Bugfix vanishing instances after elimination

openqaoa/rqaoa/rqaoa.py

@@ -388,7 +388,7 @@ def redefine_problem(problem: QUBO, spin_map: dict):
     spin_map: `dict`
         Updated spin_map with sponatenous eliminations from cancellations during spin removal process.
     """
-
+    
     # Define new QUBO problem as a dictionary
     new_problem_dict = {}
 
@@ -498,9 +498,15 @@ def redefine_problem(problem: QUBO, spin_map: dict):
                 # Delete isolated node from new problem
                 new_problem_dict.pop((node,))
 
-    # Redefine new QUBO problem from the dictionary
-    new_problem = problem_from_dict(new_problem_dict)
-
+    # For some unweighted graphs specific eliminations can lead to eliminating the whole instance before reaching cutoff.
+    if new_problem_dict == {}:
+        new_problem = problem  # set the problem to the old problem and solve classically for the smallest non-vanishing instance.
+
+    else:
+        # Redefine new QUBO problem from the dictionary
+        new_problem = problem_from_dict(new_problem_dict)
+
+
     return new_problem, spin_map
 
 

openqaoa/workflows/optimizer.py

@@ -713,26 +713,29 @@ def optimize(self, verbose=False):
             spin_map = rqaoa.spin_mapping(problem, max_terms_and_stats)
             # Eliminate spins and redefine problem
             new_problem, spin_map = rqaoa.redefine_problem(problem, spin_map)
+            if new_problem == problem:
+                print("Eliminations lead to a total reduction of the problem.\n Increasing the cutoff to solve the smallest non-vanishing problem.")
+                break
+            else:
+                # Extract final set of eliminations with correct dependencies and update tracker
+                eliminations = {(spin_map[spin][1],spin):spin_map[spin][0] for spin in sorted(spin_map.keys()) if spin != spin_map[spin][1]}
+                elimination_tracker.append(eliminations)
 
-            # Extract final set of eliminations with correct dependencies and update tracker
-            eliminations = {(spin_map[spin][1],spin):spin_map[spin][0] for spin in sorted(spin_map.keys()) if spin != spin_map[spin][1]}
-            elimination_tracker.append(eliminations)
+                # Extract new number of qubits
+                n_qubits = new_problem.n
 
-            # Extract new number of qubits
-            n_qubits = new_problem.n
+                # Save qaoa object and new problem
+                qaoa_steps.append(copy.deepcopy(q))
+                problem_steps.append(copy.deepcopy(new_problem))
 
-            # Save qaoa object and new problem
-            qaoa_steps.append(copy.deepcopy(q))
-            problem_steps.append(copy.deepcopy(new_problem))
+                # problem is updated
+                problem = new_problem
 
-            # problem is updated
-            problem = new_problem
-
-            # Compile qaoa with the problem
-            q.compile(problem, verbose=False)
+                # Compile qaoa with the problem
+                q.compile(problem, verbose=False)
 
-            # Add one step to the counter
-            counter += 1
+                # Add one step to the counter
+                counter += 1
 
         # Solve the new problem classically
         cl_energy, cl_ground_states = ground_state_hamiltonian(problem.hamiltonian)