feat(mqtt): add MqttBinaryDecoder with CONNECT/PUBLISH/SUBSCRIBE parsing and 27 tests
Port binary MQTT 3.1.1 packet body decoding from Go mqtt.go (~lines 700–1500, 2200). MqttBinaryDecoder parses CONNECT (protocol name, connect flags, keepalive, client ID, will topic/message, username, password), PUBLISH (topic, packet ID for QoS>0, DUP/ RETAIN flags, application payload), and SUBSCRIBE (packet ID, topic-filter + QoS list). Adds TranslateFilterToNatsSubject for simple MQTT wildcard conversion (+ → *, # → >, / → .). All 27 tests in MqttBinaryParserTests pass.
This commit is contained in:
Joseph Doherty
325
src/NATS.Server/Mqtt/MqttBinaryDecoder.cs
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325
src/NATS.Server/Mqtt/MqttBinaryDecoder.cs
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// Binary MQTT packet body decoder.
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// Go reference: golang/nats-server/server/mqtt.go
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// CONNECT parsing — mqttParseSub / mqttParseConnect (lines ~700–850)
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// PUBLISH parsing — mqttParsePublish (lines ~1200–1300)
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// SUBSCRIBE parsing — mqttParseSub (lines ~1400–1500)
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// Wildcard translation — mqttToNATSSubjectConversion (lines ~2200–2250)
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namespace NATS.Server.Mqtt;
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/// <summary>
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/// Decoded fields from an MQTT CONNECT packet body.
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/// Go reference: server/mqtt.go mqttParseConnect ~line 700.
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/// </summary>
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public readonly record struct MqttConnectInfo(
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string ProtocolName,
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byte ProtocolLevel,
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bool CleanSession,
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ushort KeepAlive,
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string ClientId,
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string? WillTopic,
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byte[]? WillMessage,
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byte WillQoS,
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bool WillRetain,
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string? Username,
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string? Password);
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/// <summary>
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/// Decoded fields from an MQTT PUBLISH packet body.
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/// Go reference: server/mqtt.go mqttParsePublish ~line 1200.
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/// </summary>
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public readonly record struct MqttPublishInfo(
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string Topic,
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ushort PacketId,
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byte QoS,
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bool Dup,
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bool Retain,
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ReadOnlyMemory<byte> Payload);
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/// <summary>
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/// Decoded fields from an MQTT SUBSCRIBE packet body.
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/// Go reference: server/mqtt.go mqttParseSub ~line 1400.
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/// </summary>
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public readonly record struct MqttSubscribeInfo(
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ushort PacketId,
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IReadOnlyList<(string TopicFilter, byte QoS)> Filters);
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/// <summary>
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/// Decodes the variable-header and payload of CONNECT, PUBLISH, and SUBSCRIBE
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/// MQTT 3.1.1 control packets, and translates MQTT wildcards to NATS subjects.
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/// </summary>
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public static class MqttBinaryDecoder
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{
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// -------------------------------------------------------------------------
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// CONNECT parsing
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// Go reference: server/mqtt.go mqttParseConnect ~line 700
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// -------------------------------------------------------------------------
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/// <summary>
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/// Parses the payload bytes of an MQTT CONNECT packet (everything after the
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/// fixed header and remaining-length bytes, i.e. the value of
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/// <see cref="MqttControlPacket.Payload"/>).
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/// </summary>
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/// <param name="payload">
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/// The payload bytes as returned by <see cref="MqttPacketReader.Read"/>.
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/// </param>
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/// <returns>A populated <see cref="MqttConnectInfo"/>.</returns>
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/// <exception cref="FormatException">
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/// Thrown when the packet is malformed or the protocol name is not "MQTT".
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/// </exception>
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public static MqttConnectInfo ParseConnect(ReadOnlySpan<byte> payload)
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{
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// Variable header layout (MQTT 3.1.1 spec §3.1):
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// 2-byte length prefix + protocol name bytes ("MQTT")
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// 1 byte protocol level (4 = 3.1.1, 5 = 5.0)
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// 1 byte connect flags
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// 2 bytes keepalive (big-endian)
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// Payload:
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// 2+N client ID
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// if will flag: 2+N will topic, 2+N will message
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// if username: 2+N username
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// if password: 2+N password
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var pos = 0;
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// Protocol name
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var protocolName = ReadUtf8String(payload, ref pos);
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if (protocolName != "MQTT" && protocolName != "MQIsdp")
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throw new FormatException($"Unknown MQTT protocol name: '{protocolName}'");
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if (pos + 4 > payload.Length)
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throw new FormatException("MQTT CONNECT packet too short for variable header.");
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var protocolLevel = payload[pos++];
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// Connect flags byte
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// Bit 1 = CleanSession, Bit 2 = WillFlag, Bits 3-4 = WillQoS, Bit 5 = WillRetain,
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// Bit 6 = PasswordFlag, Bit 7 = UsernameFlag
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var connectFlags = payload[pos++];
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var cleanSession = (connectFlags & 0x02) != 0;
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var willFlag = (connectFlags & 0x04) != 0;
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var willQoS = (byte)((connectFlags >> 3) & 0x03);
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var willRetain = (connectFlags & 0x20) != 0;
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var passwordFlag = (connectFlags & 0x40) != 0;
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var usernameFlag = (connectFlags & 0x80) != 0;
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// Keep-alive (big-endian uint16)
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var keepAlive = ReadUInt16BigEndian(payload, ref pos);
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// Payload fields
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var clientId = ReadUtf8String(payload, ref pos);
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string? willTopic = null;
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byte[]? willMessage = null;
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if (willFlag)
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{
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willTopic = ReadUtf8String(payload, ref pos);
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willMessage = ReadBinaryField(payload, ref pos);
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}
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string? username = null;
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if (usernameFlag)
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username = ReadUtf8String(payload, ref pos);
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string? password = null;
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if (passwordFlag)
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password = ReadUtf8String(payload, ref pos);
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return new MqttConnectInfo(
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ProtocolName: protocolName,
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ProtocolLevel: protocolLevel,
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CleanSession: cleanSession,
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KeepAlive: keepAlive,
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ClientId: clientId,
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WillTopic: willTopic,
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WillMessage: willMessage,
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WillQoS: willQoS,
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WillRetain: willRetain,
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Username: username,
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Password: password);
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}
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// -------------------------------------------------------------------------
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// PUBLISH parsing
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// Go reference: server/mqtt.go mqttParsePublish ~line 1200
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// -------------------------------------------------------------------------
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/// <summary>
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/// Parses the payload bytes of an MQTT PUBLISH packet.
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/// The <paramref name="flags"/> nibble comes from
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/// <see cref="MqttControlPacket.Flags"/> of the fixed header.
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/// </summary>
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/// <param name="payload">The payload bytes from <see cref="MqttControlPacket.Payload"/>.</param>
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/// <param name="flags">The lower nibble of the fixed header byte (DUP/QoS/RETAIN flags).</param>
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/// <returns>A populated <see cref="MqttPublishInfo"/>.</returns>
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public static MqttPublishInfo ParsePublish(ReadOnlySpan<byte> payload, byte flags)
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{
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// Fixed-header flags nibble layout (MQTT 3.1.1 spec §3.3.1):
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// Bit 3 = DUP
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// Bits 2-1 = QoS (0, 1, or 2)
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// Bit 0 = RETAIN
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var dup = (flags & 0x08) != 0;
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var qos = (byte)((flags >> 1) & 0x03);
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var retain = (flags & 0x01) != 0;
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var pos = 0;
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// Variable header: topic name (2-byte length prefix + UTF-8)
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var topic = ReadUtf8String(payload, ref pos);
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// Packet identifier — only present for QoS > 0
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ushort packetId = 0;
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if (qos > 0)
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packetId = ReadUInt16BigEndian(payload, ref pos);
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// Remaining bytes are the application payload
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var messagePayload = payload[pos..].ToArray();
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return new MqttPublishInfo(
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Topic: topic,
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PacketId: packetId,
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QoS: qos,
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Dup: dup,
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Retain: retain,
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Payload: messagePayload);
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}
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// -------------------------------------------------------------------------
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// SUBSCRIBE parsing
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// Go reference: server/mqtt.go mqttParseSub ~line 1400
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// -------------------------------------------------------------------------
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/// <summary>
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/// Parses the payload bytes of an MQTT SUBSCRIBE packet.
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/// </summary>
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/// <param name="payload">The payload bytes from <see cref="MqttControlPacket.Payload"/>.</param>
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/// <returns>A populated <see cref="MqttSubscribeInfo"/>.</returns>
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public static MqttSubscribeInfo ParseSubscribe(ReadOnlySpan<byte> payload)
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{
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// Variable header: packet identifier (2 bytes, big-endian)
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// Payload: one or more topic-filter entries, each:
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// 2-byte length prefix + UTF-8 filter string + 1-byte requested QoS
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var pos = 0;
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var packetId = ReadUInt16BigEndian(payload, ref pos);
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var filters = new List<(string, byte)>();
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while (pos < payload.Length)
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{
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var topicFilter = ReadUtf8String(payload, ref pos);
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if (pos >= payload.Length)
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throw new FormatException("MQTT SUBSCRIBE packet missing QoS byte after topic filter.");
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var filterQoS = payload[pos++];
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filters.Add((topicFilter, filterQoS));
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}
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return new MqttSubscribeInfo(packetId, filters);
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}
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// -------------------------------------------------------------------------
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// MQTT wildcard → NATS subject translation
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// Go reference: server/mqtt.go mqttToNATSSubjectConversion ~line 2200
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//
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// Simple translation (filter → NATS, wildcards permitted):
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// '+' → '*' (single-level wildcard)
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// '#' → '>' (multi-level wildcard)
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// '/' → '.' (topic separator)
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//
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// NOTE: This method implements the simple/naïve translation that the task
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// description specifies. The full Go implementation also handles dots,
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// leading/trailing slashes, and empty levels differently (see
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// MqttTopicMappingParityTests for the complete behavior). This method is
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// intentionally limited to the four rules requested by the task spec.
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// -------------------------------------------------------------------------
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/// <summary>
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/// Translates an MQTT topic filter to a NATS subject using the simple rules:
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/// <list type="bullet">
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/// <item><c>+</c> → <c>*</c> (single-level wildcard)</item>
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/// <item><c>#</c> → <c>></c> (multi-level wildcard)</item>
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/// <item><c>/</c> → <c>.</c> (separator)</item>
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/// </list>
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/// Go reference: server/mqtt.go mqttToNATSSubjectConversion ~line 2200.
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/// </summary>
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/// <param name="mqttFilter">An MQTT topic filter string.</param>
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/// <returns>The equivalent NATS subject string.</returns>
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public static string TranslateFilterToNatsSubject(string mqttFilter)
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{
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if (mqttFilter.Length == 0)
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return string.Empty;
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var result = new char[mqttFilter.Length];
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for (var i = 0; i < mqttFilter.Length; i++)
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{
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result[i] = mqttFilter[i] switch
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{
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'+' => '*',
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'#' => '>',
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'/' => '.',
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var c => c,
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};
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}
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return new string(result);
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}
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// -------------------------------------------------------------------------
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// Internal helpers
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// -------------------------------------------------------------------------
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/// <summary>
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/// Reads a 2-byte big-endian length-prefixed UTF-8 string from
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/// <paramref name="data"/> starting at <paramref name="pos"/>, advancing
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/// <paramref name="pos"/> past the consumed bytes.
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/// </summary>
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private static string ReadUtf8String(ReadOnlySpan<byte> data, ref int pos)
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{
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if (pos + 2 > data.Length)
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throw new FormatException("MQTT packet truncated reading string length prefix.");
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var length = (data[pos] << 8) | data[pos + 1];
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pos += 2;
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if (pos + length > data.Length)
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throw new FormatException("MQTT packet truncated reading string body.");
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var value = System.Text.Encoding.UTF8.GetString(data.Slice(pos, length));
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pos += length;
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return value;
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}
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/// <summary>
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/// Reads a 2-byte big-endian length-prefixed binary field (e.g. will
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/// message, password) from <paramref name="data"/>, advancing
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/// <paramref name="pos"/> past the consumed bytes.
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/// </summary>
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private static byte[] ReadBinaryField(ReadOnlySpan<byte> data, ref int pos)
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{
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if (pos + 2 > data.Length)
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throw new FormatException("MQTT packet truncated reading binary field length prefix.");
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var length = (data[pos] << 8) | data[pos + 1];
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pos += 2;
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if (pos + length > data.Length)
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throw new FormatException("MQTT packet truncated reading binary field body.");
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var value = data.Slice(pos, length).ToArray();
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pos += length;
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return value;
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}
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/// <summary>
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/// Reads a big-endian uint16 from <paramref name="data"/> at
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/// <paramref name="pos"/>, advancing <paramref name="pos"/> by 2.
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/// </summary>
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private static ushort ReadUInt16BigEndian(ReadOnlySpan<byte> data, ref int pos)
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{
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if (pos + 2 > data.Length)
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throw new FormatException("MQTT packet truncated reading uint16.");
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var value = (ushort)((data[pos] << 8) | data[pos + 1]);
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pos += 2;
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return value;
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}
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}
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