)
# Simplify each chunk in the partition
for chunk in partition:
if len(chunk) % 2 == 0:
for n in chunk:
dag.remove_op_node(n)
else:
for n in chunk[1:]:
dag.remove_op_node(n)
return dag