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

Author: rfsaliev

src/VecSim/algorithms/brute_force/brute_force.h

@@ -74,6 +74,9 @@ class BruteForceIndex : public VecSimIndexAbstract<DataType, DistType> {
     // without duplicates in tiered index). Caller should hold the flat buffer lock for read.
     virtual vecsim_stl::set<labelType> getLabelsSet() const = 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

@@ -95,7 +95,13 @@ class BruteForceIndex_Multi : public BruteForceIndex<DataType, DistType> {
         }
         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 {
         return new (this->allocator)

src/VecSim/algorithms/brute_force/brute_force_single.h

@@ -93,7 +93,17 @@ 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()) {
+            return ids;
+        }
+        ids.push_back(it->second);
+        return ids;
+    }
 
     vecsim_stl::abstract_priority_queue<DistType, labelType> *
     getNewMaxPriorityQueue() const override {