Add bounds checking to hnsw nb_neighbors (#4185)

Summary:
Pull Request resolved: #4185

Based on this users comment it seems like we should do bound checking: #4177

Reviewed By: mnorris11

Differential Revision: D69497295

fbshipit-source-id: 97025cf29c464afb0f85aa98f4b303489b7fc989

faiss/impl/HNSW.cpp

@@ -32,6 +32,7 @@ namespace faiss {
  **************************************************************/
 
 int HNSW::nb_neighbors(int layer_no) const {
+    FAISS_THROW_IF_NOT(layer_no + 1 < cum_nneighbor_per_level.size());
     return cum_nneighbor_per_level[layer_no + 1] -
             cum_nneighbor_per_level[layer_no];
 }

tests/test_hnsw.cpp

@@ -582,6 +582,16 @@ TEST_F(HNSWTest, TEST_search_neighbors_to_add) {
     }
 }
 
+TEST_F(HNSWTest, TEST_nb_neighbors_bound) {
+    omp_set_num_threads(1);
+    EXPECT_EQ(index->hnsw.nb_neighbors(0), 8);
+    EXPECT_EQ(index->hnsw.nb_neighbors(1), 4);
+    EXPECT_EQ(index->hnsw.nb_neighbors(2), 4);
+    EXPECT_EQ(index->hnsw.nb_neighbors(3), 4);
+    // picking a large number to trigger an exception based on checking bounds
+    EXPECT_THROW(index->hnsw.nb_neighbors(100), faiss::FaissException);
+}
+
 TEST_F(HNSWTest, TEST_search_level_0) {
     omp_set_num_threads(1);
     std::vector<faiss::idx_t> I(k * nq);