Design refundable transaction ledger and prioritization rules

Design and implement a transaction ledger that ingests a stream of records where each record is either a payment or a refund. Each payment has {id, userId, amount, timestamp}; each refund has {id, userId, amount, timestamp, optional paymentIdReference}. ( 1) Choose data structures to store payments and track remaining refundable amounts. ( 2) Implement APIs: addPayment(payment), addRefund(refund, rules), and getUserBalance(userId). ( 3) When a refund arrives without paymentIdReference, select which prior payment(s) to offset based on a provided priority rule set (e.g., a comparator that can express recency-first, amount-first, or other tie-breakers). ( 4) Support partial refunds and multiple refunds against the same payment; ensure refunds cannot exceed a payment’s remaining refundable amount. ( 5) Input may interleave payments and refunds; for each refund, return the list of (paymentId, refundedAmount) pairs it offsets. ( 6) Analyze time and space complexity of each operation and justify your data-structure choices.