nmslib: "Storing non-optimized index is not supported yet" for the bit_hamming space in HNSW

Hi! I ran into an unexpected issue when trying out the bit_hamming space (and INT distance type): a Storing non-optimized index is not supported yet exception when trying to save the index.

During creation skip_optimized_index is set to 0

The same code works for cosinesimil space and FLOAT distance type

Is there an underlying reason why this isn’t expected to work for the hamming space, or is it just not implemented yet/a bug?

A potentially relevant note: when creating index, I get the following message: hnsw.cc:192 (CreateIndex) [INFO] No appropriate custom distance function for Hamming (bit-storage) space At the same time, NN search seems to work reasonably. What exactly does that message mean?

Thanks!