#pragma once

#include <functional>

#include <memory>

#include "envoy/buffer/buffer.h"

#include "envoy/common/conn_pool.h"

#include "envoy/common/pure.h"

#include "envoy/event/deferred_deletable.h"

#include "envoy/upstream/upstream.h"

namespace Envoy {

namespace Tcp {

namespace ConnectionPool {

/*

 * UpstreamCallbacks for connection pool upstream connection callbacks and data. Note that

 * onEvent(Connected) is never triggered since the event always occurs before a ConnectionPool

 * caller is assigned a connection.

 */

class UpstreamCallbacks : public Network::ConnectionCallbacks {

public:

  /*

   * Invoked when data is delivered from the upstream connection while the connection is owned by a

   * ConnectionPool::Instance caller.

   * @param data supplies data from the upstream

   * @param end_stream whether the data is the last data frame

   */

  virtual void onUpstreamData(Buffer::Instance& data, bool end_stream) PURE;

};

/**

 * ConnectionState is a base class for connection state maintained across requests. For example, a

 * protocol may maintain a connection-specific request sequence number or negotiate options that

 * affect the behavior of requests for the duration of the connection. A ConnectionState subclass

 * is assigned to the ConnectionData to track this state when the connection is returned to the

 * pool so that the state is available when the connection is re-used for a subsequent request.

 * The ConnectionState assigned to a connection is automatically destroyed when the connection is

 * closed.

 */

class ConnectionState {

public:

};

using ConnectionStatePtr = std::unique_ptr<ConnectionState>;

/*

 * ConnectionData wraps a ClientConnection allocated to a caller. Open ClientConnections are

 * released back to the pool for re-use when their containing ConnectionData is destroyed.

 */

class ConnectionData {

public:

  /**

   * @return the ClientConnection for the connection.

   */

  virtual Network::ClientConnection& connection() PURE;

  /**

   * Sets the ConnectionState for this connection. Any existing ConnectionState is destroyed.

   * @param ConnectionStatePtr&& new ConnectionState for this connection.

   */

  virtual void setConnectionState(ConnectionStatePtr&& state) PURE;

  /**

   * @return T* the current ConnectionState or nullptr if no state is set or if the state's type

   *            is not T.

   */

  /**

   * Sets the ConnectionPool::UpstreamCallbacks for the connection. If no callback is attached,

   * data from the upstream will cause the connection to be closed. Callbacks cease when the

   * connection is released.

   * @param callback the UpstreamCallbacks to invoke for upstream data

   */

  virtual void addUpstreamCallbacks(ConnectionPool::UpstreamCallbacks& callback) PURE;

protected:

  /**

   * @return ConnectionState* pointer to the current ConnectionState or nullptr if not set

   */

  virtual ConnectionState* connectionState() PURE;

};

using ConnectionDataPtr = std::unique_ptr<ConnectionData>;

using PoolFailureReason = ::Envoy::ConnectionPool::PoolFailureReason;

using Cancellable = ::Envoy::ConnectionPool::Cancellable;

using CancelPolicy = ::Envoy::ConnectionPool::CancelPolicy;

/**

 * Pool callbacks invoked in the context of a newConnection() call, either synchronously or

 * asynchronously.

 */

class Callbacks {

public:

  /**

   * Called when a pool error occurred and no connection could be acquired for making the request.

   * @param reason supplies the failure reason.

   * @param transport_failure_reason supplies the details of the transport failure reason.

   * @param host supplies the description of the host that caused the failure. This may be nullptr

   *             if no host was involved in the failure (for example overflow).

   */

  virtual void onPoolFailure(PoolFailureReason reason, absl::string_view transport_failure_reason,

                             Upstream::HostDescriptionConstSharedPtr host) PURE;

  /**

   * Called when a connection is available to process a request/response. Connections may be

   * released back to the pool for re-use by resetting the ConnectionDataPtr. If the connection is

   * no longer viable for reuse (e.g. due to some kind of protocol error), the underlying

   * ClientConnection should be closed to prevent its reuse.

   *

   * @param conn supplies the connection data to use.

   * @param host supplies the description of the host that will carry the request. For logical

   *             connection pools the description may be different each time this is called.

   */

  virtual void onPoolReady(ConnectionDataPtr&& conn,

                           Upstream::HostDescriptionConstSharedPtr host) PURE;

};

/**

 * An instance of a generic connection pool.

 */

class Instance : public Envoy::ConnectionPool::Instance, public Event::DeferredDeletable {

public:

  /**

   * Immediately close all existing connection pool connections. This method can be used in cases

   * where the connection pool is not being destroyed, but the caller wishes to terminate all

   * existing connections. For example, when a health check failure occurs.

   */

  virtual void closeConnections() PURE;

  /**

   * Create a new connection on the pool.

   * @param cb supplies the callbacks to invoke when the connection is ready or has failed. The

   *           callbacks may be invoked immediately within the context of this call if there is a

   *           ready connection or an immediate failure. In this case, the routine returns nullptr.

   * @return Cancellable* If no connection is ready, the callback is not invoked, and a handle

   *                      is returned that can be used to cancel the request. Otherwise, one of the

   *                      callbacks is called and the routine returns nullptr. NOTE: Once a callback

   *                      is called, the handle is no longer valid and any further cancellation

   *                      should be done by resetting the connection.

   */

  virtual Cancellable* newConnection(Callbacks& callbacks) PURE;

};

using InstancePtr = std::unique_ptr<Instance>;

} // namespace ConnectionPool

} // namespace Tcp

} // namespace Envoy