dns_srv_data.h

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#ifndef DNS_SRV_DATA_H
#define DNS_SRV_DATA_H
 
#include <assert.h>
#include <cstdlib>
#include <list>
#include <map>
#include <string>
 
/**
 A RFC2782 compliant SRV records storage
 
 Stores host/port/weight/priority entries
 into a data structure that then allows retrieving
 these via the Dns_srv_data::pop_next() method.
 
 Entries can be stored by calling Dns_srv_data::add()
 in any order.
 
 This usage pattern is roughly as follows:
 1. Dns_srv_data construct
 2. one or more Dns_srv_data::add()
 3. one or more Dns_srv_data::pop_next()
 4. Dns_srv_data destructor
 
 @sa mysql_real_connect_dns_srv
*/
class Dns_srv_data {
  class Dns_entry {
    std::string host_;
    unsigned port_{0}, weight_{0};
    unsigned long weight_sum_{0};
 
    Dns_entry() = delete;  // disable copy constructor
 
   public:
    Dns_entry(const std::string &host, unsigned port, unsigned weight)
        : host_(host), port_(port), weight_(weight) {}
 
    unsigned port() const { return port_; }
    std::string host() const { return host_; }
    unsigned long weight_sum() const { return weight_sum_; }
    void add_weight_sum(unsigned long &weight_sum) {
      weight_sum_ = (weight_sum += weight_);
    }
  };
  using dns_entry_list_t = std::list<Dns_entry>;
  using dns_entry_data_t = std::map<unsigned, Dns_srv_data::dns_entry_list_t>;
  dns_entry_data_t data_;
 
 public:
  void clear() { data_.clear(); }
  void add(const std::string &host, unsigned port, unsigned priority,
           unsigned weight) {
    const dns_entry_data_t::iterator list = data_.find(priority);
    if (list == data_.cend())
      data_.emplace(priority,
                    dns_entry_list_t(1, Dns_entry(host, port, weight)));
    else {
      // RFC2782: put the 0 weight at the front, rest at the back
      if (weight > 0)
        list->second.emplace_back(host, port, weight);
      else
        list->second.emplace_front(host, port, weight);
    }
  }
  bool pop_next(std::string &host, unsigned &port) {
    if (data_.empty()) return true;
 
    dns_entry_list_t &list = data_.begin()->second;
    assert(!list.empty());
 
    unsigned long sum = 0;
    for (Dns_entry &elt : list) elt.add_weight_sum(sum);
 
    const unsigned long draw = (std::rand() * 1UL * sum) / RAND_MAX;
 
    dns_entry_list_t::const_iterator iter = list.cbegin();
    while (iter->weight_sum() < draw) iter++;
    assert(iter != list.end());
 
    host = iter->host();
    port = iter->port();
 
    list.erase(iter);
    if (list.empty()) data_.erase(data_.begin());
    return false;
  }
};
 
#endif  // !DNS_SRV_DATA_H