postprocess Method Documentation

Overview

The postprocess method of the Fixtures class refines raw QUBO solutions by applying a local bit-flip search to each candidate bitstring. It evaluates each modified solution against the original QUBO instance, aiming to lower the objective cost.

Method Signature

def postprocess(self, solution: QUBOSolution) -> QUBOSolution

Parameters

• solution (QUBOSolution): The QUBO solver's output, containing:
• bitstrings: Tensor of shape (num_samples, num_variables) representing candidate solutions.
• costs: Tensor of shape (num_samples,) with corresponding objective values.
• Optional counts and probabilities attributes.

Returns

• QUBOSolution: A new solution object where each bitstring has been locally optimized and costs recomputed. It includes:
• Updated bitstrings (dtype float32) of shape (num_samples, num_variables).
• Updated costs (dtype float32) of shape (num_samples,).
• solution_status: A description indicating postprocessing and listing the improved costs.
• Preserved counts and probabilities if originally present.

Behavior

No-op for Empty Solutions If solution.bitstrings is empty, the method returns the input solution unchanged.

Objective Wrapper Defines an inner function:

def qubo_objective(s_arr: np.ndarray) -> float:
    return self.instance.evaluate_solution(s_arr.tolist())

to compute the cost of any candidate bitstring via the original QUBO instance.

Local Bit-Flip Search For each bitstring in solution.bitstrings:

Convert the tensor row to a NumPy integer array.

Call bit_flip_local_search(qubo_objective, s_orig) to greedily flip bits and reduce cost.

Collect the improved bitstring and its new cost.

Assemble Tensors Stack all improved bitstrings and costs into new PyTorch tensors (dtype=torch.float32).

Update and Return

Replace solution.bitstrings and solution.costs with the improved tensors.

Setsolution.solution_statusto:

postprocessed (improved costs: [c1, c2, …])

Return the modified QUBOSolution.

Example Usage with Fixtures

import torch
from qubosolver import QUBOInstance
from qubosolver.config import SolverConfig, ClassicalConfig
from qubosolver.pipeline import Fixtures
from qubosolver.solver import QuboSolver

# Create a random 4-variable QUBO instance
graphics = torch.randn(4, 4)
graphics = (graphics + graphics.T) / 2  # Ensure symmetry
n = graphics.size(0)
off_diag_mask = ~torch.eye(n, dtype=torch.bool)
graphics[off_diag_mask] = graphics[off_diag_mask].abs() # Ensure Abs off-diagonal
qubo = QUBOInstance(coefficients=graphics)

# Configure solver to enable postprocessing
cplex = ClassicalConfig(classical_solver_type="cplex", cplex_log_path="solver.log", cplex_maxtime=300.0,)
config = SolverConfig(
    use_quantum=False,
    classical=cplex,
    do_postprocessing=True
)

# Solve with classical solver
classical_solver = QuboSolver(qubo, config)
raw_solution = classical_solver.solve()

# Apply postprocessing
fixture = Fixtures(qubo, config)
final_solution = fixture.postprocess(raw_solution)

print("Optimized bitstrings:", final_solution.bitstrings)
print("Optimized costs:", final_solution.costs)
print("Status:", final_solution.solution_status)

Optimized bitstrings: tensor([[0., 0., 1., 1.]]) Optimized costs: tensor([-0.9359]) Status: postprocessed

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Example Usage without Direct Fixture Manipulation

Instead of manually instantiating Fixtures, you can enable postprocessing directly through your solver configuration:

import torch
from qubosolver import QUBOInstance
from qubosolver.config import SolverConfig, ClassicalConfig
from qubosolver.solver import QuboSolver
import emu_mps
from qoolqit._solvers.types import BackendType


# Assume `first_qubo_coefficients` is your 2×2 QUBO matrix (e.g., identity):
first_qubo_coefficients = torch.eye(2)
instance = QUBOInstance(coefficients=first_qubo_coefficients)

# Configure solver with postprocessing enabled
cplex = ClassicalConfig(classical_solver_type="cplex", cplex_log_path="solver.log", cplex_maxtime=300.0,)
config = SolverConfig(
    classical=cplex,
    do_postprocessing=True,                  # Enable postprocessing
    use_quantum=False
)

# Instantiate and run the classical solver
classical_solver = QuboSolver(instance, config)
solution = classical_solver.solve()

print("Final bitstrings:", solution.bitstrings)
print("Final costs:", solution.costs)
print("Status:", solution.solution_status)

Final bitstrings: tensor([[0, 0]]) Final costs: tensor([0.]) Status: trivial-zero

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