#pragma once

#include <chrono>

#include "envoy/event/dispatcher.h"

#include "envoy/event/timer.h"

namespace Envoy {

namespace Config {

/**

 * Class for managing TTL expiration of xDS resources. TTLs are managed with a single timer that

 * will always be set to the time until the next TTL, ensuring that we have a constant amount of

 * timers per xDS resource type.

 *

 * We use a combination of two data structures here: a std::set that ensures that we can iterate

 * over the pending TTLs in sorted over, and a map from resource name to the set iterator which

 * allows us to efficiently clear or update TTLs. As iterator stability is required to track the

 * iterators, we use std::set over something like ``absl::btree_set``.

 *

 * As a result of these two data structures, all lookups and modifications can be performed in

 * O(log (number of TTL entries)). This comes at the cost of a higher memory overhead versus just

 * using a single data structure.

 */

class TtlManager {

public:

  TtlManager(std::function<void(const std::vector<std::string>&)> callback,

             Event::Dispatcher& dispatcher, TimeSource& time_source);

  // RAII tracker to simplify managing when we should be running the update callbacks.

  class ScopedTtlUpdate {

  public:

    ~ScopedTtlUpdate() {

      if (--parent_.scoped_update_counter_ == 0) {

        parent_.refreshTimer();

      }

    }

  private:

    ScopedTtlUpdate(TtlManager& parent) : parent_(parent) { parent_.scoped_update_counter_++; }

    friend TtlManager;

    TtlManager& parent_;

  };

  ScopedTtlUpdate scopedTtlUpdate() { return {*this}; }

  void add(std::chrono::milliseconds ttl, const std::string& name);

  void clear(const std::string& name);

private:

  void refreshTimer();

  using TtlSet = std::set<std::pair<MonotonicTime, std::string>>;

  TtlSet ttls_;

  absl::flat_hash_map<std::string, TtlSet::iterator> ttl_lookup_;

  Event::TimerPtr timer_;

  absl::optional<MonotonicTime> last_scheduled_time_;

  uint8_t scoped_update_counter_{};

  std::function<void(const std::vector<std::string>&)> callback_;

  Event::Dispatcher& dispatcher_;

  TimeSource& time_source_;

};

} // namespace Config

} // namespace Envoy