Message Reaction Feature: Design a Emoji Reaction System for a Messaging App

Question

## Round 1 - Coding

## Problem

Implement a message reaction feature for a chat app. Users can add or remove emoji reactions on any message. Each reaction type shows a count and the list of users who reacted.

```python
class ReactionStore:
    def add_reaction(self, message_id: int, user_id: int, emoji: str) -> None:
        ...
    def remove_reaction(self, message_id: int, user_id: int, emoji: str) -> None:
        ...
    def get_reactions(self, message_id: int) -> dict:
        # returns {emoji: {"count": int, "users": list[int]}}
        ...
    def top_reaction(self, message_id: int) -> str:
        ...
```

## Example

```
store = ReactionStore()
store.add_reaction(1, 101, ":thumbsup:")
store.add_reaction(1, 102, ":thumbsup:")
store.add_reaction(1, 103, ":heart:")
store.get_reactions(1)
# -> {":thumbsup:": {"count": 2, "users": [101, 102]},
#     ":heart:":     {"count": 1, "users": [103]}}
store.top_reaction(1)  -> ":thumbsup:"
store.remove_reaction(1, 101, ":thumbsup:")
store.get_reactions(1)
# -> {":thumbsup:": {"count": 1, "users": [102]}, ...}
```

## Follow-ups

1. How would you persist reactions to a database? What schema would you use?
2. A user tries to add a reaction they already added — what should happen?
3. How would you extend this to support animated reactions with a display order?
4. At scale (millions of messages), how do you keep `get_reactions` fast?

Message Reaction Feature: Design a Emoji Reaction System for a Messaging App

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