রিডিউসার ও অ্যাকশন — একটি মিনি স্টেট স্টোর বানানো

# --- reducer: একাধিক action type, প্রতিবার নতুন state dict রিটার্ন করে ---
def todo_reducer(state, action):
    if action["type"] == "ADD_TODO":
        new_todo = {"id": action["id"], "text": action["text"], "done": False}
        return {**state, "todos": state["todos"] + [new_todo]}

    elif action["type"] == "TOGGLE_TODO":
        new_todos = [
            {**todo, "done": not todo["done"]} if todo["id"] == action["id"] else todo
            for todo in state["todos"]
        ]
        return {**state, "todos": new_todos}

    elif action["type"] == "REMOVE_TODO":
        new_todos = [todo for todo in state["todos"] if todo["id"] != action["id"]]
        return {**state, "todos": new_todos}

    else:
        return state  # অচেনা action -- state অপরিবর্তিত থাকে (একই অবজেক্ট রিটার্ন)


# --- Store ক্লাস: state + dispatch() + subscribe() ---
class Store:
    def __init__(self, reducer, initial_state):
        self.reducer = reducer
        self.state = initial_state
        self.listeners = []

    def subscribe(self, listener):
        self.listeners.append(listener)

    def dispatch(self, action):
        old_state = self.state
        self.state = self.reducer(self.state, action)
        for listener in self.listeners:
            listener(self.state)
        return old_state, self.state


def on_state_change(new_state):
    print(f"  [subscriber] নোটিফিকেশন পেল -- এখন {len(new_state['todos'])}টি টুডু")


store = Store(todo_reducer, {"todos": []})
store.subscribe(on_state_change)

actions = [
    {"type": "ADD_TODO", "id": 1, "text": "বাজার করা"},
    {"type": "ADD_TODO", "id": 2, "text": "রিপোর্ট জমা দেওয়া"},
    {"type": "TOGGLE_TODO", "id": 1},
    {"type": "REMOVE_TODO", "id": 2},
]

for action in actions:
    old_state, new_state = store.dispatch(action)
    is_immutable = id(old_state) != id(new_state)
    print(f"অ্যাকশন: {action}")
    print(f"  নতুন state: {new_state}")
    print(f"  ইমিউটেবিলিটি চেক -- id(old_state) != id(new_state): {is_immutable}")
    print("-" * 60)

print(f"চূড়ান্ত state: {store.state}")