First attempt at LSH matching with nbits (facebookresearch#3679)

Summary:
Pull Request resolved: facebookresearch#3679

T195237796 Claims we should be able to incldue nbits in the LSH factory string.

Their example is:

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

This is my first attempt at modifying the regex to accept an integer for nbits. Can an expert help me understand what the domain of accepted strings should be so I can modify the regex as necessary?

Reviewed By: ramilbakhshyiev

Differential Revision: D60054776

fbshipit-source-id: e47074eb9986b7c1c702832fc0bf758f60f45290

Author: bshethmeta

faiss/index_factory.cpp

@@ -530,11 +530,12 @@ Index* parse_other_indexes(
     }
 
     // IndexLSH
-    if (match("LSH(r?)(t?)")) {
-        bool rotate_data = sm[1].length() > 0;
-        bool train_thresholds = sm[2].length() > 0;
+    if (match("LSH([0-9]*)(r?)(t?)")) {
+        int nbits = sm[1].length() > 0 ? std::stoi(sm[1].str()) : d;
+        bool rotate_data = sm[2].length() > 0;
+        bool train_thresholds = sm[3].length() > 0;
         FAISS_THROW_IF_NOT(metric == METRIC_L2);
-        return new IndexLSH(d, d, rotate_data, train_thresholds);
+        return new IndexLSH(d, nbits, rotate_data, train_thresholds);
     }
 
     // IndexLattice

tests/test_factory.py

@@ -119,6 +119,12 @@ def test_factory_NSG(self):
         assert index.nlist == 65536 and index_nsg.nsg.R == 64
         assert index.pq.M == 2 and index.pq.nbits == 8
 
+    def test_factory_lsh(self):
+        index = faiss.index_factory(128, 'LSHrt')
+        self.assertEqual(index.nbits, 128)
+        index = faiss.index_factory(128, 'LSH16rt')
+        self.assertEqual(index.nbits, 16)
+
     def test_factory_fast_scan(self):
         index = faiss.index_factory(56, "PQ28x4fs")
         self.assertEqual(index.bbs, 32)