fix: Multi-vector dynamic vamana index Save/Load functionality (#162)

* Extended _"Multi-vector dynamic vamana index"_ test section
_"Save/Load"_ to enforce multi-vector initialization and compare search
results before and after savein-loading.
* Fixed labels saving/loading in `MultiMutableVamanaIndex`

Author: rfsaliev

include/svs/index/vamana/multi.h

@@ -583,24 +583,22 @@ class MultiMutableVamanaIndex {
             return a.first < b.first;
         });
 
-        std::vector<label_type> labels(ext_lab_vec.size());
+        size_t num_labels = ext_lab_vec.size();
+        std::vector<label_type> labels(num_labels);
         std::transform(
             ext_lab_vec.begin(),
             ext_lab_vec.end(),
             labels.begin(),
             [](const auto& ext_lab) { return ext_lab.second; }
         );
-        size_t num_labels = ext_lab_vec.size();
 
         // Save auxiliary data structures.
         lib::save_to_disk(
             lib::SaveOverride([&](const lib::SaveContext& ctx) {
                 // Save labels to a file.
                 auto filename = ctx.generate_name("labels", "binary");
                 auto stream = lib::open_write(filename);
-                for (const auto& l : ext_lab_vec) {
-                    lib::write_binary(stream, l.first);
-                }
+                lib::write_binary(stream, labels);
 
                 // Save the construction parameters.
                 auto parameters = VamanaIndexParameters{
@@ -678,13 +676,10 @@ struct MultiVamanaStateLoader {
         switch (load_from) {
             case MultiMutableVamanaLoad::FROM_MULTI: {
                 auto num_labels = lib::load_at<size_t>(table, "num_labels");
-                std::vector<label_type> labels;
-                labels.reserve(num_labels);
+                std::vector<label_type> labels(num_labels);
                 auto resolved = table.resolve_at("filename");
                 auto stream = lib::open_read(resolved);
-                for (size_t i = 0; i < num_labels; ++i) {
-                    labels.push_back(lib::read_binary<label_type>(stream));
-                }
+                lib::read_binary(stream, labels);
                 return MultiVamanaStateLoader{
                     SVS_LOAD_MEMBER_AT_(table, parameters),
                     IDTranslator{},

tests/svs/index/vamana/multi.cpp

@@ -248,8 +248,12 @@ CATCH_TEMPLATE_TEST_CASE(
         auto graph_dir = dir / "graph";
         auto data_dir = dir / "data";
         std::vector<size_t> test_indices(num_points);
+        // Fill the test indices with labels in the range of num_labels
+        // to ensure that there are labels mapped to more than 1 vector.
+        const size_t per_label = 2;
+        const auto num_labels = num_points / per_label;
         for (auto& i : test_indices) {
-            i = std::rand();
+            i = std::rand() % num_labels;
         }
         auto test_index = svs::index::vamana::MultiMutableVamanaIndex(
             build_parameters, data, test_indices, Distance(), num_threads
@@ -271,6 +275,15 @@ CATCH_TEMPLATE_TEST_CASE(
         test_index_2.search(test_results_2.view(), queries.view(), search_parameters);
         auto test_recall_2 = svs::k_recall_at_n(groundtruth, test_results_2);
 
+        // Check that the results are the same
+        CATCH_REQUIRE(test_results.n_neighbors() == test_results_2.n_neighbors());
+        for (size_t i = 0; i < test_results.n_queries(); ++i) {
+            for (size_t j = 0; j < test_results.n_neighbors(); ++j) {
+                CATCH_REQUIRE(test_results.indices().at(i, j) ==
+                              test_results_2.indices().at(i, j));
+            }
+        }
+
         CATCH_REQUIRE(test_index.size() == test_index_2.size());
         CATCH_REQUIRE(test_index.dimensions() == test_index_2.dimensions());
         // Index Properties