How should I practice Transaction Fee Calculator Part 3 — Merchant / Provider Waiver?

Start by reading the full statement, solve the visible examples, then compare the pattern against related Stripe phone screen problems in FastPrep.

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Problem · String

Transaction Fee Calculator Part 3 — Merchant / Provider Waiver

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Problem statement

Source note: July 26, 2026 — 🌺 I re-audited the original author-written sources and rebuilt this practice version from the available interview reports. It is about a 65% match to the available source. The CSV fee-calculation setting, transaction ID/provider/type/status/amount fields, output columns, and the Part 3 rule that a merchant's later transactions with one payment provider become fee-free after that merchant/provider volume exceeds 10000 come directly from the sources. The source specifically warns not to combine different merchants. The exact base-fee table, cross-language signature, CSV header, whether only completed payments contribute, rounding, and boundary behavior were not fully shown, so FastPrep passes the base fee rules as input and states its deterministic adapter choices. The generated-page percentage discount tiers were removed.

Compute transaction fees in input order and apply a volume waiver independently for every (merchant_id, payment_provider) pair.

Input

csvData has header id,amount,transaction_type,payment_provider,status,merchant_id; amount is a non-negative integer.
feeRules contains "provider,rate,fixed". For a completed transaction, its ordinary fee is floor(amount * rate + fixed).

Waiver

Track the cumulative amount of earlier payment_completed transactions for each merchant/provider pair. If that earlier cumulative amount is strictly greater than 10000, the current completed transaction's fee is 0. Otherwise charge the ordinary fee, then add the current amount to the pair's cumulative amount. Thus the transaction that first pushes the total above 10000 is still charged; only later transactions are waived.

Non-completed transactions have fee 0 and do not contribute to volume. Return header id,transaction_type,payment_provider,fee followed by one row per transaction.

Practice sequence

Part 1: status gate and fee rules
Part 3: merchant/provider waiver (current)

Function

calculateFeesWithWaiver(csvData: String, feeRules: String[]) → String[]

Examples

Example 1

csvData = "id,amount,transaction_type,payment_provider,status,merchant_id\np1,6000,payment,card,payment_completed,m1\np2,5000,payment,card,payment_completed,m1\np3,100,payment,card,payment_completed,m1\np4,100,payment,card,payment_completed,m2"feeRules = ["card,0.02,30"]return = ["id,transaction_type,payment_provider,fee","p1,payment,card,150","p2,payment,card,130","p3,payment,card,0","p4,payment,card,32"]

m1/card exceeds 10000 only after p2, so p1 and p2 are charged and p3 is waived. m2 has an independent counter.

Example 2

csvData = "id,amount,transaction_type,payment_provider,status,merchant_id\na,11000,payment,card,payment_completed,m1\nb,500,payment,bank,payment_completed,m1\nc,100,payment,card,payment_failed,m1\nd,100,payment,card,payment_completed,m1"feeRules = ["card,0.01,10","bank,0.005,0"]return = ["id,transaction_type,payment_provider,fee","a,payment,card,120","b,payment,bank,2","c,payment,card,0","d,payment,card,0"]

The card total becomes 11000 after a, so later completed card transaction d is waived. Bank volume is separate, and failed transaction c neither pays a fee nor changes volume.

Constraints

The CSV uses the exact six-column header shown.
Every provider appearing in a transaction has exactly one fee rule.
Rates are non-negative decimal fractions and fixed fees are non-negative integers.
Only payment_completed rows are charged or added to volume.
The waiver key is the pair (merchant_id, payment_provider).
The threshold is strictly greater than 10000; the crossing transaction is charged.