#pragma once

#include "envoy/server/admin.h"

#include "source/server/admin/stats_params.h"

#include "source/server/admin/stats_render.h"

#include "source/server/admin/utils.h"

#include "absl/container/btree_map.h"

#include "absl/types/variant.h"

namespace Envoy {

namespace Server {

// Captures context for a streaming request, implementing the AdminHandler interface.

class StatsRequest : public Admin::Request {

  using ScopeVec = std::vector<Stats::ConstScopeSharedPtr>;

  using StatOrScopes = absl::variant<ScopeVec, Stats::TextReadoutSharedPtr, Stats::CounterSharedPtr,

                                     Stats::GaugeSharedPtr, Stats::HistogramSharedPtr>;

  // Ordered to match the StatsOrScopes variant.

  enum class StatOrScopesIndex { Scopes, TextReadout, Counter, Gauge, Histogram };

  // In order to keep the output consistent with the fully buffered behavior

  // prior to the chunked implementation that buffered each type, we iterate

  // over all scopes for each type. This enables the complex chunking

  // implementation to pass the tests that capture the buffered behavior. There

  // is not a significant cost to this, but in a future PR we may choose to

  // co-mingle the types. Note that histograms are groups together in the data

  // JSON data model, so we won't be able to fully co-mingle.

  enum class Phase {

    TextReadouts,

    CountersAndGauges,

    Histograms,

  };

public:

  using UrlHandlerFn = std::function<Admin::UrlHandler()>;

  static constexpr uint64_t DefaultChunkSize = 2 * 1000 * 1000;

  StatsRequest(Stats::Store& stats, const StatsParams& params,

               const Upstream::ClusterManager& cluster_manager,

               UrlHandlerFn url_handler_fn = nullptr);

  // Admin::Request

  Http::Code start(Http::ResponseHeaderMap& response_headers) override;

  // Streams out the next chunk of stats to the client, visiting only the scopes

  // that can plausibly contribute the next set of named stats. This enables us

  // to linearly traverse the entire set of stats without buffering all of them

  // and sorting.

  //

  // Instead we keep the a set of candidate stats to emit in stat_map_ an

  // alphabetically ordered btree, which heterogeneously stores stats of all

  // types and scopes. Note that there can be multiple scopes with the same

  // name, so we keep same-named scopes in a vector. However leaf metrics cannot

  // have duplicates. It would also be feasible to use a multi-map for this.

  //

  // So in start() above, we initially populate all the scopes, as well as the

  // metrics contained in all scopes with an empty name. So in nextChunk we can

  // emit and remove the first element of stat_map_. When we encounter a vector

  // of scopes then we add the contained metrics to the map and continue

  // iterating.

  //

  // Whenever the desired chunk size is reached we end the current chunk so that

  // the current buffer can be flushed to the network. In #19898 we will

  // introduce flow-control so that we don't buffer the all the serialized stats

  // while waiting for a slow client.

  //

  // Note that we do 3 passes through all the scopes_, so that we can emit

  // text-readouts first, then the intermingled counters and gauges, and finally

  // the histograms.

  bool nextChunk(Buffer::Instance& response) override;

  // To duplicate prior behavior for this class, we do three passes over all the stats:

  //   1. text readouts across all scopes

  //   2. counters and gauges, co-mingled, across all scopes

  //   3. histograms across all scopes.

  // It would be little more efficient to co-mingle all the stats, but three

  // passes over the scopes is OK. In the future we may decide to organize the

  // result data differently, but in the process of changing from buffering

  // the entire /stats response to streaming the data out in chunks, it's easier

  // to reason about if the tests don't change their expectations.

  void startPhase();

  // Iterates over scope_vec and populates the metric types associated with the

  // current phase.

  void populateStatsForCurrentPhase(const ScopeVec& scope_vec);

  // Populates all the metrics of the templatized type from scope_vec. Here we

  // exploit that Scope::iterate is a generic templatized function to avoid code

  // duplication.

  template <class StatType> void populateStatsFromScopes(const ScopeVec& scope_vec);

  void renderPerHostMetrics(Buffer::Instance& response);

  // Renders the templatized type, exploiting the fact that Render::generate is

  // generic to avoid code duplication.

  template <class SharedStatType>

  void renderStat(const std::string& name, Buffer::Instance& response, StatOrScopes& variant);

  // Sets the chunk size.

private:

  StatsParams params_;

  std::unique_ptr<StatsRender> render_;

  Stats::Store& stats_;

  ScopeVec scopes_;

  absl::btree_map<std::string, StatOrScopes> stat_map_;

  Phase phase_{Phase::TextReadouts};

  uint64_t phase_stat_count_{0};

  absl::string_view phase_string_{"text readouts"};

  Buffer::OwnedImpl response_;

  const Upstream::ClusterManager& cluster_manager_;

  UrlHandlerFn url_handler_fn_;

  uint64_t chunk_size_{DefaultChunkSize};

};

} // namespace Server

} // namespace Envoy