Video Game OOD: Design a Turn-Based RPG Battle System

Question

## Problem

Design the core class structure for a turn-based RPG battle system. Characters take turns attacking, using abilities, or defending. Each ability has a cooldown. A character is defeated when HP drops to 0.

```python
class Character:
    def __init__(self, name: str, hp: int, attack: int, defense: int): ...
    def take_damage(self, amount: int) -> int: """Return actual damage after defense."""
    def is_alive(self) -> bool: ...
    def use_ability(self, ability_name: str, target: 'Character') -> str: ...

class Ability:
    def __init__(self, name: str, damage: int, cooldown: int): ...
    def is_ready(self) -> bool: ...
    def activate(self) -> None: ...
    def tick(self) -> None: """Called each turn to reduce cooldown."""

class BattleSystem:
    def __init__(self, team1: list[Character], team2: list[Character]): ...
    def next_turn(self) -> str: """Execute one turn, return action log."""
    def is_battle_over(self) -> bool: ...
    def winner(self) -> str | None: ...
```

```
battle = BattleSystem([warrior], [mage])
while not battle.is_battle_over():
    print(battle.next_turn())
print(battle.winner())
```

## Follow-ups

1. How would you implement a status effect system (poison, stun) cleanly without modifying the `Character` class?
2. How do you ensure ability cooldowns are tracked correctly across multi-character teams?
3. How would you serialize and restore battle state for a save/load feature?
4. Extend to support items that any character can use on their turn.

Video Game OOD: Design a Turn-Based RPG Battle System

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