Event Notification Engine

You are tasked with implementing a core component of a marketing engine for an event marketplace. The engine needs to send targeted notifications to customers based on various campaign criteria.

Available Data

Assume the following data structures are accessible:

• Customer:
  • id (unique identifier)
  • city (customer's primary city)
  • birth_month (month of birth)
  • birth_day (day of birth)
  • latitude (geographic latitude)
  • longitude (geographic longitude)
• Event:
  • id (unique identifier)
  • title (event name)
  • city (event's city)
  • start_time (timestamp of event start)
  • latitude (geographic latitude)
  • longitude (geographic longitude)

External Service

An external NotificationService is available with the following method:

• NotificationService.send(customer_id, message): Sends a notification containing message to the specified customer_id.

Implementation Task

Implement a MarketingEngine class with a method signature similar to send_customer_notifications(customer, events, today). This method should process a given customer and a list of events to determine and send relevant notifications based on active campaigns, using today as the reference date.

Supported Campaigns

The engine must support the following notification campaigns:

1. Local Events: Notify the customer about all upcoming events that are in the same city as the customer.
2. Birthday Event: Notify the customer about the single upcoming event whose start_time is closest to the customer's next birthday after today.
3. Proximity Events: Notify the customer about the 5 upcoming events closest to the customer by geographic distance. For the purpose of this interview, you may use Euclidean distance on latitude and longitude.

Design Requirements

• Extensibility: Design the code such that new campaigns can be added in the future without requiring modifications to the core MarketingEngine logic.
• Edge Cases: Explain how your solution would handle:
  • Ties (e.g., multiple events equally close for Campaign 2 or 3).
  • Fewer than 5 matching events (for Campaign 3).
• Notification Formatting: Describe how the message parameter for NotificationService.send would be constructed.