hnswalg.h

/*-
 * -\-\-
 * voyager
 * --
 * Copyright (C) 2016 - 2023 Spotify AB
 *
 * This file is heavily based on hnswlib (https://github.com/nmslib/hnswlib,
 * Apache 2.0-licensed, no copyright author listed)
 * --
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 * -/-/-
 */

#pragma once

#include "Spaces/Space.h"
#include "hnswlib.h"
#include "visited_list_pool.h"
#include <assert.h>
#include <atomic>
#include <cstdio>
#include <cstring>
#include <fstream>
#include <list>
#include <random>
#include <shared_mutex>
#include <stdlib.h>
#include <unordered_map>
#include <unordered_set>

class IndexCannotBeShrunkError : public std::runtime_error {
public:
  IndexCannotBeShrunkError(const std::string &what)
      : std::runtime_error(what) {}
};

class IndexFullError : public std::runtime_error {
public:
  IndexFullError(const std::string &what) : std::runtime_error(what) {}
};

namespace hnswlib {
typedef unsigned int tableint;
typedef unsigned int linklistsizeint;

template <typename dist_t, typename data_t = dist_t>
class HierarchicalNSW : public AlgorithmInterface<dist_t, data_t> {
public:
  static const tableint max_update_element_locks = 65536;

  HierarchicalNSW(Space<dist_t, data_t> *s,
                  std::shared_ptr<InputStream> inputStream,
                  size_t max_elements = 0, bool search_only = false)
      : search_only_(search_only) {
    loadIndex(inputStream, s, max_elements);
  }

  HierarchicalNSW(Space<dist_t, data_t> *s, size_t max_elements, size_t M = 16,
                  size_t ef_construction = 200, size_t random_seed = 100)
      : link_list_locks_(max_elements),
        link_list_update_locks_(max_update_element_locks),
        element_levels_(max_elements) {
    max_elements_ = max_elements;

    num_deleted_ = 0;
    data_size_ = s->get_data_size();
    fstdistfunc_ = s->get_dist_func();
    dist_func_param_ = s->get_dist_func_param();
    M_ = M;
    maxM_ = M_;
    maxM0_ = M_ * 2;
    ef_construction_ = std::max(ef_construction, M_);
    ef_ = 10;

    level_generator_.seed(random_seed);
    update_probability_generator_.seed(random_seed + 1);

    size_links_level0_ = maxM0_ * sizeof(tableint) + sizeof(linklistsizeint);
    size_data_per_element_ =
        size_links_level0_ + data_size_ + sizeof(labeltype);
    offsetData_ = size_links_level0_;
    label_offset_ = size_links_level0_ + data_size_;
    offsetLevel0_ = 0;

    data_level0_memory_ =
        (char *)malloc(max_elements_ * size_data_per_element_);
    if (data_level0_memory_ == nullptr)
      throw std::runtime_error("Not enough memory");

    cur_element_count = 0;

    visited_list_pool_ = new VisitedListPool(1, max_elements);

    // initializations for special treatment of the first node
    enterpoint_node_ = -1;
    maxlevel_ = -1;

    linkLists_ = (char **)malloc(sizeof(void *) * max_elements_);
    if (linkLists_ == nullptr)
      throw std::runtime_error(
          "Not enough memory: HierarchicalNSW failed to allocate linklists");
    size_links_per_element_ =
        maxM_ * sizeof(tableint) + sizeof(linklistsizeint);
    mult_ = 1 / log(1.0 * M_);
    revSize_ = 1.0 / mult_;
  }

  struct CompareByFirst {
    constexpr bool
    operator()(std::pair<dist_t, tableint> const &a,
               std::pair<dist_t, tableint> const &b) const noexcept {
      return a.first < b.first;
    }
  };

  ~HierarchicalNSW() {
    free(data_level0_memory_);
    for (tableint i = 0; i < cur_element_count; i++) {
      if (element_levels_[i] > 0)
        free(linkLists_[i]);
    }
    free(linkLists_);
    delete visited_list_pool_;
  }

  size_t max_elements_;
  size_t cur_element_count;
  size_t size_data_per_element_;
  size_t size_links_per_element_;
  size_t num_deleted_;

  size_t M_;
  size_t maxM_;
  size_t maxM0_;
  size_t ef_construction_;

  double mult_, revSize_;
  int maxlevel_;

  std::shared_mutex resizeLock;
  VisitedListPool *visited_list_pool_;
  std::mutex cur_element_count_guard_;

  std::vector<std::mutex> link_list_locks_;

  // Locks to prevent race condition during update/insert of an element at same
  // time. Note: Locks for additions can also be used to prevent this race
  // condition if the querying of KNN is not exposed along with update/inserts
  // i.e multithread insert/update/query in parallel.
  std::vector<std::mutex> link_list_update_locks_;
  tableint enterpoint_node_;

  size_t size_links_level0_;
  size_t offsetData_, offsetLevel0_;

  char *data_level0_memory_;
  char **linkLists_;
  std::vector<int> element_levels_;

  size_t data_size_;

  bool search_only_ = false;

  size_t label_offset_;
  DISTFUNC<dist_t, data_t> fstdistfunc_;
  size_t dist_func_param_;
  std::unordered_map<labeltype, tableint> label_lookup_;

  std::default_random_engine level_generator_;
  std::default_random_engine update_probability_generator_;

  inline labeltype getExternalLabel(tableint internal_id) const {
    labeltype return_label;
    memcpy(&return_label,
           (data_level0_memory_ + internal_id * size_data_per_element_ +
            label_offset_),
           sizeof(labeltype));
    return return_label;
  }

  inline void setExternalLabel(tableint internal_id, labeltype label) const {
    memcpy((data_level0_memory_ + internal_id * size_data_per_element_ +
            label_offset_),
           &label, sizeof(labeltype));
  }

  inline labeltype *getExternalLabeLp(tableint internal_id) const {
    return (labeltype *)(data_level0_memory_ +
                         internal_id * size_data_per_element_ + label_offset_);
  }

  inline data_t *getDataByInternalId(tableint internal_id) const {
    return reinterpret_cast<data_t *>(data_level0_memory_ +
                                      internal_id * size_data_per_element_ +
                                      offsetData_);
  }

  int getRandomLevel(double reverse_size) {
    std::uniform_real_distribution<double> distribution(0.0, 1.0);
    double r = -log(distribution(level_generator_)) * reverse_size;
    return (int)r;
  }

  std::priority_queue<std::pair<dist_t, tableint>,
                      std::vector<std::pair<dist_t, tableint>>, CompareByFirst>
  searchBaseLayer(tableint ep_id, const data_t *data_point, int layer,
                  VisitedList *vl = nullptr) {
    bool wasPassedVisitedList = vl != nullptr;
    if (!wasPassedVisitedList) {
      vl = visited_list_pool_->getFreeVisitedList();
    } else {
      vl->reset();
    }

    vl_type *visited_array = vl->mass;
    vl_type visited_array_tag = vl->curV;

    std::priority_queue<std::pair<dist_t, tableint>,
                        std::vector<std::pair<dist_t, tableint>>,
                        CompareByFirst>
        top_candidates;
    std::priority_queue<std::pair<dist_t, tableint>,
                        std::vector<std::pair<dist_t, tableint>>,
                        CompareByFirst>
        candidateSet;

    dist_t lowerBound;
    if (!isMarkedDeleted(ep_id)) {
      dist_t dist = fstdistfunc_(data_point, getDataByInternalId(ep_id),
                                 dist_func_param_);
      top_candidates.emplace(dist, ep_id);
      lowerBound = dist;
      candidateSet.emplace(-dist, ep_id);
    } else {
      lowerBound = std::numeric_limits<dist_t>::max();
      candidateSet.emplace(-lowerBound, ep_id);
    }
    visited_array[ep_id] = visited_array_tag;

    while (!candidateSet.empty()) {
      std::pair<dist_t, tableint> curr_el_pair = candidateSet.top();
      if ((-curr_el_pair.first) > lowerBound &&
          top_candidates.size() == ef_construction_) {
        break;
      }
      candidateSet.pop();

      tableint curNodeNum = curr_el_pair.second;

      std::unique_lock<std::mutex> lock(link_list_locks_[curNodeNum]);

      int *data; // = (int *)(linkList0_ + curNodeNum *
                 // size_links_per_element0_);
      if (layer == 0) {
        data = (int *)get_linklist0(curNodeNum);
      } else {
        data = (int *)get_linklist(curNodeNum, layer);
        //                    data = (int *) (linkLists_[curNodeNum] + (layer -
        //                    1) * size_links_per_element_);
      }
      size_t size = getListCount((linklistsizeint *)data);
      tableint *datal = (tableint *)(data + 1);

      for (size_t j = 0; j < size; j++) {
        tableint candidate_id = *(datal + j);
        //                    if (candidate_id == 0) continue;
        if (visited_array[candidate_id] == visited_array_tag)
          continue;
        visited_array[candidate_id] = visited_array_tag;
        data_t *currObj1 = getDataByInternalId(candidate_id);

        dist_t dist1 = fstdistfunc_(data_point, currObj1, dist_func_param_);
        if (top_candidates.size() < ef_construction_ || lowerBound > dist1) {
          candidateSet.emplace(-dist1, candidate_id);

          if (!isMarkedDeleted(candidate_id))
            top_candidates.emplace(dist1, candidate_id);

          if (top_candidates.size() > ef_construction_)
            top_candidates.pop();

          if (!top_candidates.empty())
            lowerBound = top_candidates.top().first;
        }
      }
    }

    if (!wasPassedVisitedList) {
      visited_list_pool_->releaseVisitedList(vl);
    }

    return top_candidates;
  }

  mutable std::atomic<long> metric_distance_computations;
  mutable std::atomic<long> metric_hops;

  template <bool has_deletions, bool collect_metrics = false>
  std::priority_queue<std::pair<dist_t, tableint>,
                      std::vector<std::pair<dist_t, tableint>>, CompareByFirst>
  searchBaseLayerST(tableint ep_id, const data_t *data_point, size_t ef,
                    VisitedList *vl = nullptr) const {
    bool wasPassedVisitedList = vl != nullptr;
    if (!wasPassedVisitedList) {
      vl = visited_list_pool_->getFreeVisitedList();
    } else {
      vl->reset();
    }

    vl_type *visited_array = vl->mass;
    vl_type visited_array_tag = vl->curV;

    std::priority_queue<std::pair<dist_t, tableint>,
                        std::vector<std::pair<dist_t, tableint>>,
                        CompareByFirst>
        top_candidates;
    std::priority_queue<std::pair<dist_t, tableint>,
                        std::vector<std::pair<dist_t, tableint>>,
                        CompareByFirst>
        candidate_set;

    dist_t lowerBound;
    if (!has_deletions || !isMarkedDeleted(ep_id)) {
      dist_t dist = fstdistfunc_(data_point, getDataByInternalId(ep_id),
                                 dist_func_param_);
      lowerBound = dist;
      top_candidates.emplace(dist, ep_id);
      candidate_set.emplace(-dist, ep_id);
    } else {
      lowerBound = std::numeric_limits<dist_t>::max();
      candidate_set.emplace(-lowerBound, ep_id);
    }

    visited_array[ep_id] = visited_array_tag;

    while (!candidate_set.empty()) {

      std::pair<dist_t, tableint> current_node_pair = candidate_set.top();

      if ((-current_node_pair.first) > lowerBound &&
          (top_candidates.size() == ef || has_deletions == false)) {
        break;
      }
      candidate_set.pop();

      tableint current_node_id = current_node_pair.second;
      int *data = (int *)get_linklist0(current_node_id);
      size_t size = getListCount((linklistsizeint *)data);
      //                bool cur_node_deleted =
      //                isMarkedDeleted(current_node_id);
      if (collect_metrics) {
        metric_hops++;
        metric_distance_computations += size;
      }

      for (size_t j = 1; j <= size; j++) {
        int candidate_id = *(data + j);
        //                    if (candidate_id == 0) continue;
        if (!(visited_array[candidate_id] == visited_array_tag)) {

          visited_array[candidate_id] = visited_array_tag;

          data_t *currObj1 = (getDataByInternalId(candidate_id));
          dist_t dist = fstdistfunc_(data_point, currObj1, dist_func_param_);

          if (top_candidates.size() < ef || lowerBound > dist) {
            candidate_set.emplace(-dist, candidate_id);
            if (!has_deletions || !isMarkedDeleted(candidate_id))
              top_candidates.emplace(dist, candidate_id);

            if (top_candidates.size() > ef)
              top_candidates.pop();

            if (!top_candidates.empty())
              lowerBound = top_candidates.top().first;
          }
        }
      }
    }

    if (!wasPassedVisitedList) {
      visited_list_pool_->releaseVisitedList(vl);
    }
    return top_candidates;
  }

  void getNeighborsByHeuristic2(
      std::priority_queue<std::pair<dist_t, tableint>,
                          std::vector<std::pair<dist_t, tableint>>,
                          CompareByFirst> &top_candidates,
      const size_t M) {
    if (top_candidates.size() < M) {
      return;
    }

    std::priority_queue<std::pair<dist_t, tableint>> queue_closest;
    std::vector<std::pair<dist_t, tableint>> return_list;
    while (top_candidates.size() > 0) {
      queue_closest.emplace(-top_candidates.top().first,
                            top_candidates.top().second);
      top_candidates.pop();
    }

    while (queue_closest.size()) {
      if (return_list.size() >= M)
        break;
      std::pair<dist_t, tableint> curent_pair = queue_closest.top();
      dist_t dist_to_query = -curent_pair.first;
      queue_closest.pop();
      bool good = true;

      for (std::pair<dist_t, tableint> second_pair : return_list) {
        dist_t curdist = fstdistfunc_(getDataByInternalId(second_pair.second),
                                      getDataByInternalId(curent_pair.second),
                                      dist_func_param_);
        ;
        if (curdist < dist_to_query) {
          good = false;
          break;
        }
      }
      if (good) {
        return_list.push_back(curent_pair);
      }
    }

    for (std::pair<dist_t, tableint> curent_pair : return_list) {
      top_candidates.emplace(-curent_pair.first, curent_pair.second);
    }
  }

  linklistsizeint *get_linklist0(tableint internal_id) const {
    return (linklistsizeint *)(data_level0_memory_ +
                               internal_id * size_data_per_element_ +
                               offsetLevel0_);
  };

  linklistsizeint *get_linklist0(tableint internal_id,
                                 char *data_level0_memory_) const {
    return (linklistsizeint *)(data_level0_memory_ +
                               internal_id * size_data_per_element_ +
                               offsetLevel0_);
  };

  linklistsizeint *get_linklist(tableint internal_id, int level) const {
    return (linklistsizeint *)(linkLists_[internal_id] +
                               (level - 1) * size_links_per_element_);
  };

  linklistsizeint *get_linklist_at_level(tableint internal_id,
                                         int level) const {
    return level == 0 ? get_linklist0(internal_id)
                      : get_linklist(internal_id, level);
  };

  tableint mutuallyConnectNewElement(
      const data_t *data_point, tableint cur_c,
      std::priority_queue<std::pair<dist_t, tableint>,
                          std::vector<std::pair<dist_t, tableint>>,
                          CompareByFirst> &top_candidates,
      int level, bool isUpdate) {
    size_t Mcurmax = level ? maxM_ : maxM0_;
    getNeighborsByHeuristic2(top_candidates, M_);
    if (top_candidates.size() > M_)
      throw std::runtime_error(
          "Should be not be more than M_ candidates returned by the heuristic");

    std::vector<tableint> selectedNeighbors;
    selectedNeighbors.reserve(M_);
    while (top_candidates.size() > 0) {
      selectedNeighbors.push_back(top_candidates.top().second);
      top_candidates.pop();
    }

    tableint next_closest_entry_point = selectedNeighbors.back();

    {
      linklistsizeint *ll_cur;
      if (level == 0)
        ll_cur = get_linklist0(cur_c);
      else
        ll_cur = get_linklist(cur_c, level);

      if (*ll_cur && !isUpdate) {
        throw std::runtime_error(
            "The newly inserted element should have blank link list");
      }
      setListCount(ll_cur, selectedNeighbors.size());
      tableint *data = (tableint *)(ll_cur + 1);
      for (size_t idx = 0; idx < selectedNeighbors.size(); idx++) {
        if (data[idx] && !isUpdate)
          throw std::runtime_error("Possible memory corruption");
        if (level > element_levels_[selectedNeighbors[idx]])
          throw std::runtime_error(
              "Trying to make a link on a non-existent level");

        data[idx] = selectedNeighbors[idx];
      }
    }

    for (size_t idx = 0; idx < selectedNeighbors.size(); idx++) {

      std::unique_lock<std::mutex> lock(
          link_list_locks_[selectedNeighbors[idx]]);

      linklistsizeint *ll_other;
      if (level == 0)
        ll_other = get_linklist0(selectedNeighbors[idx]);
      else
        ll_other = get_linklist(selectedNeighbors[idx], level);

      size_t sz_link_list_other = getListCount(ll_other);

      if (sz_link_list_other > Mcurmax)
        throw std::runtime_error("Bad value of sz_link_list_other");
      if (selectedNeighbors[idx] == cur_c)
        throw std::runtime_error("Trying to connect an element to itself");
      if (level > element_levels_[selectedNeighbors[idx]])
        throw std::runtime_error(
            "Trying to make a link on a non-existent level");

      tableint *data = (tableint *)(ll_other + 1);

      bool is_cur_c_present = false;
      if (isUpdate) {
        for (size_t j = 0; j < sz_link_list_other; j++) {
          if (data[j] == cur_c) {
            is_cur_c_present = true;
            break;
          }
        }
      }

      // If cur_c is already present in the neighboring connections of
      // `selectedNeighbors[idx]` then no need to modify any connections or run
      // the heuristics.
      if (!is_cur_c_present) {
        if (sz_link_list_other < Mcurmax) {
          data[sz_link_list_other] = cur_c;
          setListCount(ll_other, sz_link_list_other + 1);
        } else {
          // finding the "weakest" element to replace it with the new one
          dist_t d_max = fstdistfunc_(
              getDataByInternalId(cur_c),
              getDataByInternalId(selectedNeighbors[idx]), dist_func_param_);
          // Heuristic:
          std::priority_queue<std::pair<dist_t, tableint>,
                              std::vector<std::pair<dist_t, tableint>>,
                              CompareByFirst>
              candidates;
          candidates.emplace(d_max, cur_c);

          for (size_t j = 0; j < sz_link_list_other; j++) {
            candidates.emplace(
                fstdistfunc_(getDataByInternalId(data[j]),
                             getDataByInternalId(selectedNeighbors[idx]),
                             dist_func_param_),
                data[j]);
          }

          getNeighborsByHeuristic2(candidates, Mcurmax);

          int indx = 0;
          while (candidates.size() > 0) {
            data[indx] = candidates.top().second;
            candidates.pop();
            indx++;
          }

          setListCount(ll_other, indx);
          // Nearest K:
          /*int indx = -1;
          for (int j = 0; j < sz_link_list_other; j++) {
              dist_t d = fstdistfunc_(getDataByInternalId(data[j]),
          getDataByInternalId(rez[idx]), dist_func_param_); if (d > d_max) {
                  indx = j;
                  d_max = d;
              }
          }
          if (indx >= 0) {
              data[indx] = cur_c;
          } */
        }
      }
    }

    return next_closest_entry_point;
  }

  std::mutex global;
  size_t ef_;

  void setEf(size_t ef) { ef_ = ef; }

  std::priority_queue<std::pair<dist_t, tableint>>
  searchKnnInternal(data_t *query_data, int k, VisitedList *vl = nullptr) {
    std::priority_queue<std::pair<dist_t, tableint>> top_candidates;
    if (cur_element_count == 0)
      return top_candidates;
    tableint currObj = enterpoint_node_;
    dist_t curdist = fstdistfunc_(
        query_data, getDataByInternalId(enterpoint_node_), dist_func_param_);

    for (size_t level = maxlevel_; level > 0; level--) {
      bool changed = true;
      while (changed) {
        changed = false;
        int *data;
        data = (int *)get_linklist(currObj, level);
        int size = getListCount(data);
        tableint *datal = (tableint *)(data + 1);
        for (int i = 0; i < size; i++) {
          tableint cand = datal[i];
          if (cand < 0 || cand > max_elements_)
            throw std::runtime_error("cand error");
          dist_t d = fstdistfunc_(query_data, getDataByInternalId(cand),
                                  dist_func_param_);

          if (d < curdist) {
            curdist = d;
            currObj = cand;
            changed = true;
          }
        }
      }
    }

    if (num_deleted_) {
      std::priority_queue<std::pair<dist_t, tableint>> top_candidates1 =
          searchBaseLayerST<true>(currObj, query_data, ef_, vl);
      top_candidates.swap(top_candidates1);
    } else {
      std::priority_queue<std::pair<dist_t, tableint>> top_candidates1 =
          searchBaseLayerST<false>(currObj, query_data, ef_, vl);
      top_candidates.swap(top_candidates1);
    }

    while (top_candidates.size() > k) {
      top_candidates.pop();
    }
    return top_candidates;
  };

  void resizeIndex(size_t new_max_elements) {
    if (search_only_)
      throw std::runtime_error(
          "resizeIndex is not supported in search only mode");
    std::unique_lock<std::shared_mutex> lock(resizeLock);

    if (new_max_elements < cur_element_count)
      throw IndexCannotBeShrunkError(
          "Cannot resize to " + std::to_string(new_max_elements) +
          " elements, as this index already contains " +
          std::to_string(cur_element_count) + " elements.");

    delete visited_list_pool_;
    visited_list_pool_ = new VisitedListPool(1, new_max_elements);

    element_levels_.resize(new_max_elements);

    std::vector<std::mutex>(new_max_elements).swap(link_list_locks_);

    // Reallocate base layer
    char *data_level0_memory_new = (char *)realloc(
        data_level0_memory_, new_max_elements * size_data_per_element_);
    if (data_level0_memory_new == nullptr)
      throw std::runtime_error(
          "Not enough memory: resizeIndex failed to allocate base layer");
    data_level0_memory_ = data_level0_memory_new;

    // Reallocate all other layers
    char **linkLists_new =
        (char **)realloc(linkLists_, sizeof(void *) * new_max_elements);
    if (linkLists_new == nullptr)
      throw std::runtime_error(
          "Not enough memory: resizeIndex failed to allocate other layers");
    linkLists_ = linkLists_new;

    max_elements_ = new_max_elements;
  }

  void saveIndex(const std::string &filename) {
    saveIndex(std::make_shared<FileOutputStream>(filename));
  }

  void saveIndex(std::shared_ptr<OutputStream> output) {
    writeBinaryPOD(output, offsetLevel0_);
    writeBinaryPOD(output, max_elements_);
    writeBinaryPOD(output, cur_element_count);
    writeBinaryPOD(output, size_data_per_element_);
    writeBinaryPOD(output, label_offset_);
    writeBinaryPOD(output, offsetData_);
    writeBinaryPOD(output, maxlevel_);
    writeBinaryPOD(output, enterpoint_node_);
    writeBinaryPOD(output, maxM_);

    writeBinaryPOD(output, maxM0_);
    writeBinaryPOD(output, M_);
    writeBinaryPOD(output, mult_);
    writeBinaryPOD(output, ef_construction_);

    output->write(data_level0_memory_,
                  cur_element_count * size_data_per_element_);

    for (size_t i = 0; i < cur_element_count; i++) {
      unsigned int linkListSize =
          element_levels_[i] > 0 ? size_links_per_element_ * element_levels_[i]
                                 : 0;
      writeBinaryPOD(output, linkListSize);
      if (linkListSize)
        output->write(linkLists_[i], linkListSize);
    }
  }

  void loadIndex(std::shared_ptr<InputStream> inputStream,
                 Space<dist_t, data_t> *s, size_t max_elements_i = 0) {
    size_t totalFileSize = 0;
    if (inputStream->isSeekable()) {
      totalFileSize = inputStream->getTotalLength();
    }
    readBinaryPOD(inputStream, offsetLevel0_);
    if (totalFileSize > 0 && offsetLevel0_ > totalFileSize) {
      throw std::domain_error("Index appears to contain corrupted data; level "
                              "0 offset parameter (" +
                              std::to_string(offsetLevel0_) +
                              ") exceeded size of index file (" +
                              std::to_string(totalFileSize) + ").");
    }

    readBinaryPOD(inputStream, max_elements_);
    readBinaryPOD(inputStream, cur_element_count);

    size_t max_elements = max_elements_i;
    if (max_elements < cur_element_count)
      max_elements = max_elements_;
    max_elements_ = max_elements;
    readBinaryPOD(inputStream, size_data_per_element_);
    readBinaryPOD(inputStream, label_offset_);
    readBinaryPOD(inputStream, offsetData_);
    readBinaryPOD(inputStream, maxlevel_);
    readBinaryPOD(inputStream, enterpoint_node_);

    if (enterpoint_node_ >= cur_element_count) {
      throw std::runtime_error(
          "Index seems to be corrupted or unsupported. "
          "Entry point into HNSW data structure was at element index " +
          std::to_string(enterpoint_node_) + ", but only " +
          std::to_string(cur_element_count) +
          " elements are present in the index.");
    }

    readBinaryPOD(inputStream, maxM_);
    readBinaryPOD(inputStream, maxM0_);
    readBinaryPOD(inputStream, M_);
    readBinaryPOD(inputStream, mult_);
    readBinaryPOD(inputStream, ef_construction_);

    data_size_ = s->get_data_size();
    fstdistfunc_ = s->get_dist_func();
    dist_func_param_ = s->get_dist_func_param();

    size_links_per_element_ =
        maxM_ * sizeof(tableint) + sizeof(linklistsizeint);

    size_links_level0_ = maxM0_ * sizeof(tableint) + sizeof(linklistsizeint);

    size_t expected_size_per_element =
        size_links_level0_ + data_size_ + sizeof(labeltype);
    if (size_data_per_element_ != expected_size_per_element) {
      throw std::domain_error(
          "Storage data type does not match the index data being loaded; "
          "expected " +
          std::to_string(expected_size_per_element) +
          " bytes per element, but loaded data contains " +
          std::to_string(size_data_per_element_) +
          " bytes per element. Data being loaded might not be a Voyager index, "
          "may be corrupt, or may be using a different storage data type.");
    }

    long long position = inputStream->getPosition();

    if (inputStream->isSeekable()) {
      inputStream->advanceBy(cur_element_count * size_data_per_element_);
      for (size_t i = 0; i < cur_element_count; i++) {
        if (inputStream->getPosition() < 0 ||
            inputStream->getPosition() >= (long long)totalFileSize) {
          throw std::runtime_error(
              "Index seems to be corrupted or unsupported. Seeked to " +
              std::to_string(position +
                             (cur_element_count * size_data_per_element_) +
                             (sizeof(unsigned int) * i)) +
              " bytes to read linked list, but resulting stream position was " +
              std::to_string(inputStream->getPosition()) +
              " (of total file size " + std::to_string(totalFileSize) +
              " bytes).");
        }

        unsigned int linkListSize;
        readBinaryPOD(inputStream, linkListSize);
        if (linkListSize != 0) {
          if ((size_t)inputStream->getPosition() + linkListSize >
              totalFileSize) {
            throw std::runtime_error(
                "Index seems to be corrupted or unsupported. Advancing to the "
                "next linked list requires " +
                std::to_string(linkListSize) +
                " additional bytes (from position " +
                std::to_string(inputStream->getPosition()) +
                "), but index data only has " + std::to_string(totalFileSize) +
                " bytes in total.");
          }
          inputStream->advanceBy(linkListSize);
        }
      }

      if (inputStream->getPosition() != (long long)totalFileSize)
        throw std::runtime_error(
            "Index seems to be corrupted or unsupported. After reading all "
            "linked lists, extra data remained at the end of the index.");

      inputStream->setPosition(position);
    }

    data_level0_memory_ = (char *)malloc(max_elements * size_data_per_element_);
    if (data_level0_memory_ == nullptr) {
      throw std::runtime_error(
          "Not enough memory: loadIndex failed to allocate level0 (" +
          std::to_string(max_elements * size_data_per_element_) + " bytes)");
    }

    {
      size_t bytes_to_read = cur_element_count * size_data_per_element_;
      size_t bytes_read = inputStream->read(data_level0_memory_, bytes_to_read);
      if (bytes_read != bytes_to_read) {
        throw std::runtime_error("Tried to read " +
                                 std::to_string(bytes_to_read) +
                                 " bytes from stream, but only received " +
                                 std::to_string(bytes_read) + " bytes!");
      }
    }

    linkLists_ = (char **)malloc(sizeof(void *) * max_elements);
    if (linkLists_ == nullptr)
      throw std::runtime_error(
          "Not enough memory: loadIndex failed to allocate linklists (" +
          std::to_string(sizeof(void *) * max_elements) + " bytes)");

    size_t linkListBufferSize = sizeof(void *) * max_elements;
    std::vector<char> linkListBuffer(linkListBufferSize);
    {
      size_t bytes_read = 0;

      while (true) {
        long long bytes_to_read = linkListBuffer.size() - bytes_read;

        long long bytes_read_this_iteration = inputStream->read(
            linkListBuffer.data() + bytes_read, bytes_to_read);

        if (bytes_read_this_iteration > 0) {
          bytes_read += bytes_read_this_iteration;
        }

        if (bytes_read_this_iteration == bytes_to_read) {
          // The link list data will usually be smaller than the buffer we've
          // allocated, but in case it's not, enlarge the buffer and keep
          // reading:
          try {
            linkListBuffer.resize(linkListBuffer.size() * 2);
          } catch (std::exception const &e) {
            throw std::runtime_error(
                "Failed to resize linked list buffer to "
                "double its previous size (from " +
                std::to_string(linkListBuffer.size()) + " to " +
                std::to_string(linkListBuffer.size() * 2) + ")");
          }
        } else {
          // We've hit the end of the stream (as we read fewer bytes than asked
          // for) so stop reading.
          try {
            linkListBuffer.resize(bytes_read);
          } catch (std::exception const &e) {
            throw std::runtime_error("Failed to resize linked list buffer to "
                                     "the number of bytes read (" +
                                     std::to_string(bytes_read) + ")");
          }
          break;
        }
      }
    }

    if (!search_only_) {
      std::vector<std::mutex>(max_elements).swap(link_list_locks_);
      std::vector<std::mutex>(max_update_element_locks)
          .swap(link_list_update_locks_);
    }

    visited_list_pool_ = new VisitedListPool(1, max_elements);

    element_levels_ = std::vector<int>(max_elements);
    revSize_ = 1.0 / mult_;
    ef_ = 10;

    size_t indexInLinkListBuffer = 0;
    for (size_t i = 0; i < cur_element_count; i++) {
      if (!search_only_)
        label_lookup_[getExternalLabel(i)] = i;
      unsigned int linkListSize;

      linkListSize = *((int *)(linkListBuffer.data() + indexInLinkListBuffer));
      indexInLinkListBuffer += sizeof(int);

      if (linkListSize == 0) {
        element_levels_[i] = 0;

        linkLists_[i] = nullptr;
      } else {
        element_levels_[i] = linkListSize / size_links_per_element_;
        linkLists_[i] = (char *)malloc(linkListSize);
        if (linkLists_[i] == nullptr)
          throw std::runtime_error(
              "Not enough memory: loadIndex failed to allocate linklist");

        std::memcpy(linkLists_[i],
                    (linkListBuffer.data() + indexInLinkListBuffer),
                    linkListSize);
        indexInLinkListBuffer += linkListSize;
      }
    }

    if (enterpoint_node_ > 0 && enterpoint_node_ != (tableint)-1 &&
        !linkLists_[enterpoint_node_]) {
      throw std::runtime_error(
          "Index seems to be corrupted or unsupported. "
          "Entry point into HNSW data structure was at element index " +
          std::to_string(enterpoint_node_) +
          ", but no linked list was present at that index.");
    }

    for (size_t i = 0; i < cur_element_count; i++) {
      if (isMarkedDeleted(i))
        num_deleted_ += 1;
    }

    return;
  }

  size_t getDimensionality() { return dist_func_param_; }

  std::vector<data_t> getDataByLabel(labeltype label) const {
    if (search_only_)
      throw std::runtime_error(
          "getDataByLabel is not supported in search only mode");

    tableint label_c;
    auto search = label_lookup_.find(label);
    if (search == label_lookup_.end() || isMarkedDeleted(search->second)) {
      throw std::runtime_error("Label " + std::to_string(label) +
                               " not found in index.");
    }
    label_c = search->second;

    data_t *data_ptr = getDataByInternalId(label_c);
    size_t dim = dist_func_param_;
    std::vector<data_t> data;
    for (unsigned long i = 0; i < dim; i++) {
      data.push_back(*data_ptr);
      data_ptr += 1;
    }
    return data;
  }

  data_t *getRawDataPointerByLabel(labeltype label) {
    tableint label_c;
    auto search = label_lookup_.find(label);
    if (search == label_lookup_.end() || isMarkedDeleted(search->second)) {
      throw std::runtime_error("Label not found");
    }
    label_c = search->second;

    return (data_t *)getDataByInternalId(label_c);
  }

  dist_t getDistanceByLabels(labeltype labelA, labeltype labelB) {
    tableint internalIDA, internalIDB;
    auto search = label_lookup_.find(labelA);
    if (search == label_lookup_.end() || isMarkedDeleted(search->second)) {
      throw std::runtime_error("Label not found");
    }
    internalIDA = search->second;

    search = label_lookup_.find(labelB);
    if (search == label_lookup_.end() || isMarkedDeleted(search->second)) {
      throw std::runtime_error("Label not found");
    }
    internalIDB = search->second;

    return fstdistfunc_(getDataByInternalId(internalIDA),
                        getDataByInternalId(internalIDB), dist_func_param_);
  }

  static const unsigned char DELETE_MARK = 0x01;
  // static const unsigned char REUSE_MARK = 0x10;
  /**
   * Marks an element with the given label deleted, does NOT really change the
   * current graph.
   * @param label
   */
  void markDelete(labeltype label) {
    if (search_only_)
      throw std::runtime_error(
          "markDelete is not supported in search only mode");

    auto search = label_lookup_.find(label);
    if (search == label_lookup_.end()) {
      throw std::runtime_error("Label not found");
    }
    tableint internalId = search->second;
    markDeletedInternal(internalId);
  }

  /**
   * Uses the first 8 bits of the memory for the linked list to store the mark,
   * whereas maxM0_ has to be limited to the lower 24 bits, however, still large
   * enough in almost all cases.
   * @param internalId
   */
  void markDeletedInternal(tableint internalId) {
    assert(internalId < cur_element_count);
    if (!isMarkedDeleted(internalId)) {
      unsigned char *ll_cur = ((unsigned char *)get_linklist0(internalId)) + 2;
      *ll_cur |= DELETE_MARK;
      num_deleted_ += 1;
    } else {
      throw std::runtime_error(
          "The requested to delete element is already deleted");
    }
  }

  /**
   * Remove the deleted mark of the node, does NOT really change the current
   * graph.
   * @param label
   */
  void unmarkDelete(labeltype label) {
    auto search = label_lookup_.find(label);
    if (search == label_lookup_.end()) {
      throw std::runtime_error("Label not found");
    }
    tableint internalId = search->second;
    unmarkDeletedInternal(internalId);
  }

  /**
   * Remove the deleted mark of the node.
   * @param internalId
   */
  void unmarkDeletedInternal(tableint internalId) {
    assert(internalId < cur_element_count);
    if (isMarkedDeleted(internalId)) {
      unsigned char *ll_cur = ((unsigned char *)get_linklist0(internalId)) + 2;
      *ll_cur &= ~DELETE_MARK;
      num_deleted_ -= 1;
    } else {
      throw std::runtime_error(
          "The requested to undelete element is not deleted");
    }
  }

  /**
   * Checks the first 8 bits of the memory to see if the element is marked
   * deleted.
   * @param internalId
   * @return
   */
  bool isMarkedDeleted(tableint internalId) const {
    unsigned char *ll_cur = ((unsigned char *)get_linklist0(internalId)) + 2;
    return *ll_cur & DELETE_MARK;
  }

  unsigned short int getListCount(linklistsizeint *ptr) const {
    return *((unsigned short int *)ptr);
  }

  void setListCount(linklistsizeint *ptr, unsigned short int size) const {
    *((unsigned short int *)(ptr)) = *((unsigned short int *)&size);
  }

  void addPoint(const data_t *data_point, labeltype label) {
    if (search_only_)
      throw std::runtime_error("addPoint is not supported in search only mode");

    addPoint(data_point, label, -1);
  }

  void updatePoint(const data_t *dataPoint, tableint internalId,
                   float updateNeighborProbability) {
    // update the feature vector associated with existing point with new vector
    memcpy(getDataByInternalId(internalId), dataPoint, data_size_);

    int maxLevelCopy = maxlevel_;
    tableint entryPointCopy = enterpoint_node_;
    // If point to be updated is entry point and graph just contains single
    // element then just return.
    if (entryPointCopy == internalId && cur_element_count == 1)
      return;

    int elemLevel = element_levels_[internalId];
    std::uniform_real_distribution<float> distribution(0.0, 1.0);
    for (int layer = 0; layer <= elemLevel; layer++) {
      std::unordered_set<tableint> sCand;
      std::unordered_set<tableint> sNeigh;
      std::vector<tableint> listOneHop =
          getConnectionsWithLock(internalId, layer);
      if (listOneHop.size() == 0)
        continue;

      sCand.insert(internalId);

      for (auto &&elOneHop : listOneHop) {
        sCand.insert(elOneHop);

        if (distribution(update_probability_generator_) >
            updateNeighborProbability)
          continue;

        sNeigh.insert(elOneHop);

        std::vector<tableint> listTwoHop =
            getConnectionsWithLock(elOneHop, layer);
        for (auto &&elTwoHop : listTwoHop) {
          sCand.insert(elTwoHop);
        }
      }

      for (auto &&neigh : sNeigh) {
        // if (neigh == internalId)
        //     continue;

        std::priority_queue<std::pair<dist_t, tableint>,
                            std::vector<std::pair<dist_t, tableint>>,
                            CompareByFirst>
            candidates;
        size_t size =
            sCand.find(neigh) == sCand.end()
                ? sCand.size()
                : sCand.size() - 1; // sCand guaranteed to have size >= 1
        size_t elementsToKeep = std::min(ef_construction_, size);
        for (auto &&cand : sCand) {
          if (cand == neigh)
            continue;

          dist_t distance =
              fstdistfunc_(getDataByInternalId(neigh),
                           getDataByInternalId(cand), dist_func_param_);
          if (candidates.size() < elementsToKeep) {
            candidates.emplace(distance, cand);
          } else {
            if (distance < candidates.top().first) {
              candidates.pop();
              candidates.emplace(distance, cand);
            }
          }
        }

        // Retrieve neighbours using heuristic and set connections.
        getNeighborsByHeuristic2(candidates, layer == 0 ? maxM0_ : maxM_);

        {
          std::unique_lock<std::mutex> lock(link_list_locks_[neigh]);
          linklistsizeint *ll_cur;
          ll_cur = get_linklist_at_level(neigh, layer);
          size_t candSize = candidates.size();
          setListCount(ll_cur, candSize);
          tableint *data = (tableint *)(ll_cur + 1);
          for (size_t idx = 0; idx < candSize; idx++) {
            data[idx] = candidates.top().second;
            candidates.pop();
          }
        }
      }
    }

    repairConnectionsForUpdate(dataPoint, entryPointCopy, internalId, elemLevel,
                               maxLevelCopy);
  };

  void repairConnectionsForUpdate(const data_t *dataPoint,
                                  tableint entryPointInternalId,
                                  tableint dataPointInternalId,
                                  int dataPointLevel, int maxLevel) {
    tableint currObj = entryPointInternalId;
    if (dataPointLevel < maxLevel) {
      dist_t curdist = fstdistfunc_(dataPoint, getDataByInternalId(currObj),
                                    dist_func_param_);
      for (int level = maxLevel; level > dataPointLevel; level--) {
        bool changed = true;
        while (changed) {
          changed = false;
          unsigned int *data;
          std::unique_lock<std::mutex> lock(link_list_locks_[currObj]);
          data = get_linklist_at_level(currObj, level);
          int size = getListCount(data);
          tableint *datal = (tableint *)(data + 1);
          for (int i = 0; i < size; i++) {
            tableint cand = datal[i];
            dist_t d = fstdistfunc_(dataPoint, getDataByInternalId(cand),
                                    dist_func_param_);
            if (d < curdist) {
              curdist = d;
              currObj = cand;
              changed = true;
            }
          }
        }
      }
    }

    if (dataPointLevel > maxLevel)
      throw std::runtime_error(
          "Level of item to be updated cannot be bigger than max level");

    for (int level = dataPointLevel; level >= 0; level--) {
      std::priority_queue<std::pair<dist_t, tableint>,
                          std::vector<std::pair<dist_t, tableint>>,
                          CompareByFirst>
          topCandidates = searchBaseLayer(currObj, dataPoint, level);

      std::priority_queue<std::pair<dist_t, tableint>,
                          std::vector<std::pair<dist_t, tableint>>,
                          CompareByFirst>
          filteredTopCandidates;
      while (topCandidates.size() > 0) {
        if (topCandidates.top().second != dataPointInternalId)
          filteredTopCandidates.push(topCandidates.top());

        topCandidates.pop();
      }

      // Since element_levels_ is being used to get `dataPointLevel`, there
      // could be cases where `topCandidates` could just contains entry point
      // itself. To prevent self loops, the `topCandidates` is filtered and thus
      // can be empty.
      if (filteredTopCandidates.size() > 0) {
        bool epDeleted = isMarkedDeleted(entryPointInternalId);
        if (epDeleted) {
          filteredTopCandidates.emplace(
              fstdistfunc_(dataPoint, getDataByInternalId(entryPointInternalId),
                           dist_func_param_),
              entryPointInternalId);
          if (filteredTopCandidates.size() > ef_construction_)
            filteredTopCandidates.pop();
        }

        currObj = mutuallyConnectNewElement(dataPoint, dataPointInternalId,
                                            filteredTopCandidates, level, true);
      }
    }
  }

  std::vector<tableint> getConnectionsWithLock(tableint internalId, int level) {
    std::unique_lock<std::mutex> lock(link_list_locks_[internalId]);
    unsigned int *data = get_linklist_at_level(internalId, level);
    int size = getListCount(data);
    std::vector<tableint> result(size);
    tableint *ll = (tableint *)(data + 1);
    memcpy(result.data(), ll, size * sizeof(tableint));
    return result;
  };

  tableint addPoint(const data_t *data_point, labeltype label, int level) {
    std::shared_lock<std::shared_mutex> lock(resizeLock);
    tableint cur_c = 0;
    {
      // Checking if the element with the same label already exists
      // if so, updating it *instead* of creating a new element.
      std::unique_lock<std::mutex> templock_curr(cur_element_count_guard_);
      auto search = label_lookup_.find(label);
      if (search != label_lookup_.end()) {
        tableint existingInternalId = search->second;
        templock_curr.unlock();

        std::unique_lock<std::mutex> lock_el_update(link_list_update_locks_[(
            existingInternalId & (max_update_element_locks - 1))]);

        if (isMarkedDeleted(existingInternalId)) {
          unmarkDeletedInternal(existingInternalId);
        }
        updatePoint(data_point, existingInternalId, 1.0);

        return existingInternalId;
      }

      if (cur_element_count >= max_elements_) {
        throw IndexFullError(
            "Cannot insert elements; this index already contains " +
            std::to_string(cur_element_count) +
            " elements, and its maximum size is " +
            std::to_string(max_elements_) +
            ". Call resizeIndex first to increase the maximum size of the "
            "index.");
      };

      cur_c = cur_element_count;
      cur_element_count++;
      label_lookup_[label] = cur_c;
    }

    // Take update lock to prevent race conditions on an element with
    // insertion/update at the same time.
    std::unique_lock<std::mutex> lock_el_update(
        link_list_update_locks_[(cur_c & (max_update_element_locks - 1))]);
    std::unique_lock<std::mutex> lock_el(link_list_locks_[cur_c]);
    int curlevel = getRandomLevel(mult_);
    if (level > 0)
      curlevel = level;

    element_levels_[cur_c] = curlevel;

    std::unique_lock<std::mutex> templock(global);
    int maxlevelcopy = maxlevel_;
    if (curlevel <= maxlevelcopy)
      templock.unlock();
    tableint currObj = enterpoint_node_;
    tableint enterpoint_copy = enterpoint_node_;

    memset(data_level0_memory_ + cur_c * size_data_per_element_ + offsetLevel0_,
           0, size_data_per_element_);

    // Initialisation of the data and label
    memcpy(getExternalLabeLp(cur_c), &label, sizeof(labeltype));
    memcpy(getDataByInternalId(cur_c), data_point, data_size_);

    if (curlevel) {
      linkLists_[cur_c] =
          (char *)malloc(size_links_per_element_ * curlevel + 1);
      if (linkLists_[cur_c] == nullptr)
        throw std::runtime_error(
            "Not enough memory: addPoint failed to allocate linklist");
      memset(linkLists_[cur_c], 0, size_links_per_element_ * curlevel + 1);
    }

    if ((signed)currObj != -1) {

      if (curlevel < maxlevelcopy) {

        dist_t curdist = fstdistfunc_(data_point, getDataByInternalId(currObj),
                                      dist_func_param_);
        for (int level = maxlevelcopy; level > curlevel; level--) {

          bool changed = true;
          while (changed) {
            changed = false;
            unsigned int *data;
            std::unique_lock<std::mutex> lock(link_list_locks_[currObj]);
            data = get_linklist(currObj, level);
            int size = getListCount(data);

            tableint *datal = (tableint *)(data + 1);
            for (int i = 0; i < size; i++) {
              tableint cand = datal[i];
              if (cand < 0 || cand > max_elements_)
                throw std::runtime_error("cand error");
              dist_t d = fstdistfunc_(data_point, getDataByInternalId(cand),
                                      dist_func_param_);
              if (d < curdist) {
                curdist = d;
                currObj = cand;
                changed = true;
              }
            }
          }
        }
      }

      bool epDeleted = isMarkedDeleted(enterpoint_copy);
      for (int level = std::min(curlevel, maxlevelcopy); level >= 0; level--) {
        if (level > maxlevelcopy || level < 0) // possible?
          throw std::runtime_error("Level error");

        std::priority_queue<std::pair<dist_t, tableint>,
                            std::vector<std::pair<dist_t, tableint>>,
                            CompareByFirst>
            top_candidates = searchBaseLayer(currObj, data_point, level);
        if (epDeleted) {
          top_candidates.emplace(
              fstdistfunc_(data_point, getDataByInternalId(enterpoint_copy),
                           dist_func_param_),
              enterpoint_copy);
          if (top_candidates.size() > ef_construction_)
            top_candidates.pop();
        }
        currObj = mutuallyConnectNewElement(data_point, cur_c, top_candidates,
                                            level, false);
      }

    } else {
      // Do nothing for the first element
      enterpoint_node_ = 0;
      maxlevel_ = curlevel;
    }

    // Releasing lock for the maximum level
    if (curlevel > maxlevelcopy) {
      enterpoint_node_ = cur_c;
      maxlevel_ = curlevel;
    }
    return cur_c;
  };

  std::priority_queue<std::pair<dist_t, labeltype>>
  searchKnn(const data_t *query_data, size_t k, VisitedList *vl = nullptr,
            long queryEf = -1) {
    std::shared_lock<std::shared_mutex> lock(resizeLock);
    std::priority_queue<std::pair<dist_t, labeltype>> result;
    if (cur_element_count == 0)
      return result;

    tableint currObj = enterpoint_node_;
    dist_t curdist = fstdistfunc_(
        query_data, getDataByInternalId(enterpoint_node_), dist_func_param_);

    for (int level = maxlevel_; level > 0; level--) {
      bool changed = true;
      while (changed) {
        changed = false;
        unsigned int *data;

        data = (unsigned int *)get_linklist(currObj, level);
        int size = getListCount(data);
        metric_hops++;
        metric_distance_computations += size;

        tableint *datal = (tableint *)(data + 1);
        for (int i = 0; i < size; i++) {
          tableint cand = datal[i];
          if (cand < 0 || cand > max_elements_)
            throw std::runtime_error("cand error");
          dist_t d = fstdistfunc_(query_data, getDataByInternalId(cand),
                                  dist_func_param_);

          if (d < curdist) {
            curdist = d;
            currObj = cand;
            changed = true;
          }
        }
      }
    }

    std::priority_queue<std::pair<dist_t, tableint>,
                        std::vector<std::pair<dist_t, tableint>>,
                        CompareByFirst>
        top_candidates;
    size_t effective_ef = queryEf > 0 ? queryEf : ef_;
    if (num_deleted_) {
      top_candidates = searchBaseLayerST<true, true>(
          currObj, query_data, std::max(effective_ef, k), vl);
    } else {
      top_candidates = searchBaseLayerST<false, true>(
          currObj, query_data, std::max(effective_ef, k), vl);
    }

    while (top_candidates.size() > k) {
      top_candidates.pop();
    }
    while (top_candidates.size() > 0) {
      std::pair<dist_t, tableint> rez = top_candidates.top();
      result.push(std::pair<dist_t, labeltype>(rez.first,
                                               getExternalLabel(rez.second)));
      top_candidates.pop();
    }
    return result;
  };

  void checkIntegrity() {
    int connections_checked = 0;
    std::vector<int> inbound_connections_num(cur_element_count, 0);
    for (int i = 0; i < cur_element_count; i++) {
      for (int l = 0; l <= element_levels_[i]; l++) {
        linklistsizeint *ll_cur = get_linklist_at_level(i, l);
        int size = getListCount(ll_cur);
        tableint *data = (tableint *)(ll_cur + 1);
        std::unordered_set<tableint> s;
        for (int j = 0; j < size; j++) {
          assert(data[j] > 0);
          assert(data[j] < cur_element_count);
          assert(data[j] != i);
          inbound_connections_num[data[j]]++;
          s.insert(data[j]);
          connections_checked++;
        }
        assert(s.size() == size);
      }
    }
    if (cur_element_count > 1) {
      int min1 = inbound_connections_num[0], max1 = inbound_connections_num[0];
      for (int i = 0; i < cur_element_count; i++) {
        assert(inbound_connections_num[i] > 0);
        min1 = std::min(inbound_connections_num[i], min1);
        max1 = std::max(inbound_connections_num[i], max1);
      }
      std::cout << "Min inbound: " << min1 << ", Max inbound:" << max1 << "\n";
    }
    std::cout << "integrity ok, checked " << connections_checked
              << " connections\n";
  }
};

} // namespace hnswlib