/**
 * Definition of MQTT protocol messages
 *
 * Author:
 * Tomáš Chaloupka <chalucha@gmail.com>
 *
 * License:
 * Boost Software License 1.0 (BSL-1.0)
 *
 * Permission is hereby granted, free of charge, to any person or organization obtaining a copy
 * of the software and accompanying documentation covered by this license (the "Software") to use,
 * reproduce, display, distribute, execute, and transmit the Software, and to prepare derivative
 * works of the Software, and to permit third-parties to whom the Software is furnished to do so,
 * all subject to the following:
 *
 * The copyright notices in the Software and this entire statement, including the above license
 * grant, this restriction and the following disclaimer, must be included in all copies of the Software,
 * in whole or in part, and all derivative works of the Software, unless such copies or derivative works
 * are solely in the form of machine-executable object code generated by a source language processor.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
 * INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
 * PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR ANYONE
 * DISTRIBUTING THE SOFTWARE BE LIABLE FOR ANY DAMAGES OR OTHER LIABILITY, WHETHER IN CONTRACT,
 * TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
 * OTHER DEALINGS IN THE SOFTWARE.
 */
module mqttd.messages;

import std.range;
import std.exception : enforce;
import std.traits : isIntegral;
import std.typecons : Nullable;
debug import std.stdio;

import mqttd.traits;

@safe:

/**
 * Exception thrown when package format is somehow malformed
 */
final class PacketFormatException : Exception
{
	this(string msg = null, Throwable next = null)
	{
		super(msg, next);
	}
}

enum ubyte MQTT_PROTOCOL_LEVEL_3_1_1 = 0x04;
enum string MQTT_PROTOCOL_NAME = "MQTT";

/**
 * MQTT Control Packet type
 *
 * http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Table_2.1_-
 */
enum PacketType : ubyte
{
	/// Forbidden - Reserved
	RESERVED1   = 0,
	/// Client -> Server - Client request to connect to Server
	CONNECT     = 1,
	/// Server -> Client - Connect acknowledgment
	CONNACK     = 2,
	/// Publish message
	PUBLISH     = 3,
	/// Publish acknowledgment
	PUBACK      = 4,
	/// Publish received (assured delivery part 1)
	PUBREC      = 5,
	/// Publish release (assured delivery part 2)
	PUBREL      = 6,
	/// Publish complete (assured delivery part 3)
	PUBCOMP     = 7,
	/// Client -> Server - Client subscribe request
	SUBSCRIBE   = 8,
	/// Server -> Client - Subscribe acknowledgment
	SUBACK      = 9,
	/// Client -> Server - Unsubscribe request
	UNSUBSCRIBE = 10,
	/// Server -> Client - Unsubscribe acknowledgment
	UNSUBACK    = 11,
	/// Client -> Server - PING request
	PINGREQ     = 12,
	/// Server -> Client - PING response
	PINGRESP    = 13,
	/// Client -> Server - Client is disconnecting
	DISCONNECT  = 14,
	/// Forbidden - Reserved
	RESERVED2   = 15
}

/**
 * Indicates the level of assurance for delivery of an Application Message
 * http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Table_3.11_-
 */
enum QoSLevel : ubyte
{
	/// At most once delivery
	QoS0 = 0x00,
	/// At least once delivery
	QoS1 = 0x01,
	/// Exactly once delivery
	QoS2 = 0x02,
	/// Reserved – must not be used
	Reserved = 0x03,
	/// Failure - used in SubAck packet only
	Failure = 0x80
}

/// Connect Return code values - 0 = accepted, the rest means refused (6-255 are reserved)
enum ConnectReturnCode : ubyte
{
	/// Connection accepted
	ConnectionAccepted = 0x00,
	/// The Server does not support the level of the MQTT protocol requested by the Client
	ProtocolVersion    = 0x01,
	/// The Client identifier is correct UTF-8 but not allowed by the Server
	Identifier         = 0x02,
	/// The Network Connection has been made but the MQTT service is unavailable
	ServerUnavailable  = 0x03,
	/// The data in the user name or password is malformed
	UserNameOrPassword = 0x04,
	/// The Client is not authorized to connect
	NotAuthorized      = 0x05
}

/**
 * Each MQTT Control Packet contains a fixed header.
 *
 * http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html#_Figure_2.2_-
 */
struct FixedHeader
{
	@safe pure @nogc nothrow:
	private ubyte _payload;

	/// Represented as a 4-bit unsigned value
	@property PacketType type() const
	{
		return cast(PacketType)(_payload >> 4);
	}

	/// ditto
	@property void type(in PacketType type)
	{
		_payload = cast(ubyte)((_payload & ~0xf0) | (type << 4));
	}

	/// Duplicate delivery of a PUBLISH Control Packet
	@property bool dup() const
	{
		return (_payload & 0x08) == 0x08;
	}

	/// ditto
	@property void dup(in bool value)
	{
		_payload = cast(ubyte)((_payload & ~0x08) | (value ? 0x08 : 0x00));
	}

	/// Quality Of Service for a message
	@property QoSLevel qos() const
	{
		return cast(QoSLevel)((_payload >> 1) & 0x03);
	}

	/// ditto
	@property void qos(in QoSLevel value)
	{
		_payload = cast(ubyte)((_payload & ~0x06) | (value << 1));
	}

	/// PUBLISH Retain flag
	@property bool retain() const
	{
		return (_payload & 0x01) == 0x01;
	}

	/// ditto
	@property void retain(in bool value)
	{
		_payload = cast(ubyte)(_payload & ~0x01) | (value ? 0x01 : 0x00);
	}

	/// flags to ubyte
	@property ubyte flags() const
	{
		return _payload;
	}

	@property void flags(in ubyte value)
	{
		_payload = value;
	}

	/**
	 * The Remaining Length is the number of bytes remaining within the current packet,
	 * including data in the variable header and the payload.
	 * The Remaining Length does not include the bytes used to encode the Remaining Length.
	 */
	uint length;

	alias flags this;

	this(PacketType type, bool dup, QoSLevel qos, bool retain, uint length = 0)
	{
		this.type = type;
		this.dup = dup;
		this.retain = retain;
		this.qos = qos;
		this.length = length;
	}

	this(T)(PacketType type, T flags, uint length = 0) if(isIntegral!T)
	{
		this.flags = cast(ubyte)(type << 4 | flags);
		this.type = type;
		this.length = length;
	}

	this(T)(T value) if(isIntegral!T)
	{
		this.flags = cast(ubyte)value;
	}
}

/**
 * The Connect Flags byte contains a number of parameters specifying the behavior of the MQTT connection.
 * It also indicates the presence or absence of fields in the payload.
 */
struct ConnectFlags
{
	@safe pure @nogc nothrow:
	private ubyte _payload;

	/**
	 * If the User Name Flag is set to 0, a user name MUST NOT be present in the payload.
	 * If the User Name Flag is set to 1, a user name MUST be present in the payload.
	 */
	@property bool userName() const
	{
		return (_payload & 0x80) == 0x80;
	}

	/// ditto
	@property void userName(in bool value)
	{
		_payload = cast(ubyte)((_payload & ~0x80) | (value ? 0x80 : 0x00));
	}

	/**
	 * If the Password Flag is set to 0, a password MUST NOT be present in the payload.
	 * If the Password Flag is set to 1, a password MUST be present in the payload.
	 * If the User Name Flag is set to 0, the Password Flag MUST be set to 0.
	 */
	@property bool password() const
	{
		return (_payload & 0x40) == 0x40;
	}

	/// ditto
	@property void password(in bool value)
	{
		_payload = cast(ubyte)((_payload & ~0x40) | (value ? 0x40 : 0x00));
	}

	/**
	 * This bit specifies if the Will Message is to be Retained when it is published.
	 *
	 * If the Will Flag is set to 0, then the Will Retain Flag MUST be set to 0.
	 * If the Will Flag is set to 1:
	 *      If Will Retain is set to 0, the Server MUST publish the Will Message as a non-retained message.
	 *      If Will Retain is set to 1, the Server MUST publish the Will Message as a retained message
	 */
	@property bool willRetain() const
	{
		return (_payload & 0x20) == 0x20;
	}

	/// ditto
	@property void willRetain(in bool value)
	{
		_payload = cast(ubyte)((_payload & ~0x20) | (value ? 0x20 : 0x00));
	}

	/**
	 * Specify the QoS level to be used when publishing the Will Message.
	 *
	 * If the Will Flag is set to 0, then the Will QoS MUST be set to 0 (0x00).
	 * If the Will Flag is set to 1, the value of Will QoS can be 0 (0x00), 1 (0x01), or 2 (0x02).
	 * It MUST NOT be 3 (0x03)
	 */
	@property QoSLevel willQoS() const
	{
		return cast(QoSLevel)((_payload >> 3) & 0x03);
	}

	/// ditto
	@property void willQoS(in QoSLevel value)
	{
		_payload = cast(ubyte)((_payload & ~0x18) | (value << 3));
	}

	/**
	 * If the Will Flag is set to 1 this indicates that, if the Connect request is accepted, a Will Message MUST
	 * be stored on the Server and associated with the Network Connection. The Will Message MUST be published
	 * when the Network Connection is subsequently closed unless the Will Message has been deleted by the Server
	 * on receipt of a DISCONNECT Packet.
	 *
	 * Situations in which the Will Message is published include, but are not limited to:
	 *      An I/O error or network failure detected by the Server.
	 *      The Client fails to communicate within the Keep Alive time.
	 *      The Client closes the Network Connection without first sending a DISCONNECT Packet.
	 *      The Server closes the Network Connection because of a protocol error.
	 *
	 * If the Will Flag is set to 1, the Will QoS and Will Retain fields in the Connect Flags will be used by
	 * the Server, and the Will Topic and Will Message fields MUST be present in the payload.
	 *
	 * The Will Message MUST be removed from the stored Session state in the Server once it has been published
	 * or the Server has received a DISCONNECT packet from the Client.
	 *
	 * If the Will Flag is set to 0 the Will QoS and Will Retain fields in the Connect Flags MUST be set to zero
	 * and the Will Topic and Will Message fields MUST NOT be present in the payload.
	 *
	 * If the Will Flag is set to 0, a Will Message MUST NOT be published when this Network Connection ends
	 */
	@property bool will() const
	{
		return (_payload & 0x04) == 0x04;
	}

	/// ditto
	@property void will(in bool value)
	{
		_payload = cast(ubyte)((_payload & ~0x04) | (value ? 0x04 : 0x00));
	}

	/**
	 * This bit specifies the handling of the Session state.
	 * The Client and Server can store Session state to enable reliable messaging to continue across a sequence
	 * of Network Connections. This bit is used to control the lifetime of the Session state.
	 *
	 * If CleanSession is set to 0, the Server MUST resume communications with the Client based on state from
	 * the current Session (as identified by the Client identifier).
	 * If there is no Session associated with the Client identifier the Server MUST create a new Session.
	 * The Client and Server MUST store the Session after the Client and Server are disconnected.
	 * After the disconnection of a Session that had CleanSession set to 0, the Server MUST store
	 * further QoS 1 and QoS 2 messages that match any subscriptions that the client had at the time of disconnection
	 * as part of the Session state.
	 * It MAY also store QoS 0 messages that meet the same criteria.
	 *
	 * If CleanSession is set to 1, the Client and Server MUST discard any previous Session and start a new one.
	 * This Session lasts as long as the Network Connection. State data associated with this Session MUST NOT be reused
	 * in any subsequent Session.
	 *
	 * The Session state in the Client consists of:
	 *      QoS 1 and QoS 2 messages which have been sent to the Server, but have not been completely acknowledged.
	 *      QoS 2 messages which have been received from the Server, but have not been completely acknowledged.
	 *
	 * To ensure consistent state in the event of a failure, the Client should repeat its attempts to connect with
	 * CleanSession set to 1, until it connects successfully.
	 *
	 * Typically, a Client will always connect using CleanSession set to 0 or CleanSession set to 1 and not swap
	 * between the two values. The choice will depend on the application. A Client using CleanSession set to 1 will
	 * not receive old Application Messages and has to subscribe afresh to any topics that it is interested in each
	 * time it connects. A Client using CleanSession set to 0 will receive all QoS 1 or QoS 2 messages that were
	 * published while it was disconnected. Hence, to ensure that you do not lose messages while disconnected,
	 * use QoS 1 or QoS 2 with CleanSession set to 0.
	 *
	 * When a Client connects with CleanSession set to 0, it is requesting that the Server maintain its MQTT session
	 * state after it disconnects. Clients should only connect with CleanSession set to 0, if they intend to reconnect
	 * to the Server at some later point in time. When a Client has determined that it has no further use for
	 * the session, it should do a final connect with CleanSession set to 1 and then disconnect.
	 */
	@property bool cleanSession() const
	{
		return (_payload & 0x02) == 0x02;
	}

	/// ditto
	@property void cleanSession(in bool value)
	{
		_payload = cast(ubyte)((_payload & ~0x02) | (value ? 0x02 : 0x00));
	}

	@property ubyte flags() const
	{
		return _payload;
	}

	@property void flags(ubyte value) pure
	{
		_payload = value & ~0x01;
	}

	this(bool userName, bool password, bool willRetain, QoSLevel willQoS, bool will, bool cleanSession)
	{
		this.userName = userName;
		this.password = password;
		this.willRetain = willRetain;
		this.willQoS = willQoS;
		this.will = will;
		this.cleanSession = cleanSession;
	}

	this(T)(T value) if(isIntegral!T)
	{
		this.flags = cast(ubyte)(value & ~0x01);
	}

	alias flags this;

	unittest
	{
		import std.array;

		ConnectFlags flags;

		assert(flags == ConnectFlags(false, false, false, QoSLevel.QoS0, false, false));
		assert(flags == 0);

		flags.flags = 1; //reserved - no change
		assert(flags == ConnectFlags(false, false, false, QoSLevel.QoS0, false, false));
		assert(flags == 0);

		flags.flags = 2;
		assert(flags == ConnectFlags(false, false, false, QoSLevel.QoS0, false, true));

		flags.flags = 4;
		assert(flags == ConnectFlags(false, false, false, QoSLevel.QoS0, true, false));

		flags.flags = 24;
		assert(flags == ConnectFlags(false, false, false, QoSLevel.Reserved, false, false));

		flags.flags = 32;
		assert(flags == ConnectFlags(false, false, true, QoSLevel.QoS0, false, false));

		flags.flags = 64;
		assert(flags == ConnectFlags(false, true, false, QoSLevel.QoS0, false, false));

		flags.flags = 128;
		assert(flags == ConnectFlags(true, false, false, QoSLevel.QoS0, false, false));
	}
}

/// Connect Acknowledge Flags
struct ConnAckFlags
{
	@safe pure @nogc nothrow:
	private ubyte _payload;

	/**
	 * If the Server accepts a connection with CleanSession set to 1, the Server MUST set Session Present to 0
	 * in the CONNACK packet in addition to setting a zero return code in the CONNACK packet.
	 *
	 * If the Server accepts a connection with CleanSession set to 0, the value set in Session Present depends on
	 * whether the Server already has stored Session state for the supplied client ID. If the Server has stored
	 * Session state, it MUST set Session Present to 1 in the CONNACK packet.
	 * If the Server does not have stored Session state, it MUST set Session Present to 0 in the CONNACK packet.
	 * This is in addition to setting a zero return code in the CONNACK packet.
	 *
	 * The Session Present flag enables a Client to establish whether the Client and Server have a consistent view
	 * about whether there is already stored Session state.
	 *
	 * Once the initial setup of a Session is complete, a Client with stored Session state will expect the Server
	 * to maintain its stored Session state. In the event that the value of Session Present received by the Client
	 * from the Server is not as expected, the Client can choose whether to proceed with the Session or to disconnect.
	 * The Client can discard the Session state on both Client and Server by disconnecting, connecting with
	 * Clean Session set to 1 and then disconnecting again.
	 *
	 * If a server sends a CONNACK packet containing a non-zero return code it MUST set Session Present to 0
	 */
	@property bool sessionPresent() const
	{
		return (_payload & 0x01) == 0x01;
	}

	/// ditto
	@property void sessionPresent(in bool value)
	{
		_payload = cast(ubyte)(value ? 0x01 : 0x00);
	}

	@property ubyte flags() const
	{
		return _payload;
	}

	@property void flags(ubyte value) pure
	{
		_payload = cast(ubyte)(value & 0x01); //clean reserved bits
	}

	alias flags this;

	this(T)(T value) if(isIntegral!T)
	{
		this.flags = cast(ubyte)value;
	}
}

/// The payload of a SUBSCRIBE Packet
static struct Topic
{
	/**
	 * Topic Filter indicating the Topic to which the Client wants to subscribe.
	 * The Topic Filters in a SUBSCRIBE packet payload MUST be UTF-8 encoded strings.
	 * A Server SHOULD support Topic filters that contain the wildcard characters.
	 * If it chooses not to support topic filters that contain wildcard characters it MUST reject any Subscription request whose filter contains them
	 */
	string filter;
	/// This gives the maximum QoS level at which the Server can send Application Messages to the Client.
	QoSLevel qos;
}

/**
 * After a Network Connection is established by a Client to a Server,
 * the first Packet sent from the Client to the Server MUST be a CONNECT Packet.
 *
 * A Client can only send the CONNECT Packet once over a Network Connection.
 * The Server MUST process a second CONNECT Packet sent from a Client as a protocol violation and disconnect the Client.
 *
 * The payload contains one or more encoded fields.
 * They specify a unique Client identifier for the Client, a Will topic, Will Message, User Name and Password.
 * All but the Client identifier are optional and their presence is determined based on flags in the variable header.
 */
struct Connect
{
	FixedHeader header = FixedHeader(0x10);

	/// The Protocol Name is a UTF-8 encoded string that represents the protocol name “MQTT”
	string protocolName = MQTT_PROTOCOL_NAME;

	/**
	 * The 8 bit unsigned value that represents the revision level of the protocol used by the Client.
	 * The value of the Protocol Level field for the version 3.1.1 of the protocol is 4 (0x04).
	 */
	ubyte protocolLevel = MQTT_PROTOCOL_LEVEL_3_1_1;

	/**
	 * The Connect Flags byte contains a number of parameters specifying the behavior of the MQTT connection.
	 * It also indicates the presence or absence of fields in the payload.
	 */
	ConnectFlags flags;

	/**
	 * The Keep Alive is a time interval measured in seconds. Expressed as a 16-bit word, it is the maximum time
	 * interval that is permitted to elapse between the point at which the Client finishes transmitting one Control
	 * Packet and the point it starts sending the next. It is the responsibility of the Client to ensure that the
	 * interval between Control Packets being sent does not exceed the Keep Alive value.
	 * In the absence of sending any other Control Packets, the Client MUST send a PINGREQ Packet.
	 *
	 * The Client can send PINGREQ at any time, irrespective of the Keep Alive value, and use the PINGRESP to determine
	 * that the network and the Server are working.
	 *
	 * If the Keep Alive value is non-zero and the Server does not receive a Control Packet from the Client within
	 * one and a half times the Keep Alive time period, it MUST disconnect the Network Connection to the Client as if
	 * the network had failed.
	 *
	 * If a Client does not receive a PINGRESP Packet within a reasonable amount of time after it has sent a PINGREQ,
	 * it SHOULD close the Network Connection to the Server.
	 *
	 * A Keep Alive value of zero (0) has the effect of turning off the keep alive mechanism.
	 * This means that, in this case, the Server is not required to disconnect the Client on the grounds of inactivity.
	 * Note that a Server is permitted to disconnect a Client that it determines to be inactive or non-responsive
	 * at any time, regardless of the Keep Alive value provided by that Client.
	 *
	 * The actual value of the Keep Alive is application specific; typically this is a few minutes.
	 * The maximum value is 18 hours 12 minutes and 15 seconds.
	 */
	ushort keepAlive;

	/// Client Identifier
	string clientIdentifier;

	/// Will Topic
	string willTopic;

	/// Will Message
	string willMessage;

	/// User Name
	string userName;

	/// Password
	string password;
}

/// Responce to Connect request
struct ConnAck
{
	FixedHeader header = FixedHeader(PacketType.CONNACK, 0, 2);

	ConnAckFlags flags;

	ConnectReturnCode returnCode;
}

/// A PUBLISH Control Packet is sent from a Client to a Server or from Server to a Client to transport an Application Message.
struct Publish
{
	FixedHeader header = FixedHeader(0x30);
	/**
	 * The Topic Name identifies the information channel to which payload data is published.
	 *
	 * The Topic Name MUST be present as the first field in the PUBLISH Packet Variable header.
	 * It MUST be a UTF-8 encoded string.
	 *
	 * The Topic Name in the PUBLISH Packet MUST NOT contain wildcard characters.
	 * The Topic Name in a PUBLISH Packet sent by a Server to a subscribing Client MUST match the Subscription’s
	 * Topic Filter according to the matching process.
	 * However, since the Server is permitted to override the Topic Name, it might not be the same as the Topic Name
	 * in the original PUBLISH Packet.
	 */
	string topic;
	/// The Packet Identifier field is only present in PUBLISH Packets where the QoS level is 1 or 2
	ushort packetId; // if QoS > 0
	/**
	 * The Payload contains the Application Message that is being published.
	 * The content and format of the data is application specific.
	 * The length of the payload can be calculated by subtracting the length of the variable header from
	 * the Remaining Length field that is in the Fixed Header.
	 * It is valid for a PUBLISH Packet to contain a zero length payload.
	 */
	ubyte[] payload;
}

/// A PUBACK Packet is the response to a PUBLISH Packet with QoS level 1.
struct PubAck
{
	FixedHeader header = FixedHeader(PacketType.PUBACK, 0, 2);

	/// This contains the Packet Identifier from the PUBLISH Packet that is being acknowledged.
	ushort packetId;

	this(ushort packetId)
	{
		this.packetId = packetId;
	}
}

/// A PUBREC Packet is the response to a PUBLISH Packet with QoS 2. It is the second packet of the QoS 2 protocol exchange.
struct PubRec
{
	FixedHeader header = FixedHeader(PacketType.PUBREC, 0, 2);

	/// This contains the Packet Identifier from the PUBLISH Packet that is being acknowledged.
	ushort packetId;

	this(ushort packetId)
	{
		this.packetId = packetId;
	}
}

/// A PUBREL Packet is the response to a PUBREC Packet. It is the third packet of the QoS 2 protocol exchange.
struct PubRel
{
	FixedHeader header = FixedHeader(PacketType.PUBREL, 0x02, 2);

	/// This contains the same Packet Identifier as the PUBREC Packet that is being acknowledged.
	ushort packetId;

	this(ushort packetId)
	{
		this.packetId = packetId;
	}
}

/// The PUBCOMP Packet is the response to a PUBREL Packet. It is the fourth and final packet of the QoS 2 protocol exchange.
struct PubComp
{
	FixedHeader header = FixedHeader(PacketType.PUBCOMP, 0, 2);

	/// This contains the same Packet Identifier as the PUBREC Packet that is being acknowledged.
	ushort packetId;

	this(ushort packetId)
	{
		this.packetId = packetId;
	}
}

/**
 * The SUBSCRIBE Packet is sent from the Client to the Server to create one or more Subscriptions.
 * Each Subscription registers a Client’s interest in one or more Topics.
 * The Server sends PUBLISH Packets to the Client in order to forward Application Messages that were published to Topics that match these Subscriptions.
 * The SUBSCRIBE Packet also specifies (for each Subscription) the maximum QoS with which the Server can send Application Messages to the Client.
 *
 * The payload of a SUBSCRIBE packet MUST contain at least one Topic Filter / QoS pair. A SUBSCRIBE packet with no payload is a protocol violation.
 */
struct Subscribe
{
	FixedHeader header = FixedHeader(PacketType.SUBSCRIBE, 0x02);

	/// This contains the Packet Identifier.
	ushort packetId;

	/// Topics to register to
	Topic[] topics;
}

/**
 * A SUBACK Packet is sent by the Server to the Client to confirm receipt and processing of a SUBSCRIBE Packet.
 * A SUBACK Packet contains a list of return codes, that specify the maximum QoS level that was granted in each
 * Subscription that was requested by the SUBSCRIBE.
 */
struct SubAck
{
	FixedHeader header = FixedHeader(PacketType.SUBACK, 0);

	/// This contains the Packet Identifier from the SUBSCRIBE Packet that is being acknowledged.
	ushort packetId;
	/**
	 * The payload contains a list of return codes. Each return code corresponds to a Topic Filter in the
	 * SUBSCRIBE Packet being acknowledged.
	 * The order of return codes in the SUBACK Packet MUST match the order of Topic Filters in the SUBSCRIBE Packet.
	 */
	QoSLevel[] returnCodes;
}

/// An UNSUBSCRIBE Packet is sent by the Client to the Server, to unsubscribe from topics.
struct Unsubscribe
{
	FixedHeader header = FixedHeader(PacketType.UNSUBSCRIBE, 0x02);

	/// This contains the Packet Identifier.
	ushort packetId;

	/**
	 * The list of Topic Filters that the Client wishes to unsubscribe from. The Topic Filters in an UNSUBSCRIBE packet MUST be UTF-8 encoded strings.
	 * The Payload of an UNSUBSCRIBE packet MUST contain at least one Topic Filter. An UNSUBSCRIBE packet with no payload is a protocol violation.
	 */
	string[] topics;
}

/// The UNSUBACK Packet is sent by the Server to the Client to confirm receipt of an UNSUBSCRIBE Packet.
struct UnsubAck
{
	FixedHeader header = FixedHeader(PacketType.UNSUBACK, 0, 2);

	/// This contains the same Packet Identifier as the UNSUBSCRIBE Packet that is being acknowledged.
	ushort packetId;
}

/**
 * The PINGREQ Packet is sent from a Client to the Server. It can be used to:
 *
 * Indicate to the Server that the Client is alive in the absence of any other Control Packets being sent from the Client to the Server.
 * Request that the Server responds to confirm that it is alive.
 * Exercise the network to indicate that the Network Connection is active.
 *
 * This Packet is used in Keep Alive processing
 */
struct PingReq
{
	FixedHeader header = FixedHeader(PacketType.PINGREQ, 0, 0);
}

/**
 * A PINGRESP Packet is sent by the Server to the Client in response to a PINGREQ Packet. It indicates that the Server is alive.
 * This Packet is used in Keep Alive processing.
 */
struct PingResp
{
	FixedHeader header = FixedHeader(PacketType.PINGRESP, 0, 0);
}

/**
 * The DISCONNECT Packet is the final Control Packet sent from the Client to the Server. It indicates that the Client is disconnecting cleanly.
 *
 * After sending a DISCONNECT Packet the Client:
 *      MUST close the Network Connection.
 *      MUST NOT send any more Control Packets on that Network Connection.
 *
 * On receipt of DISCONNECT the Server:
 *      MUST discard any Will Message associated with the current connection without publishing it.
 *      SHOULD close the Network Connection if the Client has not already done so.
 */
struct Disconnect
{
	FixedHeader header = FixedHeader(PacketType.DISCONNECT, 0, 0);
}