B vector
The $\bm{b}$-vectors connect kpoint $\bm{k}$ to its neighboring kpoints $\bm{k}+\bm{b}$, for calculating WF centers, spreads in reciprocal space.
The bvectors are first arranged in layers of shells, which contain bvectors having same norm. Then parallel shells are deleted, and the shells satisfying B1 condition are the final bvectors. At last, the bvectors are sorted at each kpoint, to reproduce exactly the same order as Wannier90. This ensures that we have the same order as mmn file.
To reproduce the same order as Wannier90, we need to be careful with some floating point comparison, i.e., the atol keyword arguments in the following functions. Their default value reproduce the Wannier90 order. Note the kmesh_tol in the win file also influence the search of bvectors.
In most cases, the user only need to call generate_bvectors to generate bvectors having the same order as Wannier90. Other functions are intermediate steps that are called inside generate_bvectors.