Fix auto_map_topic_alias_send logic

src/mqtt/connection/core.rs

@@ -1630,7 +1630,7 @@ where
                     } else {
                         let lru_ta = topic_alias_send.get_lru_alias();
                         topic_alias_send.insert_or_update(packet.topic_name(), lru_ta);
-                        packet = packet.remove_topic_add_topic_alias(lru_ta);
+                        packet = packet.add_topic_alias(lru_ta);
                     }
                 }
             } else if self.auto_replace_topic_alias_send {

src/mqtt/packet/v5_0/publish.rs

@@ -688,6 +688,45 @@ where
         // Set topic name to empty string
         self.topic_name_buf = MqttString::new("").unwrap();
 
+        self.add_topic_alias(topic_alias)
+    }
+
+    /// Add TopicAlias property
+    ///
+    /// This method adds a TopicAlias property to the packet properties. This
+    /// is required to associate a topic alias with a topic name on the server.
+    /// Future messages can use only the topic alias and omit the topic name.
+    /// This is useful for reducing packet size when sending multiple messages
+    /// to the same topic.
+    ///
+    /// The method removes any existing TopicAlias property before adding the new one
+    /// to ensure only one TopicAlias property exists. It automatically recalculates
+    /// the property_length and remaining_length to reflect these changes.
+    ///
+    /// # Parameters
+    ///
+    /// - `topic_alias`: The numeric topic alias to associate with the topic name
+    ///
+    /// # Returns
+    ///
+    /// The modified `GenericPublish` instance with the TopicAlias property
+    ///
+    /// # Examples
+    ///
+    /// ```ignore
+    /// use mqtt_protocol_core::mqtt;
+    ///
+    /// let publish_with_topic = mqtt::packet::v5_0::Publish::builder()
+    ///     .topic_name("sensors/temperature/room1")
+    ///     .unwrap()
+    ///     .payload(b"23.5")
+    ///     .build()
+    ///     .unwrap();
+    ///
+    /// let publish_with_alias = publish_with_topic.add_topic_alias(42);
+    /// // props() now contains TopicAlias property with value 42
+    /// ```
+    pub fn add_topic_alias(mut self, topic_alias: TopicAliasType) -> Self {
         // Add TopicAlias property to the end of properties
         let topic_alias_property =
             Property::TopicAlias(crate::mqtt::packet::TopicAlias::new(topic_alias).unwrap());

tests/connection-core-topic-alias.rs

@@ -71,8 +71,8 @@ fn auto_map_topic_alias_send() {
             ..
         } = event
         {
-            // Verify topic name is empty and topic alias is 1
-            if p.topic_name().is_empty() {
+            // Verify topic name is present and topic alias is 1
+            if p.topic_name() == "topic/a" {
                 // Check for TopicAlias property with value 1
                 for prop in p.props().iter() {
                     if let mqtt::packet::Property::TopicAlias(ta) = prop {
@@ -87,7 +87,7 @@ fn auto_map_topic_alias_send() {
     }
     assert!(
         publish_a_mapped,
-        "PUBLISH A should have empty topic name and TopicAlias=1"
+        "PUBLISH A should have valid topic name and TopicAlias=1"
     );
 
     // Send QoS0 PUBLISH B
@@ -109,8 +109,8 @@ fn auto_map_topic_alias_send() {
             ..
         } = event
         {
-            // Verify topic name is empty and topic alias is 2
-            if p.topic_name().is_empty() {
+            // Verify topic name is present and topic alias is 2
+            if p.topic_name() == "topic/b" {
                 // Check for TopicAlias property with value 2
                 for prop in p.props().iter() {
                     if let mqtt::packet::Property::TopicAlias(ta) = prop {
@@ -125,7 +125,7 @@ fn auto_map_topic_alias_send() {
     }
     assert!(
         publish_b_mapped,
-        "PUBLISH B should have empty topic name and TopicAlias=2"
+        "PUBLISH B should have valid topic name and TopicAlias=2"
     );
 
     {