Real-time Chat Application Backend

Write a Python program to implement a backend for a real-time chat application. The backend should support user registration, authentication, private messaging, group chats, and message history. Your task is to implement the ChatServer class with methods for user management and message handling.

Example 1

  
Input: 
server = ChatServer()
server.register_user("alice", "password123")
server.register_user("bob", "secure456")
alice_token = server.login("alice", "password123")
bob_token = server.login("bob", "secure456")
server.send_message(alice_token, "bob", "Hello Bob!")
messages = server.get_messages(bob_token)
  
Output: [{"from": "alice", "to": "bob", "content": "Hello Bob!", "timestamp": "2025-04-09 10:15:30"}]
  
Explanation: 
    

      
Step 1: Register two users (Alice and Bob).
      
Step 2: Both users log in to get authentication tokens.
      
Step 3: Alice sends a private message to Bob.
      
Step 4: Bob retrieves his messages, seeing Alice's message with metadata.
    
  

Example 2

  
Input: 
server = ChatServer()
server.register_user("alice", "pass123")
server.register_user("bob", "pass456")
server.register_user("charlie", "pass789")
alice_token = server.login("alice", "pass123")
bob_token = server.login("bob", "pass456")
charlie_token = server.login("charlie", "pass789")

group_id = server.create_group(alice_token, "Friends")
server.add_to_group(alice_token, group_id, "bob")
server.add_to_group(alice_token, group_id, "charlie")
server.send_group_message(alice_token, group_id, "Hello everyone!")

bob_groups = server.get_group_messages(bob_token, group_id)
charlie_groups = server.get_group_messages(charlie_token, group_id)
  
Output: 
[{"from": "alice", "group": "Friends", "content": "Hello everyone!", "timestamp": "2025-04-09 10:20:45"}]
[{"from": "alice", "group": "Friends", "content": "Hello everyone!", "timestamp": "2025-04-09 10:20:45"}]
  
Explanation: 
    

      
Step 1: Register three users and log them in.
      
Step 2: Alice creates a group and adds Bob and Charlie.
      
Step 3: Alice sends a message to the group.
      
Step 4: Both Bob and Charlie retrieve the group message successfully.
    
  

Constraints

  
1 ≤ username length ≤ 20
  
8 ≤ password length ≤ 100
  
Messages can be up to 1000 characters long
  
A user can belong to a maximum of 100 groups
  
Time Complexity: O(1) for sending messages, O(n) for retrieving messages where n is the number of messages
  
Space Complexity: O(m+u+g) where m is the number of messages, u is the number of users, and g is the number of groups