text: //
  // Copyright Aliaksei Levin (levlam@telegram.org), Arseny Smirnov (arseny30@gmail.com) 2014-2021
  //
  // Distributed under the Boost Software License, Version 1.0. (See accompanying
  // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
  //
  #pragma once
  
  #include "td/utils/common.h"
  
  #include <limits>
  #include <map>
  #include <tuple>
  
  namespace td {
  
  template <class ValueT>
  class Enumerator {
   public:
    using Key = int32;
  
    Key add(ValueT v) {
      CHECK(arr_.size() < static_cast<size_t>(std::numeric_limits<int32>::max() - 1));
      int32 next_id = static_cast<int32>(arr_.size() + 1);
      bool was_inserted;
      decltype(map_.begin()) it;
      std::tie(it, was_inserted) = map_.emplace(std::move(v), next_id);
      if (was_inserted) {
        arr_.push_back(&it->first);
      }
      return it->second;
    }
  
    const ValueT &get(Key key) const {
      auto pos = static_cast<size_t>(key - 1);
      CHECK(pos < arr_.size());
      return *arr_[pos];
    }
  
   private:
    std::map<ValueT, int32> map_;
    std::vector<const ValueT *> arr_;
  };
  
  }  // namespace td
  