Index factory doesn't support nbits for LSH

[irowberryFS]

Summary

The index factory doesn't support setting nbits for the LSH index.

Platform

Faiss version: 1.8.0

Installed from: conda

Running on:

• [x] CPU
• [ ] GPU

Interface:

• [ ] C++
• [x] Python

Reproduction instructions

Attempt to set nbits in some way for LSH index using index_factory()

index = faiss.index_factory(128, 'LSH16rt') Returns the following error. faiss/index_factory.cpp:880: could not parse index string LSHrt_16

Expecting behavior to something like this. index = faiss.IndexLSH(128, nbits)

[ramilbakhshyiev]

Thanks for reporting this. Confirmed. IndexLSH takes dimension and nbits separately while index_factory does not allow for nbits to be set and uses d as nbits.

Additional steps to repro:

import faiss
index = faiss.index_factory(128, "LSHrt")
print("d: %s, nbits: %s" % (index.d, index.nbits))
# outputs: d: 128, nbits: 128

import faiss
index = faiss.IndexLSH(128, 16)
print("d: %s, nbits: %s" % (index.d, index.nbits))
# outputs: d: 128, nbits: 16

[mdouze]

I would not label this as a bug. Most use cases of LSH do not need the dimension to be specified as a pre-transform reduces to the proper dimension and the LSH object just converts to binary. See eg. https://github.com/facebookresearch/faiss/wiki/Vector-codec-benchmarks#sift1m

[asadoughi]

Merged with https://github.com/facebookresearch/faiss/pull/3679