import json
import datetime

class Calendar:
    def __init__(self, filename="calendar_data.json"):
        self.filename = filename
        self.reminders = self.load_calendar()
        self.current_date = datetime.date.today()

    def load_calendar(self):
        """Load reminders from a file."""
        try:
            with open(self.filename, "r") as file:
                return json.load(file)
        except (FileNotFoundError, json.JSONDecodeError):
            return {}

    def save_calendar(self):
        """Save reminders to a file."""
        with open(self.filename, "w") as file:
            json.dump(self.reminders, file, indent=4)

    def add_reminder(self, date, reminder):
        """Add a reminder for a specific date."""
        if date not in self.reminders:
            self.reminders[date] = []
        self.reminders[date].append(reminder)
        self.save_calendar()
        print(f"Reminder added for {date}: {reminder}")

    def show_reminders(self, date):
        """Show reminders for a specific date."""
        if date in self.reminders:
            print(f"Reminders for {date}:")
            for i, reminder in enumerate(self.reminders[date], 1):
                print(f"{i}. {reminder}")
        else:
            print(f"No reminders for {date}.")

    def browse_calendar(self, days):
        """Move forward or backward in the calendar by a given number of days."""
        self.current_date += datetime.timedelta(days=days)
        print(f"Current date: {self.current_date}")
        self.show_reminders(str(self.current_date))

if __name__ == "__main__":
    calendar = Calendar()
    while True:
        print("\nCalendar Menu:")
        print("1. Add Reminder")
        print("2. Show Reminders")
        print("3. Browse Calendar")
        print("4. Exit")
        choice = input("Choose an option: ")
        
        if choice == "1":
            date = input("Enter date (YYYY-MM-DD): ")
            reminder = input("Enter reminder for event name: ")
            calendar.add_reminder(date, reminder)
        elif choice == "2":
            date = input("Enter date (YYYY-MM-DD): ")
            calendar.show_reminders(date)
        elif choice == "3":
            days = int(input("Enter number of days to move (positive for forward, negative for backward): "))
            calendar.browse_calendar(days)
        elif choice == "4":
            calendar.save_calendar()
            break
        else:
            print("Invalid option. Please try again.")