Design two-user communications handler with exceptions

Q: Design two-user communications handler with exceptions

This question evaluates object-oriented design, state management, and custom exception handling skills for enforcing a single active communication pair and related invariants between two users.

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This is a Medium difficulty Coding & Algorithms question, commonly asked during HR Screen rounds at Akuna Capital.

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Question

Implement a two-user communications handler. Create:
(A) CommsHandler(CommsHandlerABC) with methods connect(self, user1: Caller, user2: Caller) -> str, hangup(self, user1: Caller, user2: Caller) -> str, and clear_all(self) -> None; and
(B) CommunicationException(Exception) that accepts a message string. Behavior: Only two users can communicate at any given time; no other users can connect until the current communication ends. connect: If user1 is the same as user2, raise CommunicationException with the message "{user1.name} cannot connect with {user2.name}". If user1 and user2 are different and the line is in use, raise CommunicationException("Connection in use. Please try later"). If the line is free, store the active pair and return "Connection established between {user1.name} and {user2.name}". hangup: If user1 is the same as user2, raise CommunicationException with the message "{user1.name} cannot hangup with {user2.name}". If user1 and user2 are different and currently communicating, disconnect them and return "{user1.name} and {user2.name} are disconnected". clear_all: Clear the communication channel regardless of current users. Assume a provided Caller class with a name attribute.

PracHub · Accepted Answer

def solution(operations):
    results = []
    active_pair = None  # stored as a frozenset of two user names

for op in operations:
        action = op[0]

if action == 'connect':
            _, user1, user2 = op
            if user1 == user2:
                results.append(f"{user1} cannot connect with {user2}")
            elif active_pair is not None:
                results.append("Connection in use. Please try later")
            else:
                active_pair = frozenset((user1, user2))
                results.append(f"Connection established between {user1} and {user2}")

elif action == 'hangup':
            _, user1, user2 = op
            if user1 == user2:
                results.append(f"{user1} cannot hangup with {user2}")
            elif active_pair == frozenset((user1, user2)):
                active_pair = None
                results.append(f"{user1} and {user2} are disconnected")
            else:
                results.append(None)

elif action == 'clear_all':
            active_pair = None
            results.append(None)

return results

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