Add flexible-layer mapper (FlexlayerSwap) pass

qiskit/transpiler/coupling.py

@@ -187,6 +187,17 @@ def shortest_undirected_path(self, physical_qubit1, physical_qubit2):
             raise CouplingError(
                 "Nodes %s and %s are not connected" % (str(physical_qubit1), str(physical_qubit2)))
 
+    def undirected_neighbors(self, physical_qubit):
+        """List up neighbors of the `physical_qubit` in the undirected coupling graph.
+
+        Args:
+            physical_qubit (int): A physical qubit whose neighors should be listed up
+
+        Returns:
+            iterator: The neighbors of the physical qubit
+        """
+        return self.graph.to_undirected().neighbors(physical_qubit)
+
     @property
     def is_symmetric(self):
         """

qiskit/transpiler/layout.py

@@ -44,6 +44,7 @@ def __repr__(self):
         for key, val in self._p2v.items():
             str_list.append("{k}: {v},".format(k=key, v=val))
         if str_list:
+            str_list.sort()
             str_list[-1] = str_list[-1][:-1]
         return "Layout({\n" + "\n".join(str_list) + "\n})"
 

qiskit/transpiler/passes/__init__.py

@@ -51,3 +51,4 @@
 from .mapping.lookahead_swap import LookaheadSwap
 from .remove_diagonal_gates_before_measure import RemoveDiagonalGatesBeforeMeasure
 from .mapping.stochastic_swap import StochasticSwap
+from .mapping.flexlayer_swap import FlexlayerSwap

qiskit/transpiler/passes/mapping/algorithm/__init__.py

@@ -0,0 +1,15 @@
+# -*- coding: utf-8 -*-
+
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2019.
+#
+# 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.
+
+"""Module containing algorithms used in transpiler pass."""

qiskit/transpiler/passes/mapping/algorithm/ancestors.py

@@ -0,0 +1,49 @@
+# -*- coding: utf-8 -*-
+
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2019.
+#
+# 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.
+
+"""
+Utility for speeding up a function to find ancestors in DAG.
+"""
+
+from functools import lru_cache
+
+import networkx as nx
+
+CACHE_SIZE = 2 ** 10
+
+
+class Ancestors:
+    """
+    Utility class for speeding up a function to find ancestors in DAG.
+    """
+
+    def __init__(self, G: nx.DiGraph):
+        self._graph = G.reverse()
+
+    @lru_cache(maxsize=CACHE_SIZE)
+    def ancestors(self, n: int) -> set:
+        """
+        Ancestor nodes of the node `n`
+        Args:
+            n: Index of the node in the `self._graph`
+        Returns:
+            Set of indices of the ancestor nodes.
+        """
+        ret = set()
+        for n_succ in self._graph.successors(n):
+            if n_succ in ret:
+                continue
+            ret.add(n_succ)
+            ret.update(self.ancestors(n_succ))
+        return ret