Implement prioritized refund allocation engine

Q: Implement prioritized refund allocation engine

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Question

Implement a refund allocation function that takes: (a) a list of payments, each with a unique paymentId, method ∈ {CREDIT, CREDIT_CARD, PAYPAL}, ISO-8601 date (yyyy-mm-dd), and amountPaid; (b) a list of existing refunds, each linked to a paymentId with amountRefunded; and (c) a new refundRequest amount. Output a list of allocations [(paymentId, method, amount)] that satisfies all rules:

Fully exhaust a payment's remaining refundable amount before moving to another payment.
Prioritize methods in this order: CREDIT, then CREDIT_CARD, then PAYPAL.
Within the same method, refund more recent payments before older ones (most recent date first).
A payment’s remaining refundable amount = amountPaid − sum(existing refunds linked to it), never negative.
If refundRequest exceeds total refundable amount, return allocations for what is available and the shortfall. Assume inputs are strings to be parsed; dates are comparable by calendar order and there are no time zones. Provide function signature, explain data structures (e.g., grouping by method, sorting by date desc, tracking remaining amounts), and analyze complexity.

Implement prioritized refund allocation engine

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