text: //
  // Copyright Aliaksei Levin (levlam@telegram.org), Arseny Smirnov (arseny30@gmail.com) 2014-2019
  //
  // 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/Span.h"
  
  #include <utility>
  #include <vector>
  
  namespace td {
  
  template <class T>
  class VectorQueue {
   public:
    template <class S>
    void push(S &&s) {
      vector_.push_back(std::forward<S>(s));
    }
    T pop() {
      try_shrink();
      return std::move(vector_[read_pos_++]);
    }
    void pop_n(size_t n) {
      read_pos_ += n;
      try_shrink();
    }
    T &front() {
      return vector_[read_pos_];
    }
    T &back() {
      return vector_.back();
    }
    bool empty() const {
      return size() == 0;
    }
    size_t size() const {
      return vector_.size() - read_pos_;
    }
    T *data() {
      return vector_.data() + read_pos_;
    }
    const T *data() const {
      return vector_.data() + read_pos_;
    }
    Span<T> as_span() const {
      return {data(), size()};
    }
    MutableSpan<T> as_mutable_span() {
      return {data(), size()};
    }
  
   private:
    std::vector<T> vector_;
    size_t read_pos_{0};
  
    void try_shrink() {
      if (read_pos_ * 2 > vector_.size() && read_pos_ > 4) {
        vector_.erase(vector_.begin(), vector_.begin() + read_pos_);
        read_pos_ = 0;
      }
    }
  };
  
  }  // namespace td
  