Add more unit testing for HNSW [3/n] (#4059)

Summary:
Pull Request resolved: #4059

Add unit test to compare reference version with optimized version.

Reviewed By: mengdilin

Differential Revision: D66793367

fbshipit-source-id: 8da25e79f66d079f76d237c10fc3db4a0def767d

faiss/impl/HNSW.cpp

@@ -351,6 +351,8 @@ void add_link(
     }
 }
 
+} // namespace
+
 /// search neighbors on a single level, starting from an entry point
 void search_neighbors_to_add(
         HNSW& hnsw,
@@ -359,10 +361,8 @@ void search_neighbors_to_add(
         int entry_point,
         float d_entry_point,
         int level,
-        VisitedTable& vt) {
-    // selects a version
-    const bool reference_version = false;
-
+        VisitedTable& vt,
+        bool reference_version) {
     // top is nearest candidate
     std::priority_queue<NodeDistFarther> candidates;
 
@@ -385,7 +385,14 @@ void search_neighbors_to_add(
         size_t begin, end;
         hnsw.neighbor_range(currNode, level, &begin, &end);
 
-        // select a version, based on a flag
+        // The reference version is not used, but kept here because:
+        // 1. It is easier to switch back if the optimized version has a problem
+        // 2. It serves as a starting point for new optimizations
+        // 3. It helps understand the code
+        // 4. It ensures the reference version is still compilable if the
+        // optimized version changes
+        // The reference and the optimized versions' results are compared in
+        // test_hnsw.cpp
         if (reference_version) {
             // a reference version
             for (size_t i = begin; i < end; i++) {
@@ -470,8 +477,6 @@ void search_neighbors_to_add(
     vt.advance();
 }
 
-} // namespace
-
 /// Finds neighbors and builds links with them, starting from an entry
 /// point. The own neighbor list is assumed to be locked.
 void HNSW::add_links_starting_from(

faiss/impl/HNSW.h

@@ -281,4 +281,14 @@ std::priority_queue<HNSW::Node> search_from_candidate_unbounded(
         VisitedTable* vt,
         HNSWStats& stats);
 
+void search_neighbors_to_add(
+        HNSW& hnsw,
+        DistanceComputer& qdis,
+        std::priority_queue<HNSW::NodeDistCloser>& results,
+        int entry_point,
+        float d_entry_point,
+        int level,
+        VisitedTable& vt,
+        bool reference_version = false);
+
 } // namespace faiss

tests/test_hnsw.cpp

@@ -541,3 +541,43 @@ TEST_F(HNSWTest, TEST_search_from_candidates) {
     EXPECT_EQ(reference_stats.n1, stats.n1);
     EXPECT_EQ(reference_stats.n2, stats.n2);
 }
+
+TEST_F(HNSWTest, TEST_search_neighbors_to_add) {
+    omp_set_num_threads(1);
+
+    faiss::VisitedTable vt(index->ntotal);
+    faiss::VisitedTable reference_vt(index->ntotal);
+
+    std::priority_queue<faiss::HNSW::NodeDistCloser> link_targets;
+    std::priority_queue<faiss::HNSW::NodeDistCloser> reference_link_targets;
+
+    faiss::search_neighbors_to_add(
+            index->hnsw,
+            *dis,
+            link_targets,
+            index->hnsw.entry_point,
+            (*dis)(index->hnsw.entry_point),
+            index->hnsw.max_level,
+            vt,
+            false);
+
+    faiss::search_neighbors_to_add(
+            index->hnsw,
+            *dis,
+            reference_link_targets,
+            index->hnsw.entry_point,
+            (*dis)(index->hnsw.entry_point),
+            index->hnsw.max_level,
+            reference_vt,
+            true);
+
+    EXPECT_EQ(link_targets.size(), reference_link_targets.size());
+    while (!link_targets.empty()) {
+        auto val = link_targets.top();
+        auto reference_val = reference_link_targets.top();
+        EXPECT_EQ(val.d, reference_val.d);
+        EXPECT_EQ(val.id, reference_val.id);
+        link_targets.pop();
+        reference_link_targets.pop();
+    }
+}