diff --git a/tdutils/CMakeLists.txt b/tdutils/CMakeLists.txt
index b53e9dd99..18f42d5d4 100644
--- a/tdutils/CMakeLists.txt
+++ b/tdutils/CMakeLists.txt
@@ -298,6 +298,7 @@ set(TDUTILS_TEST_SOURCE
   ${CMAKE_CURRENT_SOURCE_DIR}/test/filesystem.cpp
   ${CMAKE_CURRENT_SOURCE_DIR}/test/gzip.cpp
   ${CMAKE_CURRENT_SOURCE_DIR}/test/HazardPointers.cpp
+  ${CMAKE_CURRENT_SOURCE_DIR}/test/HashSet.cpp
   ${CMAKE_CURRENT_SOURCE_DIR}/test/heap.cpp
   ${CMAKE_CURRENT_SOURCE_DIR}/test/HttpUrl.cpp
   ${CMAKE_CURRENT_SOURCE_DIR}/test/json.cpp
diff --git a/tdutils/td/utils/FlatHashMap.h b/tdutils/td/utils/FlatHashMap.h
new file mode 100644
index 000000000..50466fd3f
--- /dev/null
+++ b/tdutils/td/utils/FlatHashMap.h
@@ -0,0 +1,376 @@
+#pragma once
+
+#include <unordered_map>
+#include <utility>
+#include <vector>
+
+namespace td {
+
+template <class KeyT, class ValueT, class HashT = std::hash<KeyT>, class EqualT = std::equal_to<KeyT>>
+class FlatHashMapImpl {
+  struct Node {
+    KeyT first{};
+    union {
+      ValueT second;
+    };
+    const auto &key() const {
+      return first;
+    }
+    auto &value() {
+      return second;
+    }
+
+    Node() {
+    }
+    ~Node() {
+      if (!empty()) {
+        second.~ValueT();
+      }
+    }
+    Node(Node &&other) {
+      *this = std::move(other);
+    }
+    Node &operator=(Node &&other) {
+      DCHECK(empty());
+      DCHECK(!other.empty());
+      first = std::move(other.first);
+      other.first = KeyT{};
+      new (&second) ValueT(std::move(other.second));
+      other.second.~ValueT();
+      return *this;
+    }
+    bool empty() const {
+      return is_key_empty(key());
+    }
+    void clear() {
+      DCHECK(!empty());
+      first = KeyT();
+      second.~ValueT();
+      DCHECK(empty());
+    }
+    template <class... ArgsT>
+    void emplace(KeyT key, ArgsT &&...args) {
+      DCHECK(empty());
+      first = std::move(key);
+      new (&second) ValueT(std::forward<ArgsT>(args)...);
+      CHECK(!empty());
+    }
+  };
+  using Self = FlatHashMapImpl<KeyT, ValueT, HashT, EqualT>;
+  using NodeIterator = typename std::vector<Node>::iterator;
+  using ConstNodeIterator = typename std::vector<Node>::const_iterator;
+
+ public:
+  struct Iterator {
+   public:
+    using iterator_category = std::bidirectional_iterator_tag;
+    using difference_type = std::ptrdiff_t;
+    using value_type = Node;
+    using pointer = Node *;
+    using reference = Node &;
+
+    friend class FlatHashMapImpl;
+    Iterator &operator++() {
+      do {
+        ++it_;
+      } while (it_ != map_->nodes_.end() && it_->empty());
+      return *this;
+    }
+    Iterator &operator--() {
+      do {
+        --it_;
+      } while (it_->empty());
+      return *this;
+    }
+    Node &operator*() {
+      return *it_;
+    }
+    Node *operator->() {
+      return &*it_;
+    }
+    bool operator==(const Iterator &other) const {
+      DCHECK(map_ == other.map_);
+      return it_ == other.it_;
+    }
+    bool operator!=(const Iterator &other) const {
+      DCHECK(map_ == other.map_);
+      return it_ != other.it_;
+    }
+
+    Iterator() = default;
+    Iterator(const Iterator &other) = default;
+    Iterator &operator=(const Iterator &other) = default;
+    Iterator(Iterator &&other) = default;
+    Iterator &operator=(Iterator &&other) = default;
+    Iterator(NodeIterator it, Self *map) : it_(std::move(it)), map_(map) {
+    }
+
+   private:
+    NodeIterator it_;
+    Self *map_;
+  };
+  struct ConstIterator {
+   public:
+    using iterator_category = std::bidirectional_iterator_tag;
+    using difference_type = std::ptrdiff_t;
+    using value_type = Node;
+    using pointer = const Node *;
+    using reference = const Node &;
+
+    friend class FlatHashMapImpl;
+    ConstIterator &operator++() {
+      do {
+        ++it_;
+      } while (it_ != map_->nodes_.end() && it_->empty());
+      return *this;
+    }
+    ConstIterator &operator--() {
+      do {
+        --it_;
+      } while (it_->empty());
+      return *this;
+    }
+    const Node &operator*() {
+      return *it_;
+    }
+    const Node *operator->() {
+      return &*it_;
+    }
+    bool operator==(const ConstIterator &other) const {
+      DCHECK(map_ == other.map_);
+      return it_ == other.it_;
+    }
+    bool operator!=(const ConstIterator &other) const {
+      DCHECK(map_ == other.map_);
+      return it_ != other.it_;
+    }
+
+    ConstIterator() = default;
+    ConstIterator(const ConstIterator &other) = default;
+    ConstIterator &operator=(const ConstIterator &other) = default;
+    ConstIterator(ConstIterator &&other) = default;
+    ConstIterator &operator=(ConstIterator &&other) = default;
+    ConstIterator(ConstNodeIterator it, const Self *map) : it_(std::move(it)), map_(map) {
+    }
+    ConstIterator(Iterator iterator) : it_(std::move(iterator.it_)), map_(iterator.map_) {
+    }
+
+   private:
+    ConstNodeIterator it_;
+    const Self *map_;
+  };
+
+  using iterator = Iterator;
+  using key_type = KeyT;
+  using value_type = std::pair<const KeyT, ValueT>;
+
+  FlatHashMapImpl() = default;
+  FlatHashMapImpl(FlatHashMapImpl &&other) : nodes_(std::move(other.nodes_)), used_nodes_(other.used_nodes_) {
+    other.used_nodes_ = 0;
+  }
+  FlatHashMapImpl &operator=(FlatHashMapImpl &&other) {
+    nodes_ = std::move(other.nodes_);
+    used_nodes_ = other.used_nodes_;
+    other.used_nodes_ = 0;
+    return *this;
+  }
+  template <class ItT>
+  FlatHashMapImpl(ItT begin, ItT end) {
+    assign(begin, end);
+  }
+  FlatHashMapImpl(const FlatHashMapImpl &other) : FlatHashMapImpl(other.begin(), other.end()) {
+  }
+  FlatHashMapImpl &operator=(const FlatHashMapImpl &other) {
+    assign(other.begin(), other.end());
+    return *this;
+  }
+
+  template <class ItT>
+  void assign(ItT begin, ItT end) {
+    resize(std::distance(begin, end));  // TODO: should be conditional
+    for (; begin != end; ++begin) {
+      emplace(begin->first, begin->second);
+    }
+  }
+
+  Iterator find(const KeyT &key) {
+    if (empty()) {
+      return end();
+    }
+    auto it = find_bucket_for_insert(key);
+    if (it->empty()) {
+      return end();
+    }
+    return Iterator(it, this);
+  }
+  ConstIterator find(const KeyT &key) const {
+    if (empty()) {
+      return end();
+    }
+    auto it = find_bucket_for_insert(key);
+    if (it->empty()) {
+      return end();
+    }
+    return ConstIterator(it, this);
+  }
+  size_t size() const {
+    return used_nodes_;
+  }
+  bool empty() const {
+    return size() == 0;
+  }
+  auto begin() {
+    if (empty()) {
+      return end();
+    }
+    auto it = nodes_.begin();
+    while (it->empty()) {
+      it++;
+    }
+    return Iterator(it, this);
+  }
+  auto end() {
+    return Iterator(nodes_.end(), this);
+  }
+  auto begin() const {
+    if (empty()) {
+      return end();
+    }
+    auto it = nodes_.begin();
+    while (it->empty()) {
+      it++;
+    }
+    return ConstIterator(it, this);
+  }
+  auto end() const {
+    return ConstIterator(nodes_.end(), this);
+  }
+
+  template <class... ArgsT>
+  std::pair<Iterator, bool> emplace(KeyT key, ArgsT &&...args) {
+    if (should_resize()) {
+      resize(used_nodes_ + 1);
+    }
+    auto it = find_bucket_for_insert(key);
+    if (it->empty()) {
+      it->emplace(std::move(key), std::forward<ArgsT>(args)...);
+      used_nodes_++;
+      return std::make_pair(Iterator(it, this), true);
+    }
+    return std::make_pair(Iterator(it, this), false);
+  }
+
+  ValueT &operator[](const KeyT &key) {
+    DCHECK(!is_key_empty(key));
+
+    if (should_resize()) {
+      resize(used_nodes_ + 1);
+    }
+
+    auto it = find_bucket_for_insert(key);
+    if (it->empty()) {
+      it->emplace(key);
+      used_nodes_++;
+    }
+    return it->second;
+  }
+
+  size_t erase(const KeyT &key) {
+    auto it = find(key);
+    if (it == end()) {
+      return 0;
+    }
+    erase(it);
+    return 1;
+  }
+  size_t count(const KeyT &key) const {
+    return find(key) != end();
+  }
+
+  void clear() {
+    used_nodes_ = 0;
+    nodes_.clear();
+  }
+
+  void erase(Iterator it) {
+    DCHECK(it != end());
+    DCHECK(!is_key_empty(it->key()));
+    size_t empty_i = it.it_ - nodes_.begin();
+    auto empty_bucket = empty_i;
+    DCHECK(0 <= empty_i < nodes_.size());
+    nodes_[empty_bucket].clear();
+    used_nodes_--;
+
+    for (size_t test_i = empty_i + 1;; test_i++) {
+      auto test_bucket = test_i;
+      if (test_bucket >= nodes_.size()) {
+        test_bucket -= nodes_.size();
+      }
+
+      if (is_key_empty(nodes_[test_bucket].key())) {
+        break;
+      }
+
+      auto want_i = HashT()(nodes_[test_bucket].key()) % nodes_.size();
+      if (want_i < empty_i) {
+        want_i += nodes_.size();
+      }
+
+      if (want_i <= empty_i || want_i > test_i) {
+        nodes_[empty_bucket] = std::move(nodes_[test_bucket]);
+        empty_i = test_i;
+        empty_bucket = test_bucket;
+      }
+    }
+  }
+
+ private:
+  static bool is_key_empty(const KeyT &key) {
+    return key == KeyT();
+  }
+  std::vector<Node> nodes_;
+  size_t used_nodes_{};
+
+  bool should_resize() const {
+    return (used_nodes_ + 1) * 10 > nodes_.size() * 6;
+  }
+  size_t calc_bucket(const KeyT &key) const {
+    return HashT()(key) % nodes_.size();
+  }
+  auto find_bucket_for_insert(const KeyT &key) {
+    size_t bucket = calc_bucket(key);
+    while (!(nodes_[bucket].key() == key) && !is_key_empty(nodes_[bucket].key())) {
+      bucket++;
+      if (bucket == nodes_.size()) {
+        bucket = 0;
+      }
+    }
+    return nodes_.begin() + bucket;
+  }
+  auto find_bucket_for_insert(const KeyT &key) const {
+    size_t bucket = calc_bucket(key);
+    while (!EqualT()(nodes_[bucket].key(), key) && !is_key_empty(nodes_[bucket].key())) {
+      bucket++;
+      if (bucket == nodes_.size()) {
+        bucket = 0;
+      }
+    }
+    return nodes_.begin() + bucket;
+  }
+  void resize(size_t size) {
+    auto old_nodes = std::move(nodes_);
+    nodes_.resize(td::max(old_nodes.size(), size) * 2 + 1);  // TODO: some other logic
+    for (auto &node : old_nodes) {
+      if (is_key_empty(node.key())) {
+        continue;
+      }
+      *find_bucket_for_insert(node.key()) = std::move(node);
+    }
+  }
+};
+//template <class KeyT, class ValueT, class HashT = std::hash<KeyT>, class EqualT = std::equal_to<KeyT>>
+//using FlatHashMap = FlatHashMapImpl<KeyT, ValueT, HashT, EqualT>;
+template <class KeyT, class ValueT, class HashT = std::hash<KeyT>, class EqualT = std::equal_to<KeyT>>
+using FlatHashMap = std::unordered_map<KeyT, ValueT, HashT, EqualT>;
+
+}  // namespace td
diff --git a/tdutils/test/HashSet.cpp b/tdutils/test/HashSet.cpp
new file mode 100644
index 000000000..f8572b2e4
--- /dev/null
+++ b/tdutils/test/HashSet.cpp
@@ -0,0 +1,25 @@
+#include "td/utils/tests.h"
+#include "td/utils/FlatHashMap.h"
+#include <array>
+#include <string>
+
+TEST(FlatHashMap, basic) {
+  td::FlatHashMap<int, int> map;
+  map[1] = 2;
+  ASSERT_EQ(2, map[1]);
+  ASSERT_EQ(1, map.find(1)->first);
+  ASSERT_EQ(2, map.find(1)->second);
+  // ASSERT_EQ(1, map.find(1)->key());
+  // ASSERT_EQ(2, map.find(1)->value());
+  for (auto &kv : map) {
+    ASSERT_EQ(1, kv.first);
+    ASSERT_EQ(2, kv.second);
+  }
+  map.erase(map.find(1));
+  auto map_copy = map;
+
+  td::FlatHashMap<int, std::array<std::unique_ptr<std::string>, 20>> x;
+  auto y = std::move(x);
+  x[12];
+  x.erase(x.find(12));
+}
\ No newline at end of file