Exception handling for noiseless circuits

[navneethramakrishnan]

For a noiseless Stim circuit, the BPOSD decoder does not decode and instead outputs the error message

IndexError: index 0 is out of bounds for axis 0 with size 0

However, the PyMatching decoder does not have such an error and decodes correctly.

Example code below

import numpy as np
from stimbposd import BPOSD
import pymatching

test_circ = stim.Circuit.generated(
    "repetition_code:memory",
    rounds=25,
    distance=9,
    before_round_data_depolarization=0,
    before_measure_flip_probability=0)

num_shots = 100

sampler = test_circ.compile_detector_sampler()
shots, observables = sampler.sample(num_shots, separate_observables=True)

#decoder = pymatching.Matching.from_detector_error_model(test_circ.detector_error_model(decompose_errors=True))
decoder = BPOSD(test_circ.detector_error_model(), max_bp_iters=20)


predicted_observables = decoder.decode_batch(shots)
num_mistakes = np.sum(np.any(predicted_observables != observables, axis=1))

print(f"{num_mistakes}/{num_shots}")