Reset visited set to buffer contents when a batch search resumes

include/svs/index/vamana/dynamic_search_buffer.h

@@ -221,6 +221,18 @@ template <typename Idx, typename Cmp = std::less<>> class MutableBuffer {
         best_unvisited_ = 0;
     }
 
+    /// Keep only the ids now in the buffer in the visited set, so dropped ids can be
+    /// re-inserted.
+    void reset_visited_to_contents() {
+        if (!visited_set_enabled()) {
+            return;
+        }
+        visited_->reset();
+        for (const auto& neighbor : candidates_) {
+            visited_->emplace(neighbor.id());
+        }
+    }
+
     size_t capacity() const { return candidates_.capacity(); }
 
     /// @brief Return the number of valid elements currently contained in the buffer.

include/svs/index/vamana/iterator.h

@@ -54,6 +54,8 @@ template <std::integral I> struct RestartInitializer {
             return;
         }
 
+        // Let ids dropped from the buffer be inserted again in the larger window.
+        buffer.reset_visited_to_contents();
         // Otherwise, sort the buffer to prepare for the next search.
         buffer.sort();
     }

include/svs/index/vamana/search_buffer.h

@@ -287,6 +287,18 @@ template <typename Idx, typename Cmp = std::less<>> class SearchBuffer {
         best_unvisited_ = 0;
     }
 
+    /// Keep only the ids now in the buffer in the visited set, so dropped ids can be
+    /// re-inserted.
+    void reset_visited_to_contents() {
+        if (!visited_set_enabled()) {
+            return;
+        }
+        visited_->reset();
+        for (size_t i = 0; i < size_; ++i) {
+            visited_->emplace(candidates_[i].id());
+        }
+    }
+
     /// @brief Return the current number of valid elements in the buffer.
     size_t size() const { return size_; }