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MQTT packet model and wire protocol codec that incrementally decodes and encodes MQTT streams without intermediate buffering
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// Copyright 2015-2024 Nstream, inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package swim.mqtt;

import swim.codec.Decoder;
import swim.codec.DecoderException;
import swim.codec.InputBuffer;
import swim.collections.FingerTrieSeq;

final class MqttSubAckPacketDecoder extends Decoder {

  final MqttDecoder mqtt;
  final int packetFlags;
  final int packetId;
  final FingerTrieSeq subscriptions;
  final int remaining;
  final int step;

  MqttSubAckPacketDecoder(MqttDecoder mqtt, int packetFlags, int packetId,
                          FingerTrieSeq subscriptions,
                          int remaining, int step) {
    this.mqtt = mqtt;
    this.packetFlags = packetFlags;
    this.packetId = packetId;
    this.subscriptions = subscriptions;
    this.remaining = remaining;
    this.step = step;
  }

  MqttSubAckPacketDecoder(MqttDecoder mqtt) {
    this(mqtt, 0, 0, FingerTrieSeq.empty(), 0, 1);
  }

  @Override
  public Decoder feed(InputBuffer input) {
    return MqttSubAckPacketDecoder.decode(input, this.mqtt, this.packetFlags,
                                          this.packetId, this.subscriptions,
                                          this.remaining, this.step);
  }

  static Decoder decode(InputBuffer input, MqttDecoder mqtt,
                                          int packetFlags, int packetId,
                                          FingerTrieSeq subscriptions,
                                          int remaining, int step) {
    if (step == 1 && input.isCont()) {
      packetFlags = input.head() & 0x0f;
      input = input.step();
      step = 2;
    }
    while (step >= 2 && step <= 5 && input.isCont()) {
      final int b = input.head();
      input = input.step();
      remaining |= (b & 0x7f) << (7 * (step - 2));
      if ((b & 0x80) == 0) {
        step = 6;
        break;
      } else if (step < 5) {
        step += 1;
      } else {
        return Decoder.error(new MqttException("packet length too long"));
      }
    }
    if (step == 6 && remaining > 0 && input.isCont()) {
      packetId = input.head() << 8;
      input = input.step();
      remaining -= 1;
      step = 7;
    }
    if (step == 7 && remaining > 0 && input.isCont()) {
      packetId |= input.head();
      input = input.step();
      remaining -= 1;
      if (remaining > 0) {
        step = 8;
      } else {
        step = 9;
      }
    }
    while (step == 8 && remaining > 0 && input.isCont()) {
      final int code = input.head();
      input = input.step();
      remaining -= 1;
      subscriptions = subscriptions.appended(mqtt.subStatus(code));
      if (remaining == 0) {
        step = 9;
        break;
      }
    }
    if (step == 9 && remaining == 0) {
      return Decoder.done(mqtt.subAckPacket(packetFlags, packetId, subscriptions));
    }
    if (remaining < 0) {
      return Decoder.error(new MqttException("packet length too short"));
    } else if (input.isDone()) {
      return Decoder.error(new DecoderException("incomplete"));
    } else if (input.isError()) {
      return Decoder.error(input.trap());
    }
    return new MqttSubAckPacketDecoder(mqtt, packetFlags, packetId,
                                       subscriptions, remaining, step);
  }

  static Decoder decode(InputBuffer input, MqttDecoder mqtt) {
    return MqttSubAckPacketDecoder.decode(input, mqtt, 0, 0,
                                          FingerTrieSeq.empty(), 0, 1);
  }

}