DoubleFactorized block encoding fails when sf_error_threshold is zero

[max-radin]

I'm seeing the error below when initializing DoubleFactorized objects with sf_error_threshold set to zero. (Context: I'm setting this to zero to make the truncation consistent with OpenFermion's logic for estimating truncation error.) It looks like the issue is just that num_ops does not get initialized if none of the eigenvalues are thrown out.

I think this is not a big blocker for our work though because it seems like we can work around it by just setting sf_error_threshold to some very small value.

Traceback (most recent call last):
  File "/Users/maxradin/Development/qb-gsee-benchmark/examples/get_qre.py", line 14, in <module>
    circuit, n_shots, allowable_error = get_df_qpe_circuit(
                                        ^^^^^^^^^^^^^^^^^^^
  File "/Users/maxradin/Development/qb-gsee-benchmark/src/qb_gsee_benchmark/qre.py", line 250, in get_df_qpe_circuit
    encoding = DoubleFactorized(
               ^^^^^^^^^^^^^^^^^
  File "/Users/maxradin/anaconda3/lib/python3.11/site-packages/pyLIQTR/BlockEncodings/DoubleFactorized.py", line 67, in __init__
    one_body_array, two_body_array, self.Xi_l_data, self.givens_angle_tensor = self.PI.yield_DF_Info(df_error_threshold=df_error_threshold,sf_error_threshold=sf_error_threshold)
                                                                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/maxradin/anaconda3/lib/python3.11/site-packages/pyLIQTR/ProblemInstances/ChemicalHamiltonian.py", line 193, in yield_DF_Info
    DF_frags = self.DF_fragments(sf_error_threshold)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/maxradin/anaconda3/lib/python3.11/site-packages/pyLIQTR/ProblemInstances/ChemicalHamiltonian.py", line 130, in DF_fragments
    return DF_decomposition(h0, one_body_tensor, two_body_tensor,tol=sf_error_threshold)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/maxradin/anaconda3/lib/python3.11/site-packages/pyLIQTR/utils/df_utils.py", line 238, in DF_decomposition
    vals = all_vals[:num_ops]
                     ^^^^^^^
UnboundLocalError: cannot access local variable 'num_ops' where it is not associated with a value

[kjm538]

It looks like the issue is just that num_ops does not get initialized if none of the eigenvalues are thrown out.

Looks like you're right. I'll push the fix with the next release, thanks for pointing it out.