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To implement an MQTT topic handler using a trie structure in Python, we define a `Node` class to represent each level of the topic hierarchy. A `MQTTRegistry` manages these nodes and registers functions to match specific patterns (with wildcards like `+` and `#`). Finally, we create a decorator that allows users to register handlers for MQTT topics.

---

### ✅ Key Components

- **Node Class**: Represents a node in the trie with:
  - Children: dictionary mapping topic parts.
  - Wildcard nodes (`plus_wildcard`, `hash_wildcard`).
  - List of registered handler functions.

- **MQTTRegistry Class**:
  - Stores root node and manages adding handlers to patterns.
  - Implements a recursive DFS-based match logic that supports wildcard matching for MQTT topics.

- **Decorator**: A function `@mqtt_handler(topic_pattern)` used to register functions as topic handlers with their corresponding pattern.

---

### 🧠 Matching Logic

- The `+` wildcard matches exactly one level of the topic (e.g., `a/+/b`).
- The `#` wildcard matches any number of levels, but **must appear at the end** of a pattern.
- During matching, we explore all possible paths that can match the incoming topic.

---

### 📦 Complete Implementation

```python
class Node:
    def __init__(self):
        self.children = {}
        self.plus_wildcard = None
        self.hash_wildcard = None
        self.handlers = []

class MQTTRegistry:
    def __init__(self):
        self.root = Node()

    def add_handler(self, topic_pattern, handler_func):
        parts = self.split_topic(topic_pattern)
        current_node = self.root
        i = 0

        while i < len(parts):
            part = parts[i]
            if part == '+':
                # Handle '+' wildcard: move to the plus_wildcard node
                if not current_node.plus_wildcard:
                    current_node.plus_wildcard = Node()
                current_node = current_node.plus_wildcard
                i += 1
            elif part == '#':
                # Handle '#' wildcard: match any remaining parts
                if not current_node.hash_wildcard:
                    current_node.hash_wildcard = Node()
                current_node = current_node.hash_wildcard
                i += 1
            else:
                # Regular topic level
                if part not in current_node.children:
                    current_node.children[part] = Node()
                current_node = current_node.children[part]
                i += 1

        # Register the handler at this node
        current_node.handlers.append(handler_func)

    def split_topic(self, topic):
        return topic.split('/')

    def match_topic(self, topic):
        parts = self.split_topic(topic)
        matched_handlers = []

        def dfs(node, index):
            if index == len(parts):
                # All parts of the incoming topic have been processed
                matched_handlers.extend(node.handlers)
                return

            current_part = parts[index]

            # 1. Check for plus wildcard (match any single part)
            if node.plus_wildcard:
                dfs(node.plus_wildcard, index + 1)

            # 2. Check for hash wildcard (match any remaining parts)
            if node.hash_wildcard:
                matched_handlers.extend(node.hash_wildcard.handlers)

            # 3. Check for exact match with current topic part
            if current_part in node.children:
                dfs(node.children[current_part], index + 1)

        dfs(self.root, 0)
        return matched_handlers

# Decorator to register handlers for MQTT topics
def mqtt_handler(topic_pattern):
    def decorator(func):
        registry.add_handler(topic_pattern, func)
        return func
    return decorator

# Initialize the registry
registry = MQTTRegistry()
```

---

### 🧪 Example Usage

```python
@mqtt_handler("a/+/b")
def handler1(msg):
    print(f"Handler 1: {msg}")

@mqtt_handler("a/#")
def handler2(msg):
    print(f"Handler 2: {msg}")

@mqtt_handler("c/d")
def handler3(msg):
    print(f"Handler 3: {msg}")

# Simulate message dispatch
message_topic = "a/x/b"
message_data = "This is a test message."

print(f"\nProcessing topic: {message_topic}")
matched_handlers = registry.match_topic(message_topic)

for handler in matched_handlers:
    handler(message_data)
```

---

### 📌 Expected Output

```
Processing topic: a/x/b
Handler 1: This is a test message.
Handler 2: This is a test message.
```

This demonstrates how both `a/+/b` and `a/#` match the incoming topic `"a/x/b"`.

---

### ⚠️ Notes for Production

- The current `match_topic` logic does not fully respect MQTT rules (e.g., `#` must be at the end of a pattern).
- For production, consider:
  - Enforcing that `#` can only appear once and at the end.
  - Improving DFS to track which parts have been processed for exact match with wildcards.

---

### 🔗 Summary

This implementation provides a clean way to register MQTT topic handlers using a trie structure. It supports both single-level (`+`) and multi-level (`#`) wildcards, making it suitable for real-time message routing in systems like IoT or event-driven architectures.