// Copyright (c) 2022 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef QUICHE_QUIC_LOAD_BALANCER_ENCODER_H_
#define QUICHE_QUIC_LOAD_BALANCER_ENCODER_H_

#include "quic/core/crypto/quic_random.h"
#include "quic/load_balancer/load_balancer_config.h"
#include "quic/load_balancer/load_balancer_server_id.h"

namespace quic {

namespace test {
class LoadBalancerEncoderPeer;
}

// Default length of a 4-tuple connection ID.
inline constexpr uint8_t kLoadBalancerUnroutableLen = 8;

// Interface which receives notifications when the current config is updated.
class QUIC_EXPORT_PRIVATE LoadBalancerEncoderVisitorInterface {
 public:
  virtual ~LoadBalancerEncoderVisitorInterface() {}

  // Called when a config is added where none existed.
  //
  // Connections that support address migration should retire unroutable
  // connection IDs and replace them with routable ones using the new config,
  // while avoiding sending a sudden storm of packets containing
  // RETIRE_CONNECTION_ID and NEW_CONNECTION_ID frames.
  virtual void OnConfigAdded(const uint8_t config_id) = 0;
  // Called when the config is changed.
  //
  // Existing routable connection IDs should be retired before the decoder stops
  // supporting that config. The timing of this event is deployment-dependent
  // and might be tied to the arrival of a new config at the encoder.
  virtual void OnConfigChanged(const uint8_t old_config_id,
                               const uint8_t new_config_id) = 0;
  // Called when a config is deleted. The encoder will generate unroutable
  // connection IDs from now on.
  //
  // New connections will not be able to support address migration until a new
  // config arrives. Existing connections can retain connection IDs that use the
  // deleted config, which will only become unroutable once the decoder also
  // deletes it. The time of that deletion is deployment-dependent and might be
  // tied to the arrival of a new config at the encoder.
  virtual void OnConfigDeleted(const uint8_t config_id) = 0;
};

// Manages QUIC-LB configurations to properly encode a given server ID in a
// QUIC Connection ID.
class QUIC_EXPORT_PRIVATE LoadBalancerEncoder {
 public:
  // Returns a newly created encoder with no active config, if
  // |unroutable_connection_id_length| is valid. |visitor| specifies an optional
  // interface to receive callbacks when connection IDs need to be retired.
  // If |encode_length| is true, then the first byte of any generated
  // connection ids will encode the length. Otherwise, those bits will be
  // random. |unroutable_connection_id_length| specifies the length of
  // connection IDs to be generated when there is no active config. It must not
  // be 0 and must not be larger than the RFC9000 maximum of 20.
  static absl::optional<LoadBalancerEncoder> Create(
      QuicRandom& random, LoadBalancerEncoderVisitorInterface* const visitor,
      const bool len_self_encoded,
      const uint8_t unroutable_connection_id_len = kLoadBalancerUnroutableLen);

  // Attempts to replace the current config and server_id with |config| and
  // |server_id|. If the length |server_id| does not match the server_id_length
  // of |config| or the ID of |config| matches the ID of the current config,
  // returns false and leaves the current config unchanged. Otherwise, returns
  // true. When the encoder runs out of nonces, it will delete the config and
  // begin generating unroutable connection IDs.
  bool UpdateConfig(const LoadBalancerConfig& config,
                    const LoadBalancerServerId server_id);

  // Delete the current config and generate unroutable connection IDs from now
  // on.
  void DeleteConfig();

  // Returns the number of additional connections IDs that can be generated with
  // the current config, or 0 if there is no current config.
  absl::uint128 num_nonces_left() const { return num_nonces_left_; }

  // Returns true if there is an active configuration.
  bool IsEncoding() const { return config_.has_value(); }
  // Returns true if there is an active configuration that uses encryption.
  bool IsEncrypted() const {
    return config_.has_value() && config_->IsEncrypted();
  }

  // If there's an active config, generates a connection ID using it. If not,
  // generates an unroutable connection_id. If there's an error, returns a zero-
  // length Connection ID.
  QuicConnectionId GenerateConnectionId();

 private:
  friend class test::LoadBalancerEncoderPeer;

  LoadBalancerEncoder(QuicRandom& random,
                      LoadBalancerEncoderVisitorInterface* const visitor,
                      const bool len_self_encoded,
                      const uint8_t unroutable_connection_id_len)
      : random_(random),
        len_self_encoded_(len_self_encoded),
        visitor_(visitor),
        unroutable_connection_id_len_(unroutable_connection_id_len) {}

  QuicConnectionId MakeUnroutableConnectionId(uint8_t first_byte);

  QuicRandom& random_;
  const bool len_self_encoded_;
  LoadBalancerEncoderVisitorInterface* const visitor_;
  const uint8_t unroutable_connection_id_len_;

  absl::optional<LoadBalancerConfig> config_;
  absl::uint128 seed_, num_nonces_left_ = 0;
  absl::optional<LoadBalancerServerId> server_id_;
};

}  // namespace quic

#endif  // QUICHE_QUIC_LOAD_BALANCER_ENCODER_H_