Obtain a reduced coupling_map for a subgraph of a device (#2225)

* reduced_coupling_map

* style

* new headers

* make reduce a CouplingMap method

* get rid of 'backend'

* fix test for python version dependent ordering

* fix idx

qiskit/transpiler/coupling.py

@@ -23,8 +23,9 @@ CNOT gates. The object has a distance function that can be used to map quantum c
 onto a device with this coupling.
 """
 import numpy as np
+import scipy.sparse as sp
+import scipy.sparse.csgraph as cs
 import networkx as nx
-
 from qiskit.transpiler.exceptions import CouplingError
 
 
@@ -185,6 +186,45 @@ class CouplingMap:
             raise CouplingError(
                 "Nodes %s and %s are not connected" % (str(physical_qubit1), str(physical_qubit2)))
 
+    def reduce(self, mapping):
+        """Returns a reduced coupling map that
+        corresponds to the subgraph of qubits
+        selected in the mapping.
+
+        Args:
+            mapping (list): A mapping of reduced qubits to device
+                            qubits.
+
+        Returns:
+            CouplingMap: A reduced coupling_map for the selected qubits.
+
+        Raises:
+            CouplingError: Reduced coupling map must be connected.
+        """
+        reduced_qubits = len(mapping)
+        inv_map = [None]*(max(mapping)+1)
+        for idx, val in enumerate(mapping):
+            inv_map[val] = idx
+
+        reduced_cmap = []
+
+        for edge in self.get_edges():
+            if edge[0] in mapping and edge[1] in mapping:
+                reduced_cmap.append([inv_map[edge[0]], inv_map[edge[1]]])
+
+        # Verify coupling_map is connected
+        rows = np.array([edge[0] for edge in reduced_cmap], dtype=int)
+        cols = np.array([edge[1] for edge in reduced_cmap], dtype=int)
+        data = np.ones_like(rows)
+
+        mat = sp.coo_matrix((data, (rows, cols)),
+                            shape=(reduced_qubits, reduced_qubits)).tocsr()
+
+        if cs.connected_components(mat)[0] != 1:
+            raise CouplingError('coupling_map must be connected.')
+
+        return CouplingMap(reduced_cmap)
+
     def __str__(self):
         """Return a string representation of the coupling graph."""
         string = ""

test/python/transpiler/test_coupling.py

@@ -16,6 +16,7 @@
 
 from qiskit.transpiler import CouplingMap
 from qiskit.transpiler.exceptions import CouplingError
+from qiskit.test.mock import FakeRueschlikon
 from qiskit.test import QiskitTestCase
 
 
@@ -78,3 +79,22 @@ class CouplingTest(QiskitTestCase):
         self.assertEqual(coupling.physical_qubits, qubits_expected)
         self.assertEqual(coupling.get_edges(), edges_expected)
         self.assertEqual(2, coupling.distance(0, 2))
+
+    def test_successful_reduced_map(self):
+        """Generate a reduced map
+        """
+        fake = FakeRueschlikon()
+        cmap = fake.configuration().coupling_map
+        coupling_map = CouplingMap(cmap)
+        out = coupling_map.reduce([12, 11, 10, 9]).get_edges()
+        ans = [(1, 2), (3, 2), (0, 1)]
+        self.assertEqual(set(out), set(ans))
+
+    def test_failed_reduced_map(self):
+        """Generate a bad disconnected reduced map
+        """
+        fake = FakeRueschlikon()
+        cmap = fake.configuration().coupling_map
+        coupling_map = CouplingMap(cmap)
+        with self.assertRaises(CouplingError):
+            coupling_map.reduce([12, 11, 10, 3])