Design: Delivery Cost System (Object-Oriented)

Goal

Build a class-based system to track delivery work by drivers, compute costs, manage payments, and answer a concurrency query.

Entities and Operations

Assume timestamps are epoch milliseconds (or a comparable monotonic time unit) and each driver has a fixed hourly rate. Costs are computed as:

• cost = hourly_rate × duration_hours
• duration_hours = (end_time − start_time) / 3600000

Implement a DeliveryCostSystem with the following methods:

1. registerDriver(driverId, hourlyRate)
   • Register a driver and their hourly rate.
2. recordDelivery(driverId, startTime, endTime)
   • Record a delivery interval for a driver.
3. getTotalCost() -> number
   • Return the total cost of all recorded deliveries (paid or unpaid).
4. payUpTo(timestamp)
   • Mark all deliveries ending on or before timestamp as paid.
5. getTotalUnpaid() -> number
   • Return the total unpaid cost of all recorded deliveries.
6. maxSimultaneousDriverInPast24Hours() -> integer
   • Return the maximum number of drivers that were simultaneously on a delivery within the last 24 hours relative to "now".

Clarifications (assume unless specified otherwise)

• A driver may not have overlapping deliveries (at most one active delivery per driver at any time).
• End times are exclusive for overlap checks (intervals are [start, end)).
• If a delivery is recorded after a payUpTo that already covers its end time, it should be considered paid immediately.
• Use currency-safe arithmetic (e.g., store cents as integers or use decimal types).
• Single-threaded in-memory design is sufficient for this exercise.