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HiveMQ CE is a Java-based open source MQTT broker that fully supports MQTT 3.x and MQTT 5

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/*
 * Copyright 2019-present HiveMQ GmbH
 *
 * 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 com.hivemq.codec.encoder.mqtt3;

import com.hivemq.bootstrap.ClientConnectionContext;
import com.hivemq.codec.encoder.MqttEncoder;
import com.hivemq.extension.sdk.api.annotations.NotNull;
import com.hivemq.mqtt.message.connack.Mqtt3CONNACK;
import com.hivemq.mqtt.message.connack.Mqtt3ConnAckReturnCode;
import io.netty.buffer.ByteBuf;

/**
 * @author Dominik Obermaier
 */
public class Mqtt3ConnackEncoder implements MqttEncoder {

    private static final byte CONNACK_FIXED_HEADER = 0b0010_0000;
    private static final byte CONNACK_REMAINING_LENGTH = 0b0000_0010;
    private static final byte CONNACK_FLAGS_EMPTY = 0b0000_0000;
    private static final byte CONNACK_FLAGS_SP_SET = 0b0000_0001;
    private static final int ENCODED_CONNACK_SIZE = 4;

    @Override
    public void encode(
            final @NotNull ClientConnectionContext clientConnectionContext,
            final @NotNull Mqtt3CONNACK msg,
            final @NotNull ByteBuf out) {

        out.writeByte(CONNACK_FIXED_HEADER);
        //The remaining length is always static for CONNACKs
        out.writeByte(CONNACK_REMAINING_LENGTH);

        final Mqtt3ConnAckReturnCode returnCode = msg.getReturnCode();
        switch (clientConnectionContext.getProtocolVersion()) {
            case MQTTv3_1:
                out.writeByte(CONNACK_FLAGS_EMPTY);
                break;
            case MQTTv3_1_1:
                if (returnCode == Mqtt3ConnAckReturnCode.ACCEPTED && msg.isSessionPresent()) {
                    out.writeByte(CONNACK_FLAGS_SP_SET);
                } else {
                    out.writeByte(CONNACK_FLAGS_EMPTY);
                }
                break;
        }
        out.writeByte(returnCode.getCode());
    }

    @Override
    public int bufferSize(
            final @NotNull ClientConnectionContext clientConnectionContext, final @NotNull Mqtt3CONNACK connack) {
        return ENCODED_CONNACK_SIZE;
    }
}