Fix StarConnectivtyPlacer/Router._check_star_connectivity for len(nodes)>5

src/qibo/transpiler/placer.py

@@ -6,7 +6,7 @@
 from qibo.backends import _check_backend_and_local_state
 from qibo.config import raise_error
 from qibo.models import Circuit
-from qibo.transpiler._exceptions import PlacementError
+from qibo.transpiler._exceptions import ConnectivityError, PlacementError
 from qibo.transpiler.abstract import Placer, Router
 from qibo.transpiler.asserts import assert_placement
 from qibo.transpiler.router import _find_connected_qubit
@@ -93,13 +93,18 @@ def __call__(self, circuit: Circuit):
 
     def _check_star_connectivity(self):
         """Check if the connectivity graph is a star graph."""
+        if len(self.connectivity.nodes) != 5:
+            raise_error(
+                ConnectivityError,
+                f"This connectivity graph is not a star graph. Length of nodes provided: {len(self.connectivity.nodes)} != 5.",
+            )
         for node in self.connectivity.nodes:
             if self.connectivity.degree(node) == 4:
                 self.middle_qubit = node
             elif self.connectivity.degree(node) != 1:
                 raise_error(
-                    ValueError,
-                    "This connectivity graph is not a star graph.",
+                    ConnectivityError,
+                    "This connectivity graph is not a star graph. There is a node with degree different from 1 or 4.",
                 )
 
 

src/qibo/transpiler/router.py

@@ -91,13 +91,18 @@ def __call__(self, circuit: Circuit):
 
     def _check_star_connectivity(self):
         """Check if the connectivity graph is a star graph."""
+        if len(self.connectivity.nodes) != 5:
+            raise_error(
+                ConnectivityError,
+                f"This connectivity graph is not a star graph. Length of nodes provided: {len(self.connectivity.nodes)} != 5.",
+            )
         for node in self.connectivity.nodes:
             if self.connectivity.degree(node) == 4:
                 self.middle_qubit = node
             elif self.connectivity.degree(node) != 1:
                 raise_error(
-                    ValueError,
-                    "This connectivity graph is not a star graph.",
+                    ConnectivityError,
+                    "This connectivity graph is not a star graph. There is a node with degree different from 1 or 4.",
                 )
 
 

tests/test_transpiler_placer.py

@@ -3,7 +3,7 @@
 
 from qibo import gates
 from qibo.models import Circuit
-from qibo.transpiler._exceptions import PlacementError, TranspilerPipelineError
+from qibo.transpiler._exceptions import ConnectivityError, PlacementError
 from qibo.transpiler.asserts import assert_placement
 from qibo.transpiler.pipeline import restrict_connectivity_qubits
 from qibo.transpiler.placer import (
@@ -197,3 +197,23 @@ def test_star_connectivity_placer_error(first, star_connectivity):
     with pytest.raises(ValueError):
         placer = StarConnectivityPlacer(chip)
         placer(circ)
+
+
+def test_star_connectivity_plus_disconnected_edges(star_connectivity):
+    connectivity = star_connectivity()
+    connectivity.add_edge(5, 6)
+    placer = StarConnectivityPlacer(connectivity=connectivity)
+    error_msg = (
+        "This connectivity graph is not a star graph. Length of nodes provided: 7 != 5."
+    )
+    with pytest.raises(ConnectivityError, match=error_msg):
+        placer(Circuit(5))
+
+
+def test_incorrect_star_connectivity(star_connectivity):
+    connectivity = star_connectivity()
+    connectivity.add_edge(3, 4)
+    placer = StarConnectivityPlacer(connectivity=connectivity)
+    error_msg = "This connectivity graph is not a star graph. There is a node with degree different from 1 or 4."
+    with pytest.raises(ConnectivityError, match=error_msg):
+        placer(Circuit(5))

tests/test_transpiler_router.py

@@ -609,3 +609,23 @@ def test_logical_to_physical_setter():
     circuit_map.logical_to_physical = [2, 0, 1, 3]
     assert circuit_map.logical_to_physical == [2, 0, 1, 3]
     assert circuit_map.physical_to_logical == [1, 2, 0, 3]
+
+
+def test_star_connectivity_plus_disconnected_edges(star_connectivity):
+    connectivity = star_connectivity()
+    connectivity.add_edge(5, 6)
+    placer = StarConnectivityPlacer(connectivity=connectivity)
+    error_msg = (
+        "This connectivity graph is not a star graph. Length of nodes provided: 7 != 5."
+    )
+    with pytest.raises(ConnectivityError, match=error_msg):
+        placer(Circuit(5))
+
+
+def test_incorrect_star_connectivity(star_connectivity):
+    connectivity = star_connectivity()
+    connectivity.add_edge(3, 4)
+    placer = StarConnectivityPlacer(connectivity=connectivity)
+    error_msg = "This connectivity graph is not a star graph. There is a node with degree different from 1 or 4."
+    with pytest.raises(ConnectivityError, match=error_msg):
+        placer(Circuit(5))