[SVS] Add multi-vector support to the Tiered SVS Index (#699)

* [SVS] Add multi-vector support to the Tiered SVS Index

* Code refine

* Add tests and fixes for swaps journal processing

* Code review s1e1: Extract complex logic to dedicated funcs

* Update algorithm for swaps journal processing

* Improve code coverage

* Code review s2e1

Author: rfsaliev

src/VecSim/algorithms/brute_force/brute_force.h

@@ -59,9 +59,6 @@ class BruteForceIndex : public VecSimIndexAbstract<DataType, DistType> {
 
     const RawDataContainer *getVectorsContainer() const { return this->vectors; }
 
-    const labelType getLabelByInternalId(idType internal_id) const {
-        return idToLabelMapping.at(internal_id);
-    }
     // Remove a specific vector that is stored under a label from the index by its internal id.
     virtual int deleteVectorById(labelType label, idType id) = 0;
     // Remove a vector and return a map between internal ids and the original internal ids of the
@@ -71,6 +68,9 @@ class BruteForceIndex : public VecSimIndexAbstract<DataType, DistType> {
     // Check if a certain label exists in the index.
     virtual bool isLabelExists(labelType label) = 0;
 
+    // Unsafe (assume index data guard is held in MT mode).
+    virtual vecsim_stl::vector<idType> getElementIds(size_t label) const = 0;
+
     virtual ~BruteForceIndex() = default;
 #ifdef BUILD_TESTS
     void fitMemory() override {

src/VecSim/algorithms/brute_force/brute_force_multi.h

@@ -94,7 +94,15 @@ class BruteForceIndex_Multi : public BruteForceIndex<DataType, DistType> {
             keys.insert(it.first);
         }
         return keys;
-    };
+    }
+
+    inline vecsim_stl::vector<idType> getElementIds(size_t label) const override {
+        auto it = labelToIdsLookup.find(label);
+        if (it == labelToIdsLookup.end()) {
+            return vecsim_stl::vector<idType>{this->allocator}; // return an empty collection
+        }
+        return it->second;
+    }
 
     inline vecsim_stl::abstract_priority_queue<DistType, labelType> *
     getNewMaxPriorityQueue() const override {

src/VecSim/algorithms/brute_force/brute_force_single.h

@@ -93,7 +93,16 @@ class BruteForceIndex_Single : public BruteForceIndex<DataType, DistType> {
             keys.insert(it.first);
         }
         return keys;
-    };
+    }
+
+    vecsim_stl::vector<idType> getElementIds(size_t label) const override {
+        vecsim_stl::vector<idType> ids(this->allocator);
+        auto it = labelToIdLookup.find(label);
+        if (it != labelToIdLookup.end()) {
+            ids.push_back(it->second);
+        }
+        return ids;
+    }
 
     vecsim_stl::abstract_priority_queue<DistType, labelType> *
     getNewMaxPriorityQueue() const override {

src/VecSim/algorithms/hnsw/hnsw_tiered.h

@@ -571,7 +571,7 @@ void TieredHNSWIndex<DataType, DistType>::executeInsertJob(HNSWInsertJob *job) {
         // corresponding job id. Note that after calling deleteVectorById, the last id's label
         // shouldn't be available, since it is removed from the lookup.
         labelType last_vec_label =
-            this->frontendIndex->getLabelByInternalId(this->frontendIndex->indexSize() - 1);
+            this->frontendIndex->getVectorLabel(this->frontendIndex->indexSize() - 1);
         int deleted = this->frontendIndex->deleteVectorById(job->label, job->id);
         if (deleted && job->id != this->frontendIndex->indexSize()) {
             // If the vector removal caused a swap with the last id, update the relevant insert job.

src/VecSim/algorithms/svs/svs_tiered.h

@@ -127,7 +127,7 @@ BFParams NewBFParams(const TieredIndexParams *params) {
     return BFParams{.type = svs_params.type,
                     .dim = svs_params.dim,
                     .metric = svs_params.metric,
-                    .multi = false, // multi not supported
+                    .multi = svs_params.multi,
                     .blockSize = svs_params.blockSize};
 }
 
@@ -155,7 +155,7 @@ inline VecSimIndex *NewIndex(const TieredIndexParams *params) {
                                                   .blockSize = bf_params.blockSize,
                                                   .multi = bf_params.multi,
                                                   .logCtx = params->primaryIndexParams->logCtx};
-    auto frontendIndex = static_cast<BruteForceIndex_Single<DataType, float> *>(
+    auto frontendIndex = static_cast<BruteForceIndex<DataType, float> *>(
         BruteForceFactory::NewIndex(&bf_params, abstractInitParams, false));
 
     // Create new tiered svs index